Best Performance Practices For Hibernate5 & Spring Boot 2

Hibernate & Spring Boot Samples

How To Store UTC Timezone In MySQL

Description: This application is a sample of how to store date, time, and timestamps in UTC time zone in MySQL.

Key points:
- spring.jpa.properties.hibernate.jdbc.time_zone=UTC
- spring.datasource.url=jdbc:mysql://localhost:3306/screenshotdb?useLegacyDatetimeCode=false

View Binding/Extracted Params Via Log4J 2

Description: View the prepared statement binding/extracted parameters via Log4J 2 logger setting.

Key points:
- for Maven, in pom.xml, exclude Spring Boot's Default Logging
- for Maven, in pom.xml, Add Log4j 2 Dependency
- in log4j2.xml add, <Logger name="org.hibernate.type.descriptor.sql" level="trace"/>

Output example:

How To View Query Details Via "datasource-proxy"

Description: View the query details (query type, binding parameters, batch size, execution time, etc) via datasource-proxy

Key points:
- for Maven, add in pom.xml the datasource-proxy dependency
- create an bean post processor to intercept the DataSource bean
- wrap the DataSource bean via ProxyFactory and an implementation of MethodInterceptor

Output example:

How To Batch Inserts Via saveAll(Iterable<S> entities) In MySQL

Description: Batch inserts via SimpleJpaRepository#saveAll(Iterable<S> entities) method in MySQL

Key points:
- in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
- in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
- in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
- in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
- in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
- in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
- in entity, use the assigned generator since MySQL IDENTITY will cause batching to be disabled
- in entity, add @Version property to avoid extra-SELECTs fired before batching (also prevent lost updates in multi-request transactions). Extra-SELECTs are the effect of using merge() instead of persist(); behind the scene, saveAll() uses save(), which in case of non-new entities (have IDs) will call merge(), which instruct Hibernate to fire a SELECT statement to make sure that there is no record in the database having the same identifier
- pay attention on the amount of inserts passed to saveAll() to not "overwhelm" the persistence context; normally the EntityManager should be flushed and cleared from time to time, but during the saveAll() execution you simply cannot do that, so if in saveAll() there is a list with a high amount of data, all that data will hit the persistence context (1st level cache) and will be in-memory until flush time. Using relatively small amount of data should be ok

Output example:

Batch Inserts Via EntityManager in MySQL

Description: Batch inserts via EntityManager in MySQL. This way you can easily control the flush() and clear() of the persistence context (1st level cache). This is not possible via SpringBoot, saveAll(Iterable<S> entities). Another advantage is that you can call persist() instead of merge() - this is used behind the scene by the SpringBoot saveAll(Iterable<S> entities) and save(S entity).

Output example:

How To Batch Inserts Via JpaContext/EntityManager In MySQL

Description: Batch inserts via JpaContext/EntityManager in MySQL.

Output example:

How To Exploit Session-Level Batching (Hibernate 5.2 Or Higher) In MySQL

Description: Batch inserts via Hibernate session-level batching (Hibernate 5.2 or higher) in MySQL.

Key points:
- in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
- in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
- in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
- in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
- in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
- in entity, use the assigned generator since MySQL IDENTITY will cause batching to be disabled
- the Hibernate Session is obtained by un-wrapping it via EntityManager#unwrap(Session.class)
- the batching size is set via Session#setJdbcBatchSize(Integer size) and get via Session#getJdbcBatchSize()
- in DAO, flush and clear the persistence context from time to time; this way you avoid to "overwhelm" the persistence context

Output example:

How To Use Direct Fetching Via Spring Data/EntityManager/Session

Description: Direct fetching via Spring Data, EntityManager and Hibernate Session examples.

Key points:
- direct fetching via Spring Data uses findById()
- direct fetching via EntityManager uses find()
- direct fetching via Hibernate Session uses get()

DTOs Via Spring Data Projections

Note: You may also like to read the recipe, "How To Enrich DTOs With Virtual Properties Via Spring Projections"

Description: Fetch only the needed data from the database via Spring Data Projections (DTOs)

Key points:
- write an interface (projection) containing getters only for the columns that should be fetched from the database
- write the proper query returning a List<projection>
- if is applicable, limit the number of returned rows (e.g., via LIMIT) - here, we can use query builder mechanism built into Spring Data repository infrastructure

Note: Using projections is not limited to use query builder mechanism built into Spring Data repository infrastructure. We can fetch projections via JPQL or native queries as well. For example, in this application we use a JPQL.

Output example (select first 2 rows; select only "name" and "age"):

Attribute Lazy Loading

Description: By default, the attributes of an entity are loaded eagerly (all at once). But, we can load them lazy as well. This is useful for column types that store large amounts of data: CLOB, BLOB, VARBINARY, etc or details that should be loaded on demand. In this application, we have an entity named Author. Its properties are: id, name, genre, avatar and age. And, we want to load the avatar lazy. So, the avatar should be loaded on demand.

Key points:
- in pom.xml, activate Hibernate bytecode instrumentation (e.g. use Maven bytecode enhancement plugin)
- in entity, annotate the columns that should be loaded lazy with @Basic(fetch = FetchType.LAZY)
- in application.properties, disable Open Session in View

Check as well:
- Default Values For Lazy Loaded Attributes
- Attribute Lazy Loading And Jackson Serialization

How To Populate A Child-Side Parent Association Via Proxy

Description: A Proxy can be useful when a child entity can be persisted with a reference to its parent. In such cases, fetching the parent entity from the database (execute the SELECT statement) is a performance penalty and a pointless action. Hibernate can set the underlying foreign key value for an uninitialized Proxy.

Key points:
- rely on EntityManager#getReference()
- in Spring, use JpaRepository#getOne() -> used in this example
- in Hibernate, use load()
- here, we have two entities, Tournament and TennisPlayer, and a tournament can have multiple players (@OneToMany)
- we fetch the tournament via a Proxy (this will not trigger a SELECT), we create a new tennis player, we set the Proxy as the tournament for this player and we save the player (this will trigger an INSERT in the tennis players table, tennis_player)

Output example:
- the console output will reveal that only an INSERT is triggered, not the SELECT

How To Quickly Reproduce The N+1 Performance Issue

Description: The N+1 is an issue of lazy fetching (but, eager is not exempt). This application reproduce the N+1 behavior.

Key points:
- define two entities, Author and Book in a lazy bidirectional @OneToMany relationship
- fetch all Book lazy, so without Author (results in 1 query)
- loop the fetched Book collection and for each entry fetch the corresponding Author (results N queries)
- or, fetch all Author lazy, so without Book (results in 1 query)
- loop the fetched Author collection and for each entry fetch the corresponding Book (results N queries)

Output example:

Optimize SELECT DISTINCT Via Hibernate HINT_PASS_DISTINCT_THROUGH Hint

Description: Starting with Hibernate 5.2.2, we can optimize JPQL (HQL) query entites of type SELECT DISTINCT via HINT_PASS_DISTINCT_THROUGH hint.

Key points:
- use @QueryHints(value = @QueryHint(name = HINT_PASS_DISTINCT_THROUGH, value = "false"))

Output example:

How To Enable Dirty Tracking

Description: Prior to Hibernate version 5, the dirty checking mechanism relies on Java Reflection API. Starting with Hibernate version 5, the dirty checking mechanism relies on bytecode enhancement. This approach sustain a better performance, especially when you have a relatively large number of entitites.

Key points:
- add the corresponding plugin in pom.xml (use Maven bytecode enhancement plugin)

Output example:

The bytecode enhancement effect can be seen on User.class here

How To Use Java 8 Optional In Entities And Queries

Description: This application is an example of how is correct to use the Java 8 Optional in entities and queries.

Key points:
- use the Spring Data built-in query-methods that return Optional (e.g., findById())
- write your own queries that return Optional
- use Optional in entities getters
- in order to run different scenarios check the file, data-mysql.sql

How To Use OneToMany Bidirectional Correctly

Description: This application is a proof of concept of how is correct to implement the bidirectional @OneToMany association.

Key points:
- always cascade from parent to child
- use mappedBy on the parent
- use orphanRemoval on parent in order to remove children without references
- use helper methods on parent to keep both sides of the association in sync
- use lazy fetching on both side of the association
- as entities identifiers, use assigned identifiers (business key, natural key (@NaturalId)) and/or database-generated identifiers and override (on child-side) properly the equals() and hashCode() methods as here
- if toString() need to be overridden, then pay attention to involve only for the basic attributes fetched when the entity is loaded from the database

How To Use Query Fetching

Description: This application is a proof of concept of how to write a query via JpaRepository, EntityManager and Session.

Key points:
- for JpaRepository use @Query or Spring Data Query Creation
- for EntityManager and Session use the createQuery() method

How To Avoid MySQL & Hibernate 5 AUTO Generator Type

Description: In MySQL & Hibernate 5, the GenerationType.AUTO generator type will result in using the TABLE generator. This adds a significant performance penalty. Turning this behavior to IDENTITY generator can be obtained by using GenerationType.IDENTITY or the native generator.

Key points:
- use GenerationType.IDENTITY instead of GenerationType.AUTO
- use the native generator - exemplified in this application

Output example:

How To Avoid The Redundant save() Call

Description: This application is an example when calling save() for a managed entity is redundant.

Key points:
- Hibernate triggers UPDATE statements for managed entities without the need to explicitly call the save() method
- behind the scene, this redundancy implies a performance penalty as well

How To Handle PostgreSQL (BIG)SERIAL And Batching Inserts

Description: In PostgreSQL, using GenerationType.IDENTITY will disable insert batching. The (BIG)SERIAL is acting "almost" like MySQL, AUTO_INCREMENT. In this application, we use the GenerationType.SEQUENCE which enable insert batching, and we optimize it via the hi/lo optimization algorithm.

Key points:
- use GenerationType.SEQUENCE instead of GenerationType.IDENTITY
- rely on the hi/lo algorithm to fetch multiple identifiers in a single database roundtrip (you can go even further and use the Hibernate pooled and pooled-lo identifier generators (these are optimizations of hi/lo))

Output example:

How To Write JPA Inheritance - Single Table

Description: This application is a sample of JPA Single Table inheritance strategy (SINGLE_TABLE)

Key points:
- this is the default inheritance strategy (@Inheritance(strategy=InheritanceType.SINGLE_TABLE))
- all the classes in a hierarchy are mapped to a single table in a the database

Output example (below is a single table obtained from 4 entities):

How To Count And Assert SQL Statements

Description: This application is a sample of counting and asserting SQL statements triggered "behind the scene". Is very useful to count the SQL statements in order to ensure that your code is not generating more SQLs that you may think (e.g., N+1 can be easily detected by asserting the number of expected statements).

Key points:
- for Maven, in pom.xml, add dependencies for datasource-proxy and Vlad Mihalcea's db-util
- create the ProxyDataSourceBuilder with countQuery()
- reset the counter via SQLStatementCountValidator.reset()
- assert INSERT, UPDATE, DELETE and SELECT via assertInsert/Update/Delete/Select/Count(long expectedNumberOfSql)

Output example (when the number of expected SQLs is not equal with the reality an exception is thrown):

How To Use JPA Callbacks

Description: This application is a sample of enabling the JPA callbacks (Pre/PostPersist, Pre/PostUpdate, Pre/PostRemove and PostLoad).

Key points:
- in entity, write callback methods and use the proper annotations
- callback methods annotated on the bean class must return void and take no arguments

Output example:

How To Use @MapsId For Sharing Identifier In @OneToOne Relationship

Description: Instead of classical unidirectional/bidirectional @OneToOne better rely on an unidirectional @OneToOne and @MapsId. This application is a proof of concept.

Key points:
- use @MapsId on child side
- use @JoinColumn to customize the name of the Primary Key column
- basically, for @OneToOne associations, @MapsId will share the Primary Key with the parent table (id property acts as both Primary Key and Foreign Key)

How To Extract DTOs Via SqlResultSetMapping

Description: Fetching more data than needed is prone to performance penalities. Using DTOs allows us to extract only the needed data. In this application we rely on SqlResultSetMapping and EntityManager.

Key points:
- use SqlResultSetMapping and EntityManager
- for using Spring Data Projections check this recipe

How To Extract DTOs Via SqlResultSetMapping & NamedNativeQuery

Key points:
- use SqlResultSetMapping, NamedNativeQuery and EntityManager
- for using Spring Data Projections check this recipe

How To Extract DTOs Via javax.persistence.Tuple And Native SQL

Description: Fetching more data than needed is prone to performance penalities. Using DTOs allows us to extract only the needed data. In this application we rely on javax.persistence.Tuple and native SQL.

Key points:
- use java.persistence.Tuple and EntityManager.createNativeQuery()
- for using Spring Data Projections check this recipe

How To Extract DTOs Via javax.persistence.Tuple And JPQL

How To Extract DTOs Via Constructor Expression And JPQL

Description: Fetching more data than needed is prone to performance penalities. Using DTOs allows us to extract only the needed data. In this application we rely on Constructor Expression and JPQL.

Key points:
- use a proper constructor in the DTO class
- and use a query as select new com.jpa.CarDto(c.name, c.color) from Car c
- for using Spring Data Projections check this recipe

How To Extract DTOs Via ResultTransformer And Native SQL

Description: Fetching more data than needed is prone to performance penalities. Using DTOs allows us to extract only the needed data. In this application we rely on Hibernate, ResultTransformer and native SQL.

Key points:
- use AliasToBeanConstructorResultTransformer for DTOs without setters, with constructor
- use Transformers.aliasToBean() for DTOs with setters
- use EntityManager.createNativeQuery() and unwrap(org.hibernate.query.NativeQuery.class)
- starting with Hibernate 5.2, ResultTransformer is deprecated, but until a replacement will be available (in Hibernate 6.0) it can be used (read further)
- for using Spring Data Projections check this recipe

How To Extract DTOs Via ResultTransformer And JPQL

Key points:
- use AliasToBeanConstructorResultTransformer for DTOs without setters, with constructor
- use Transformers.aliasToBean() for DTOs with setters
- use EntityManager.createQuery() and unwrap(org.hibernate.query.Query.class)
- starting with Hibernate 5.2, ResultTransformer is deprecated, but until a replacement will be available (in Hibernate 6.0) it can be used (read further)
- for using Spring Data Projections check this recipe

How To Extract DTOs Via Blaze-Persistence Entity Views

Description: Fetching more data than needed is prone to performance penalities. Using DTOs allows us to extract only the needed data. In this application we rely on Blaze-Persistence entity views.

Key points:
- add in pom.xml dependencies specific to Blaze-Persistence
- configure Blaze-Persistence, CriteriaBuilderFactory and EntityViewManager
- write an entity view via an interface in Blaze-Persistence fashion
- write a Spring-centric repository by extending EntityViewRepository
- call method of this repository such as, findAll(), findOne(), etc
- for using Spring Data Projections check this recipe

How @ElementCollection Without @OrderColumn Works

Description: This application reveals the possible performance penalties of using @ElementCollection. In this case, without @OrderColumn. As you can see in the next recipe (34) adding @OrderColumn can mitigate some performance penalties.

Key points:
- an @ElementCollection doesn't have a Primary Key
- an @ElementCollection is mapped in a separate table
- avoid @ElementCollection when you have a lot of inserts and deletes in/from it since the database has to delete all existing rows in order to add a new one or delete one
- the more items we have in this table the greater the performance penalty

Output example:

How @ElementCollection With @OrderColumn Works

Description: This application reveals the performance penalties of using @ElementCollection. In this case, with @OrderColumn. But, as you can see in this application, by adding @OrderColumn can mitigate some performance penalties when operations take place near the collection tail (e.g., add/remove at/from the end of the collection). Mainly, all elements situated before the adding/removing entry are left untouched, so the performance penalty can be ignored if we affect rows close to the collection tail.

Key points:
- an @ElementCollection doesn't have a Primary Key
- an @ElementCollection is mapped in a separate table
- pefer @ElementCollection with @OrderColumn when you have a lot of inserts and deletes from the collection tail
- the more items are inserted/removed from the beginning of this table the greater the performance penalty

Output example:

How To Avoid Lazy Initialization Caused By Open Session In View Anti-Pattern (1 Session / 1 HTTP Request-Response)

Description: The Open-Session in View anti-pattern is activated by default in SpringBoot. If you prefer to use it then it is recommended to mitigate its performance penalties as much as possible. One optimization consist in marking the Connection as read-only which would allow the database server to avoid writing to the transaction log. Another optimization consist in explicitly setting the lazy properties of the fetched entities when you don't want them to be lazy initialized.

Key points:
- fetch a entity and set the lazy properties explicitly
- you can do this is the service or controller layer, depending where it fits better to your case, but outside of an explicit transaction
- why is this working? why we can set the property of a managed entity and not trigger the flush? well, the answer can be found in the documentation of OpenSessionInViewFilter which specifies that:

NOTE: This filter will by default not flush the Hibernate Session, with the flush mode set to FlushMode.NEVER. It assumes to be used in combination with service layer transactions that care for the flushing: The active transaction manager will temporarily change the flush mode to FlushMode.AUTO during a read-write transaction, with the flush mode reset to FlushMode.NEVER at the end of each transaction. If you intend to use this filter without transactions, consider changing the default flush mode (through the "flushMode" property).

Output example:

How To Use Spring Projections(DTOs) And Inner Joins

Description: This application is a proof of concept for using Spring Projections(DTOs) and inner joins written via JPQL and native SQL (for MySQL).

Key points:
- define two entities (e.g., Author and Book in a lazy bidirectional @OneToMany relationship)
- populate the database with some test data (e.g., check the file resources/data-mysql.sql)
- write interfaces (projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
- write inner joins queries using JPQL/SQL

How To Use Spring Projections(DTOs) And Left Joins

Description: This application is a proof of concept for using Spring Projections(DTOs) and left joins written via JPQL and native SQL (for MySQL).

How To Use Spring Projections(DTOs) And Right Joins

Description: This application is a proof of concept for using Spring Projections(DTOs) and right joins written via JPQL and native SQL (for MySQL).

How To Use Spring Projections(DTOs) And Inclusive Full Joins (PostgreSQL)

Description: This application is a proof of concept for using Spring Projections(DTOs) and inclusive full joins written via JPQL and native SQL (for PostgreSQL).

How To Use Spring Projections(DTOs) And Exclusive Left Joins

Description: This application is a proof of concept for using Spring Projections(DTOs) and exclusive left joins written via JPQL and native SQL (for MySQL).

How To Use Spring Projections(DTOs) And Exclusive Right Joins

Description: This application is a proof of concept for using Spring Projections(DTOs) and exclusive right joins written via JPQL and native SQL (for MySQL).

How To Use Spring Projections(DTOs) And Exclusive Full Joins (PostgreSQL)

Description: This application is a proof of concept for using Spring Projections(DTOs) and exclusive full joins written via JPQL and native SQL (for PostgreSQL).

Why You Should Avoid Time-Consuming Tasks In Spring Boot Post-Commits

Description: This application is a proof of concept for using Spring post-commit hooks.

Key points:
- avoid time-consuming task in post-commits since the database connection will remain open until this code finshes

How To Exploit Spring Projections(DTOs) And Join Unrelated Entities in Hibernate 5.1+

Description: This application is a proof of concept for using Spring Projections (DTOs) and join unrelated entities. Hibernate 5.1 introduced explicit joins on unrelated entities and the syntax and behaviour are similar to SQL JOIN statements.

Key points:
- define serveral entities (e.g., Author and Book unrelated entities)
- populate the database with some test data (e.g., check the file resources/data-mysql.sql)
- write interfaces (projections) that contains getters for the columns that should be fetched from the database (e.g., BookstoreDto)
- write joins queries using JPQL/SQL (e.g., queries all authors names and book titles of the given price)

Why To Avoid Lombok @EqualsAndHashCode And @Data In Entities And How To Override equals() And hashCode()

Description: Entities should implement equals() and hashCode() as here. The main idea is that Hibernate requires that an entity is equal to itself across all its state transitions (transient, attached, detached and removed). Using Lombok @EqualsAndHashCode (or @Data) will not respect this requirment.

Key points:
AVOID THESE APPROACHES
- Using Lombok default behavior of @EqualsAndHashCode
(entity: LombokDefaultBook, test: LombokDefaultEqualsAndHashCodeTest)
- Using Lombok @EqualsAndHashCode with primary key only
(entity: LombokIdBook, test: LombokEqualsAndHashCodeWithIdOnlyTest)
- Rely on default equals() and hashCode()
(entity: DefaultBook, test: DefaultEqualsAndHashCodeTest)
- Rely on default equals() and hashCode() containing only the database-generated identifier
(entity: IdBook, test: IdEqualsAndHashCodeTest)

PREFER THESE APPROACHES
- Rely on business key (entity: BusinessKeyBook, test: BusinessKeyEqualsAndHashCodeTest)
- Rely on @NaturalId (entity: NaturalIdBook, test: NaturalIdEqualsAndHashCodeTest)
- Rely on manually assigned identifiers (entity: IdManBook, test: IdManEqualsAndHashCodeTest)
- Rely on database-generated identifiers (entity: IdGenBook, test: IdGenEqualsAndHashCodeTest)

How To Avoid LazyInitializationException Via JOIN FETCH

Description: Typically, when we get a LazyInitializationException we tend to modify the relationship fetching type from LAZY to EAGER. That is bad! This is a code smell. Best way to avoid this exception is to rely on JOIN FETCH (if you plan to modify the fetched entities) or JOIN + DTOs (if the fetched data is only read). JOIN FETCH allows associations or collections of values to be initialized along with their parent objects using a single SELECT. This application is a JOIN FETCH example with entities. But, with some constraints, JOIN FETCH can be used with DTOs as well. An example is available here.

Key points:
- define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
- write a JPQL JOIN FETCH to fetch an author including his books
- write a JPQL JOIN FETCH to fetch a book including its author

Output example:

How To Merge Entity Collections

Description: This is a Spring Boot example based on the following article. Is a functional implementation of the Vlad's example. It is highly recommended to read that article.

Key points:
- Remove the existing database rows that are no longer found in the incoming collection
- Update the existing database rows which can be found in the incoming collection
- Add the rows found in the incoming collection, which cannot be found in the current database snapshot

How To Delay Connection Acquisition As Needed (Hibernate 5.2.10)

Description: This is a Spring Boot example that exploits Hibernate 5.2.10 capability of delaying the connection acquisition as needed. Normally, a database connection is aquried immediately after calling a method annotated with @Transactional. If this method contains some time-consuming tasks before the first SQL statement then the connection is holded for nothing. But, Hibernate 5.2.10 allows us to delay the connection acquisition as needed. This example rely on HikariCP as the default connection pool for Spring Boot.

Key points:
- set spring.datasource.hikari.auto-commit=false in application.properties
- set spring.jpa.properties.hibernate.connection.provider_disables_autocommit=true in application.properties

Output example:

How To Fetch Primary Keys Via Hibernate hi/lo Algorithm

Description: This is a Spring Boot example of using the hi/lo algorithm for fetching 1000 PKs in 10 database roundtrips for batching 1000 inserts in batches of 10 inserts. The hi/lo algorithm is an optimization algorithm for generating sequences of identifiers.

Key points:
- use the SEQUENCE generator type (e.g., in PostgreSQL)
- configure the hi/lo algorithm as in Player.java entity

Output example:

How To Correctly Write a Bidirectional @ManyToMany Association

Description: This application is a proof of concept of how is correct to implement the bidirectional @ManyToMany association.

Key points:
- choose and set the owner of the relationship via mappedBy
- materialize the relationships collections via Set not List
- use helper methods on the owner of the relationship to keep both sides of the association in sync
- on the owner of the relationship use CascadeType.PERSIST and CascadeType.MERGE, but avoid CascadeType.REMOVE/ALL
- on the owner of the relationship set up join table
- @ManyToMany is lazy by default; keep it this way!
- as entities identifiers, use assigned identifiers (business key, natural key (@NaturalId)) and/or database-generated identifiers and override properly (on both sides) the equals() and hashCode() methods as here
- if toString() need to be overridden, then pay attention to involve only for the basic attributes fetched when the entity is loaded from the database

Prefer Set Instead of List in @ManyToMany Relationships

Description: This is a Spring Boot example of removing rows in case of a bidirectional @ManyToMany using a List and a Set. The conclusion is that Set is much better! This applies to unidirectional as well!

Key points:
- using Set is much more efficent than List

Output example:

How To View Query Details Via log4jdbc

Description: View the query details via log4jdbc

Key points:
- for Maven, in pom.xml, add log4jdbc dependency

Output sample:

How To View Binding Params Via TRACE

Description: View the prepared statement binding/extracted parameters via TRACE

Key points:
- in application.properties add: logging.level.org.hibernate.type.descriptor.sql=TRACE

Output sample:

How To Store java.time.YearMonth As Integer Or Date Via Hibernate Types Library

Description: Hibernate Types is a set of extra types not supported by Hibernate by default. One of these types is java.time.YearMonth. This is a Spring Boot application that uses Hibernate Type to store this YearMonth in a MySQL database as integer or Date.

Key points:
- for Maven, add Hibernate Types as a dependency in pom.xml
- in entity use @TypeDef to map typeClass to defaultForType

Output example:

How To Execute SQL Functions With Multiple Parameters in a JPQL Query

Description: Trying to use SQL functions (MySQL, PostgreSQL, etc) in JPQL queries may result in exceptions if Hibernate will not recognize them and cannot parse the JPQL query. For example, the MySQL, concat_ws function is not recognized by Hibernate. This application is a Spring Boot application based on Hibernate 5.3, that registers the concat_ws function via MetadataBuilderContributor and inform Hibernate about it via, metadata_builder_contributor property. This example uses @Query and EntityManager as well, so you can see two use cases.

Key points:
- use Hibernate 5.3 (or, to be precisely, 5.2.18) (e.g., use Spring Boot 2.1.0.RELEASE)
- implement MetadataBuilderContributor and register the concat_ws MySQL function
- in application.properties, set spring.jpa.properties.hibernate.metadata_builder_contributor to point out to MetadataBuilderContributor implementation

Output example:

Log Slow Queries Via "datasource-proxy"

Description: This application is a sample of logging only slow queries via datasource-proxy. A slow query is a query that has an exection time bigger than a specificed threshold in milliseconds.

Output example:

Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<dto>

Description: This application fetches data as Page<dto> via Spring Boot offset pagination. Most of the time, the data that should be paginated is read-only data. Fetching the data into entities should be done only if we plan to modify that data, therefore, fetching read only data as Page<entity> is not preferable since it may end up in a significant performance penalty. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

Key points:
- create a Spring projection (DTO) to contains getters only for the data that should be fetched
- write a repository that extends PagingAndSortingRepository
- fetch data via a JPQL or native query (that includes counting) into a List<dto>, and a Pageable
- use the fetched List<dto> and Pageable to create a Page<dto>

Offset Pagination - Trigger SELECT COUNT Subquery And Return List<dto>

Description: This application fetches data as List<dto> via Spring Boot offset pagination. Most of the time, the data that should be paginated is read-only data. Fetching the data into entities should be done only if we plan to modify that data, therefore, fetching read only data as List<entity> is not preferable since it may end up in a significant performance penalty. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

How To Configure HikariCP Via application.properties

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up HikariCP via application.properties. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorServicefor simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:
- in application.properties, rely on spring.datasource.hikari.* to configure HikariCP

Output sample:

How To Configure HikariCP Via DataSourceBuilder

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Description: This is a kickoff application that set up HikariCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in application.properties, configure HikariCP via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

Output sample:

Running a SpringBoot Application Under Payara Server Using a Payara Data Source (JDBC Resource and Connection Pool)

This application is detailed in this DZone article.

How To Configure BoneCP Via DataSourceBuilder

Description: This is a kickoff application that set up BoneCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in pom.xml add the BoneCP dependency
- in application.properties, configure BoneCP via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

Output sample:

How To Configure ViburDBCP Via DataSourceBuilder

Description: This is a kickoff application that set up ViburDBCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in pom.xml add the ViburDBCP dependency
- in application.properties, configure ViburDBCP via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

Output sample:

How To Configure C3P0 Via DataSourceBuilder

Description: This is a kickoff application that set up C3P0 via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in pom.xml add the C3P0 dependency
- in application.properties, configure C3P0 via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

Output sample:

How To Configure DBCP2 Via DataSourceBuilder

Description: This is a kickoff application that set up DBCP2 via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in pom.xml add the DBCP2 dependency
- in application.properties, configure DBCP2 via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

How To Configure Tomcat Connection Pool Via DataSourceBuilder

Description: This is a kickoff application that set up Tomcat Connection Pool via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purpose the application uses an ExecutorService for simulating concurrent users.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in pom.xml add the Tomcat Connection Pool dependency
- in application.properties, configure Tomcat Connection Pool via a custom prefix, e.g., app.datasource.*
- write a @Bean that returns the DataSource

Output sample:

How To Configure Two Data Sources With Two Connection Pools

Description: This is a kickoff application that uses two data sources (two MySQL databases, one named players_db and one named coaches_db) with two connection pools (each database uses its own HikariCP connection pool with different settings). Based on the above recipes is pretty easy to configure two connection pools from two different providers as well.

Key points:
- in pom.xml, add the spring-boot-configuration-processor dependency
- in application.properties, configure two HikariCP connection pools via a two custom prefixes, e.g., app.datasource.ds1 and app.datasource.ds2
- write a @Bean that returns the first DataSource and mark it as @Primary
- write another @Bean that returns the second DataSource
- configure two EntityManagerFactory and point out the packages to scan for each of them
- put the domains and repositories for each EntityManager in the right packages

Output sample:

How To Provide a Fluent API Via Setters For Building Entities

Note: If you want yo provide a Fluent API without altering setters then consider this recipe.

Description: This is a sample application that alter the entities setters methods in order to empower a Fluent API.

Key points:
- in entities, return this instead of void in setters

Fluent API example:

How To Provide a Fluent API Via Additional Methods For Building Entities

Note: If you want yo provide a Fluent API by altering setters then consider this recipe.

Description: This is a sample application that add in entities additional methods (e.g., for setName, we add name) methods in order to empower a Fluent API.

Key points:
- in entities, add for each setter an additional method that return this instead of void

Fluent API example:

How To Implement Slice<T> findAll()

Most probably this is all you want: How To Fetch Slice<entity>/Slice<dto> Via fetchAll/fetchAllDto

Some implementations of Slice<T> findAll():

This is a thin implementation based on a hard-coded SQL: "SELECT e FROM " + entityClass.getSimpleName() + " e;"
This is just another minimalist implementation based on CriteriaBuilder instead of hard-coded SQL
This is an implementation that allows us to provide a Sort, so sorting results is possible
This is an implementation that allows us to provide a Sort and a Spring Data Specification
This is an implementation that allows us to provide a Sort, a LockModeType, a QueryHints and a Spring Data Specification
This is an implementation that allows us to provide a Spring Data Pageable and/or Specification by extending the SimpleJpaRepository from Spring Data. Bascially, this implementation is the only one that returns Page<T> instead of Slice<T>, but it doesn't trigger the extra SELECT COUNT since it was eliminated by overriding the Page<T> readPage(...) method from SimpleJpaRepository. The main drawback is that by returing a Page<T> you don't know if there is a next page or the current one is the last. Nevertheless, there are workarounds to have this as well. In this implementation you cannot set LockModeType or query hints.

Story: Spring Boot provides an offset based built-in paging mechanism that returns a Page or Slice. Each of these APIs represents a page of data and some metadata. The main difference is that Page contains the total number of records, while Slice can only tell if there is another page available. For Page, Spring Boot provides a findAll() method capable to take as arguments a Pageable and/or a Specification or Example. In order to create a Page that contains the total number of records, this method triggers an SELECT COUNT extra-query next to the query used to fetch the data of the current page. This can be a performance penalty since the SELECT COUNT query is triggered every time we request a page. In order to avoid this extra-query, Spring Boot provides a more relaxed API, the Slice API. Using Slice instead of Page removes the need of this extra SELECT COUNT query and returns the page (records) and some metadata without the total number of records. So, while Slice doesn't know the total number of records, it still can tell if there is another page available after the current one or this is the last page. The problem is that Slice work fine for queries containing the SQL, WHERE clause (including those that uses the query builder mechanism built into Spring Data), but it doesn't work for findAll(). This method will still return a Page instead of Slice therefore the SELECT COUNT query is triggered for Slice<T> findAll(...);.

Description: This is a suite of samples applications that provides different versions of a Slice<T> findAll(...) method. We have from a minimalist implementation that relies on a hardcoded query as: "SELECT e FROM " + entityClass.getSimpleName() + " e"; (this recipe), to a custom implementation that supports sorting, specification, lock mode and query hints to an implementation that relies on extending SimpleJpaRepository.

Key points:
- write an abstract class that expose the Slice<T> findAll(...) methods (SlicePagingRepositoryImplementation)
- implement the findAll() methods to return Slice<T> (or Page<T>, but without the total number of elements)
- return a SliceImpl (Slice<T>) or a PageImpl (Page<T>) without the total number of elements
- implement a new readSlice() method or override the SimpleJpaRepository#readPage() page to avoid SELECT COUNT
- pass the entity class (e.g., Author.class) to this abstract class via a class repository (AuthorRepository)

Offset Pagination - Trigger COUNT(*) OVER And Return List<dto>

Description: Typically, in offset pagination, there is one query needed for fetching the data and one for counting the total number of records. But, we can fetch this information in a single database rountrip via a SELECT COUNT subquery nested in the main SELECT. Even better, for databases vendors that support Window Functions there is a solution relying on COUNT(*) OVER() as in this application that uses this window function in a native query against MySQL 8. So, prefer this one instead of SELECT COUNT subquery.

Key points:
- create a DTO projection that contains getters for the data that should be fetched and an extra-column for mapping the return of the COUNT(*) OVER() window function
- write a native query relying on this window function

Example:

How To Implement Keyset Pagination in Spring Boot

Description: When we rely on an offset paging we have the performance penalty induced by throwing away n records before reached the desired offset. Larger n leads to a significant performance penalty. When we have a large n is better to rely on keyset pagination which maintain a "constant" time for large datasets. In order to understand how bad offset can perform please check this article:

Screenshot from that article (offset pagination):

Need to know if there are more records?
By its nature, keyset doesn't use a SELECT COUNT to fetch the number of total records. But, we a little tweak we can easily say if there are more records, therefore to show a button of type Next Page. Mainly, if you need such a thing then consider this application.

public AuthorView fetchNextPage(long id, int limit) {
List<Author> authors = authorRepository.fetchAll(id, limit + 1);

     if (authors.size() == (limit + 1)) {
          authors.remove(authors.size() - 1);
          return new AuthorView(authors, true);
     }

return new AuthorView(authors, false);
}

Or, like this (rely on Author.toString() method):

public Map<List<Author>, Boolean> fetchNextPage(long id, int limit) {
List<Author> authors = authorRepository.fetchAll(id, limit + 1);

     if(authors.size() == (limit + 1)) {
          authors.remove(authors.size() -1);
          return Collections.singletonMap(authors, true);
     }

return Collections.singletonMap(authors, false);
}

A Previous Page button can be implemented easily based on the first record.

Key points:
- choose the column(s) to act as the latest visited record (e.g., id)
- use the column(s) in the WHERE and ORDER BY clauses of your SQL

How To Implement Offset Pagination in Spring Boot

Description: This is a classical Spring Boot offset pagination example. However, is not advisable to use this approach in production because of its performance penalties explained further.

When we rely on an offset pagination, we have the performance penalty induced by throwing away n records before reaching the desired offset. Larger n leads to a significant performance penalty. Another penalty is the extra-SELECT needed to count the total number of records. In order to understand how bad offset pagination can perform please check this article. A screenshot from that article is below: Nevertheless, maybe this example is a little bit extreme. For relatively small datasets, offset pagination is not so bad (it is close in performance to keyset pagination), and, since Spring Boot provides built-in support for offset pagination via the Page API, it is very easy to use it. However, depending on the case, we can optimize a little bit the offset pagination as in the following examples:

Fetch a page as a Page:

Fetch a page as a List:

But: If offset pagination is causing you performance issues and you decide to go with keyset pagination then please check here (keyset pagination).

Key points of classical offset pagination:
- write a repository that extends PagingAndSortingRepository
- call or write methods that returns Page<entity>

Examples of classical offset pagination:
- call the built-in findAll(Pageable) without sorting:
repository.findAll(PageRequest.of(page, size));
- call the built-in findAll(Pageable) with sorting:
repository.findAll(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "name")));
- use Spring Data query creation to define new methods in your repository:
Page<Author> findByName(String name, Pageable pageable);
Page<Author> queryFirst10ByName(String name, Pageable pageable);

How To Optimize Batch Inserts of Parent-Child Relationships In MySQL

Description: Let's suppose that we have a one-to-many relationship between Author and Book entities. When we save an author, we save his/her books as well thanks to cascading all/persist. We want to create a bunch of authors with books and save them in the database (e.g., a MySQL database) using the batch technique. By default, this will result in batching each author and the books per author. In order to batch authors and books, we need to order inserts as in this application.

Key points:
- beside all setting specific to batching inserts in MySQL, we need to set up in application.properties the following property: spring.jpa.properties.hibernate.order_inserts=true

Example without ordered inserts:

Example with ordered inserts:

How To Batch Updates In MySQL

Implementations:

Description: Batch updates in MySQL.

Key points:
- in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
- in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL, statements get rewritten into a single String buffer and sent in a single request)
- in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
- in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
- in case of using a parent-child relationship with cascade all/persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_updates=true to optimize the batching by ordering updates
- before Hibernate 5, we need to set in application.properties a setting for enabling batching for versioned entities during update and delete operations (entities that contains @Version for implicit optimistic locking); this setting is: spring.jpa.properties.hibernate.jdbc.batch_versioned_data=true; starting with Hibernate 5, this setting should be true by default

Output example for single entity:

Output example for parent-child relationship:

How To Batch Deletes That Don't Involve Associations In MySQL

Description: Batch deletes that don't involve associations in MySQL.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use classical delete batching. They rely on Query.executeUpdate() to trigger bulk operations. These operations are fast, but Hibernate doesn’t know which entities are removed, therefore, the persistent context is not synchronized accordingly. The first one simply triggers a delete from entity_name statement and is very useful for deleting all records. The second one triggers a delete from entity_name where id=? or id=? or id=? ... statement, therefore, is prone to cause issues if the generated DELETE statement exceedes the maximum accepted size. This drawback can be controlled by deleting the data in chunks that results in DELETE statements that don't exceed the maximum accepted size. The maximum accepted size depends on RDBMS (e.g., for MySQL, execute SHOW VARIABLES LIKE 'max_allowed_packet';). Nevertheless, this approach is very fast. Is faster than classical delete batching which can be achieved via the deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). The delete()/deleteAll() methods rely on EntityManager.remove() therefore the persistent context is synchronized accordingly.

Key points for classical delete batching:
- for classical delete batching rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method
- in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
- in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL, statements get rewritten into a single String buffer and sent in a single request)
- in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
- in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
- before Hibernate 5, we need to set in application.properties a setting for enabling batching for versioned entities during update and delete operations (entities that contains @Version for implicit optimistic locking); this setting is: spring.jpa.properties.hibernate.jdbc.batch_versioned_data=true; starting with Hibernate 5, this setting should be true by default

Output example:

How To Batch Deletes In MySQL Via orphanRemoval=true

Description: Batch deletes in MySQL via orphanRemoval=true.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use batching and don't take advantage of orphanRemoval=true. They trigger bulk operations and the persistent context is not synchronized accordingly. The first one simply triggers a delete from entity_name statement, while the second one triggers a delete from entity_name where id=? or id=? or id=? ... statement. Using these methods for deleting parent-entities and the associated entites (child-entities) requires explicit calls for both sides. For batching rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or even better, on delete() method. Behind the scene, deleteAll() methods uses delete(). The delete() and deleteAll() methods rely on EntityManager.remove(), therefore, the persistent context is synchronized.

Key points for using deleteAll()/delete():
- in this example, we have a Author entity and each author can have several Book (one-to-many)
- first, we use orphanRemoval=true and CascadeType.ALL
- second, we dissociate all Book from the corresponding Author
- third, we explicitly (manually) flush the persistent context; is time for orphanRemoval=true to enter into the scene; thanks to this setting, all disassociated books will be deleted; the generated DELETE statements are batched (if orphanRemoval is set to false, a bunch of updates will be executed instead of deletes)
- forth, we delete all Author via the deleteAll() or delete() method (since we have dissaciated all Book, the Author deletion will take advantage of batching as well)

How To Batch Deletes In MySQL Via SQL "ON DELETE CASCADE"

Description: Batch deletes in MySQL via ON DELETE CASCADE. Auto-generated database schema will contain the ON DELETE CASCADE directive.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use classical delete batching. They trigger bulk operations via Query.executeUpdate(), therefore, the persistent context is not synchronized accordingly. The first one simply triggers a delete from entity_name statement, while the second one triggers a delete from entity_name where id=? or id=? or id=? ... statement. Both of them take advantage on ON DELETE CASCADE and are very efficient. For classical delete batching rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). Mixing batching with database automatic actions (ON DELETE CASCADE) will result in a partially synchronized persistent context.

Key points:
- in this application, we have a Author entity and each author can have several Book (one-to-many)
- first, we remove orphanRemoval or set it to false
- second, we use only CascadeType.PERSIST and CascadeType.MERGE
- third, we set @OnDelete(action = OnDeleteAction.CASCADE) next to @OneToMany
- fourth, we set spring.jpa.properties.hibernate.dialect to org.hibernate.dialect.MySQL5InnoDBDialect
- fifth, we run through each deleteFoo() method

Output example:

How To Use Hibernate @NaturalId in SpringBoot

Alternative implementation: In case that you want to avoid extending SimpleJpaRepository check this implementation.

Description: This is a SpringBoot application that maps a natural business key using Hibernate @NaturalId.

Key points:
- in the entity (e.g., Product), mark the properties (business keys) that should act as natural IDs with @NaturalId; commonly, there is a single such property, but multiple are suppored as well as here.
- for non-mutable ids, mark the columns as @NaturalId(mutable = false) and @Column(nullable = false, updatable = false, unique = true, ...)
- for mutable ids, mark the columns as @NaturalId(mutable = true) and @Column(nullable = false, updatable = true, unique = true, ...)
- override the equals() and hashCode() using the natural id(s)
- define a @NoRepositoryBean interface (NaturalRepository) to define two methods, named findBySimpleNaturalId() and findByNaturalId()
- provide an implementation for this interface (NaturalRepositoryImpl) relying on Hibernate, Session, bySimpleNaturalId() and byNaturalId() methods
- use @EnableJpaRepositories(repositoryBaseClass = NaturalRepositoryImpl.class) to register this implementation as the base class
- for the entity, write a classic repository
- inject this class in your services and call findBySimpleNaturalId() or findByNaturalId()

How To Set Up p6spy in Spring Boot

Description: This is a Spring Boot application that uses P6Spy. P6Spy is a framework that enables database data to be seamlessly intercepted and logged with no code changes to the application.

Key points:
- in pom.xml, add the P6Spy Maven dependency
- in application.properties, set up JDBC URL as, jdbc:p6spy:mysql://localhost:3306/db_users
- in application.properties, set up driver class name as, com.p6spy.engine.spy.P6SpyDriver
- in the application root folder add the file spy.properties (this file contains P6Spy configurations); in this application, the logs will be outputed to console, but you can easy switch to a file; more details about P6Spy configurations can be found in documentation

Output sample:

How To Retry Transactions After OptimisticLockException Shaped Via @Version

Note: Optimistic locking via @Version works for detached entities as well.

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. When such exception occur, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:
- in pom.xml, add the db-util dependency
- configure the OptimisticConcurrencyControlAspect bean
- mark the method (not annotated with @Transactional) that is prone to throw (or that calls a method that is prone to throw (this method can be annotated with @Transactional)) an optimistic locking exception with @Retry(times = 10, on = OptimisticLockingFailureException.class)

Output sample:

How To Retry Transaction After OptimisticLockException Shaped Via Hibernate Version-less Optimistic Locking

Note: Optimistic locking via Hibernate version-less doesn't work for detached entities (don't close the persistent context).

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception (e.g., in Spring Boot, OptimisticLockingFailureException) via Hibernate version-less optimistic locking. When such exception occur, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:
- in pom.xml, add the db-util library dependency
- configure the OptimisticConcurrencyControlAspect bean
- annotate the corresponding entity (e.g., Inventory) with @DynamicUpdate and @OptimisticLocking(type = OptimisticLockType.DIRTY)
- mark the method (not annotated with @Transactional) that is prone to throw (or that calls a method that is prone to throw (this method can be annotated with @Transactional)) an optimistic locking exception with @Retry(times = 10, on = OptimisticLockingFailureException.class)

How To Enrich DTOs With Virtual Properties Via Spring Projections

Note: You may also like to read the recipe, "How To Create DTOs Via Spring Data Projections"

Description: Fetch only the needed data from the database via Spring Data Projections (DTOs) and enrich the result via virtual properties.

Key points:
- we fetch from the database only the user name and city
- in the projection interface, UserDetail, use the @Value and Spring SpEL to point to a backing property from the domain model (in this case, the domain model property city is exposed via the virtual property livingin)
- in the projection interface, UserDetail, use the @Value and Spring SpEL to enrich the result with two virtual properties that don't have a match in the domain model (in this case, sessionid and status)

Output example:

How To Use Query Creation Mechanism For JPA To Limit Result Size

Description: Spring Data comes with the query creation mechanism for JPA that is capable to interpret a query method name and convert it into a SQL query. This is possible as long as we respect the naming conventions of this mechanism. This is an application that exploit this mechanism to write queries that limit the result size. Basically, the name of the query method instructs Spring Data how to add the LIMIT (or similar clauses depending on the RDBMS) clause to the generated SQL queries.

Key points:
- define a Spring Data classic repository (e.g., AuthorRepository)
- write query methods respecting the query creation mechanism for JPA naming conventions

Examples:
- List<Author> findFirst5ByAge(int age);
- List<Author> findFirst5ByAgeGreaterThanEqual(int age);
- List<Author> findFirst5ByAgeLessThan(int age);
- List<Author> findFirst5ByAgeOrderByNameDesc(int age);
- List<Author> findFirst5ByGenreOrderByAgeAsc(String genre);
- List<Author> findFirst5ByAgeGreaterThanEqualOrderByNameAsc(int age);
- List<Author> findFirst5ByGenreAndAgeLessThanOrderByNameDesc(String genre, int age);
- List<AuthorDto> findFirst5ByOrderByAgeAsc();
- Page<Author> queryFirst10ByName(String name, Pageable p);
- Slice<Author> findFirst10ByName(String name, Pageable p);

The list of supported keywords is listed below:

How To Generate A Schema Via schema-.sql In MySQL

Note: As a rule, in real applications avoid generating schema via, hibernate.ddl-auto. Use schema-.sql file or better Flyway or Liquibase.

Description: This application is an example of using schema-.sql to generate a schema(database) in MySQL.

Key points:
- in application.properties, set the JDBC URL. E.g., spring.datasource.url=jdbc:mysql://localhost:3306/db_cars?createDatabaseIfNotExist=true
- in application.properties, disable DDL auto, e.g., spring.jpa.hibernate.ddl-auto=none
- in aaplication.properties, instruct Spring Boot to initialize the schema from schema-mysql.sql file

How To Generate Two Schema Via schema-.sql And Match Tables And Schemas Via @Table In MySQL

Note: As a rule, in real applications avoid generating schema via, hibernate.ddl-auto. Use schema-.sql file or better Flyway or Liquibase.

Description: This application is an example of using schema-.sql to generate two schemas(databases) in MySQL. The tables and schemas are matched at entity mapping via @Table.

Key points:
- in application.properties, set the JDBC URL without schema, e.g., spring.datasource.url=jdbc:mysql://localhost:3306
- in application.properties, disable DDL auto, e.g., spring.jpa.hibernate.ddl-auto=none
- in aaplication.properties, instruct Spring Boot to initialize the schema from schema-mysql.sql file
- in Car entity, specify that the schema is db_cars and the table is, lux_cars, @Table(name = "lux_cars", catalog = "db_cars")
- in Driver entity, specify that the schema is db_drivers and the table is, lux_drivers, @Table(name = "lux_drivers", catalog = "db_drivers")

Output example:

Persisting a Car results in the following SQL: insert into db_cars.lux_cars (name) values (?)
Persisting a Driver results the following SQL: insert into db_drivers.lux_drivers (name) values (?)

How To Stream Result Set Via Spring Data In MySQL

Note: For web-applications, pagination should be the way to go, not streaming. But, if you choose streaming then keep in mind the golden rule: keep th result set as small as posible. Also, keep in mind that the Execution Plan might not be as efficient as when using SQL-level pagination.

Description: This application is an example of streaming the result set via Spring Data and MySQL. This example can be adopted for databases that fetches the entire result set in a single roundtrip causing performance penalties.

Key points:
- rely on forward-only result set (default in Spring Data)
- rely on read-only statement (add @Transactional(readOnly=true))
- set the fetch-size set (e.g. 30, or row-by-row; Integer.MIN_VALUE (recommended in MySQL))

How To Migrate Schema Using Flyway in MySQL When Schema Exists

Note: For production don't rely on hibernate.ddl-auto to create your schema. Set hibernate.ddl-auto to none or validate and rely on Flyway or Liquibase.

Description: This application is an example of migrating a MySQL schema when the schema exists (is created before migration).

Key points:
- for Maven, in pom.xml, add the Flyway dependency
- in application.properties, set spring.jpa.hibernate.ddl-auto=none
- in application.properties, set the JDBC URL with the schema, e.g., jdbc:mysql://localhost:3306/db_cars
- each SQL file containing the schema update add it in classpath:db/migration
- each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

How To Auto-Create And Migrate Schema Using Flyway In SpringBoot And MySQL (kickoff)

Note: For production don't rely on hibernate.ddl-auto to create your schema. Set hibernate.ddl-auto to none or validate and rely on Flyway or Liquibase.

Description: These two applications are kickoffs of using Flyway in SpringBoot for auto-creating and migrating database schema in MySQL. The main difference between them is that in the first one we need to indicate the used schema via @Table(catalog = '...') since is not present in the JDBC URL. Basically, we remove the schema name from connection URL and use flyway.schemas option to provide the schema name and Flyway will create the missing schema for us. The second application is more friendly, since it doesn't have this restriction because it set the database name in the JDBC URL as usual, and uses the MySQL specific createDatabaseIfNotExist=true for creating the missing schema before Flyway enters into the scene.

First application key points (using "spring.flyway.schemas"):
- for Maven, in pom.xml, add the Flyway dependency
- in application.properties, set spring.jpa.hibernate.ddl-auto=none
- in application.properties, set the JDBC URL without specifying the schema, e.g., jdbc:mysql://localhost:3306/
- in application.properties, set the schemas that should be migrated, e.g., spring.flyway.schemas=db_cars
- each SQL file containing the schema update add it in classpath:db/migration
- each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

Second application key points (using "createDatabaseIfNotExist=true"):
- for Maven, in pom.xml, add the Flyway dependency
- in application.properties, set spring.jpa.hibernate.ddl-auto=none
- in application.properties, set the JDBC URL with the schema, e.g., jdbc:mysql://localhost:3306/db_cars?createDatabaseIfNotExist=true
- each SQL file containing the schema update add it in classpath:db/migration
- each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

Output of migrationg history example:

How To Auto-Create And Migrate Schemas For Two Data Sources (MySQL and PostgreSQL) Using Flyway

Note: For production don't rely on hibernate.ddl-auto to create your schema. Set hibernate.ddl-auto to none or validate and rely on Flyway or Liquibase.

Description: This application is an example of auto-creating and migrating schemas for MySQL and PostgreSQL. In addition, each data source uses its own HikariCP connection pool. In case of MySQL, where schema=database, we auto-create the schema (players_db) based on createDatabaseIfNotExist=true. In case of PostgreSQL, where a database can have multiple schemas, we use the default postgres database and auto-create in it the schema, coaches_db. For this we rely on Flyway, which is capable to create a missing schema.

Key points:
- for Maven, in pom.xml, add the Flyway dependency
- in application.properties, configure the JDBC URL for MySQL as, jdbc:mysql://localhost:3306/players_db?createDatabaseIfNotExist=true&useSSL=false and for PostgreSQL as, jdbc:postgresql://localhost:5432/postgres?currentSchema=coaches_db
- in application.properties, set spring.flyway.enabled=false to disable default behavior
- programmatically create a DataSource for MySQL and one for PostgreSQL
- programmatically create a FlywayDataSource for MySQL and one for PostgreSQL
- programmatically create an EntityManagerFactory for MySQL and one for PostgreSQL
- for MySQL, place the migration SQLs files in db\migration\mysql
- for PostgreSQL, place the migration SQLs files in db\migration\postgresql

How To Auto-Create And Migrate Two Schemas In PostgreSQL Using Flyway

Note: For production don't rely on hibernate.ddl-auto to create your schema. Set hibernate.ddl-auto to none or validate and rely on Flyway or Liquibase.

Description: This application is an example of auto-creating and migrating two schemas in PostgreSQL using Flyway. In addition, each data source uses its own HikariCP connection pool. In case of PostgreSQL, where a database can have multiple schemas, we use the default postgres database and auto-create in it two schemas, players_db and coaches_db. For this we rely on Flyway, which is capable to create the missing schemas.

Key points:
- for Maven, in pom.xml, add the Flyway dependency
- in application.properties, configure the JDBC URL for players_db as jdbc:postgresql://localhost:5432/postgres?currentSchema=players_db and for coaches_db as jdbc:postgresql://localhost:5432/postgres?currentSchema=coaches_db
- in application.properties, set spring.flyway.enabled=false to disable default behavior
- programmatically create two DataSource, one for players_db and one for coaches_db
- programmatically create two FlywayDataSource, one for players_db and one for coaches_db
- programmatically create two EntityManagerFactory, one for players_db and one for coaches_db
- for players_db, place the migration SQLs files in db\migration\playersdb
- for coaches_db, place the migration SQLs files in db\migration\coachesdb

How To JOIN FETCH an @ElementCollection

Description: This application is an example applying JOIN FETCH to fetch an @ElementCollection.

Key points:
- by default, @ElementCollection is loaded lazy, keep it lazy
- use JOIN FETCH in the repository

How To Map An Entity To a Query (@Subselect) in a Spring Boot Application

Note: Consider using @Subselect only if using DTO + extra queries or map a database view to an entity is not a solution.

Description: This application is an example of mapping an entity to a query via Hibernate, @Subselect. Mainly, we have two entities in a bidirectional one-to-many association. An Author has wrote several Book. The idea is to write a read-only query to fetch from Author only some fields (e.g., DTO), but to have the posibility to call getBooks() and fetch the Book in a lazy manner as well. As you know, a classic DTO cannot be used, since such DTO is not managed and we cannot fetch managed associations. Via Hibernate, @Subselect we can map a read-only, immutable entity to a query. Being an entity, it can lazy load the managed associations.

Key points:
- define a new entity that contains only the needed fields from the Author (including association to Book)
- for these fields, define only getters
- mark the entity as @Immutable since no write operations are allowed
- flush pending state transitions for the used entities by @Synchronize
- use @Subselect to write the needed query, map an entity to an SQL query

How To Use Hibernate Soft Deletes in a Spring Boot Application

Description: This application is an example of using Hibernate soft deletes in a Spring Boot application.

Key points:
- in entities (e.g., Tournament entity) that should take advantage of soft deletes define a dedicated column to store the deletion status (e.g., deleted)
- these entities should be marked with Hibernate, @Where annotation like this: @Where(clause = "deleted = false")
- these entities should be marked with Hibernate, @SQLDelete annotation to trigger UPDATE SQLs in place of DELETE SQLs, as follows: @SQLDelete(sql = "UPDATE tournament SET deleted = true WHERE id = ?")
- for fetching all entities including those marked as deleted or for fetching only the entities marked as deleted we need to rely on SQL native queries

Output example:

How To Programmatically Configure HikariCP Via DataSourceBuilder

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Key points:
- write a @Bean that returns the DataSource configured programmatically via DataSourceBuilder

Spring Data JPA Auditing

Description: Auditing is useful for maintaining history records. This can later help us in tracking user activities.

Key points:
- create an abstract base entity (e.g., BaseEntity) and annotate it with @MappedSuperclass and @EntityListeners({AuditingEntityListener.class})
- in this base entity, add the following fields that will be automatically persisted: @CreatedDate protected LocalDateTime createdAt;, @LastModifiedDate protected LocalDateTime updatedAt;, @CreatedBy protected U createdBy; and @LastModifiedBy protected U modifiedBy;
- enable auditing via @EnableJpaAuditing(auditorAwareRef = "auditorAware")
- provide an implementation for AuditorAware (this is needed for persisting the user that performed the modification; use Spring Security to return the currently logged-in user)
- expose this implementation via @Bean
- entites that should be audited should extend the base entity

Hibernate Envers Auditing

Description: Auditing is useful for maintaining history records. This can later help us in tracking user activities.

Key points:
- in pom.xml add the dependency hibernate-envers
- each entity that should be audited should be annotated with @Audited
- optionally, annotate entities with @AuditTable to rename the table used for auditing

Attributes Lazy Loading Via Subentities

Description: By default, the attributes of an entity are loaded eager (all at once). This application is an alternative to Attribute Lazy Loading from here. This application uses a base class to isolate the attributes that should be loaded eagerly and subentities (entities that extends the base class) for isolating the attributes that should be loaded on demand.

Key points:
- create the base class (this is not an entity), BaseAuthor, and annotate it with @MappedSuperclass
- create AuthorShallow subentity of BaseAuthor and don't add any attribute in it (this will inherit the attributes from the superclass)
- create AuthorDeep subentity of BaseAuthor and add to it the attributes that should be loaded on demand (e.g., avatar)
- map both subentities to the same table via @Table(name = "author")
- provide the typical repositories, AuthorShallowRepository and AuthorDeepRepository

Run the following requests (via BookstoreController):
- fetch all authors shallow (without avatars): localhost:8080/authors/shallow
- fetch all authors deep (with avatars): localhost:8080/authors/deep

DTOs Via Constructor And Spring Data Query Builder Mechanism

Key points:
- write a proper constructor in the DTO class
- rely on Spring Data Query Builder Mechanism for expression the SQL
- for using Spring Data Projections check this recipe

JOIN FETCH And DTOs

Description: Combining JOIN FETCH and DTOs can be done under several constrains. Mainly, the JPQL containing the JOIN FETCH cannot be used to fetch only some columns from the involved entities (in such cases, JOIN is the proper choice). It must fetch all attributes of the involved entities.

Key points:
- define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
- define the proper DTOs classes (e.g., BookDto and AuthorDto)
- the BookDto and AuthorDto may map only the needed columns, but the triggered SQL will fetch all of them anyway
- write a JPQL JOIN FETCH to fetch an author including his books

Constrains:
- this is ok: SELECT a FROM Author a JOIN FETCH a.books
- this is not ok: SELECT a.age as age FROM Author a JOIN FETCH a.books -> org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list
- this is not ok: SELECT a FROM Author a JOIN FETCH a.books.title -> org.hibernate.QueryException: illegal attempt to dereference collection [author0_.id.books] with element property reference [title]

LEFT JOIN FETCH

Description: Let's assume that we have two entities engaged in a one-to-many (or many-to-many) lazy bidirectional (or unidirectional) relationhip (e.g., Author has more Book). And, we want to trigger a single SELECT that fetches all Author and the corresponding Book. This is a job for JOIN FETCH which is converted behind the scene into a INNER JOIN. Being an INNER JOIN, the SQL will return only Author that have Book. If we want to return all Author, including those that doesn't have Book, then we can rely on LEFT JOIN FETCH. Similar, we can fetch all Book, including those with no registered Author.

Key points:
- define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
- write a JPQL LEFT JOIN FETCH to fetch all authors and books (fetch authors even if they don't have registered books)
- write a JPQL LEFT JOIN FETCH to fetch all books and authors (fetch books even if they don't have registered authors)

JOIN VS. JOIN FETCH

Description: This is an application meant to reveal the differences between JOIN and JOIN FETCH. The important thing to keep in mind is that, in case of LAZY fetching, JOIN will not be capable to initialize the associations/collections along with their parent objects using a single SQL SELECT. On the other hand, JOIN FETCH is capable to accomplish this kind of task. But, don't underestimate JOIN, because JOIN is the proper choice when we need to combine/join the columns of two (or more) tables in the same query, but we don't need to initialize the association on the returned entity (e.g., very useful for fetching DTOs).

Key points:
- define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
- write a JPQL JOIN and JOIN FETCH to fetch an author including his books
- write a JPQL JOIN and JOIN FETCH to fetch a book including its author

Notice that:
- via JOIN, fetching each Author of a Book (or each Book of an Author) may require additional SELECT(s) being prone to N+1 performance penalty
- via JOIN FETCH, fetching each Author of a Book (or each Book of an Author) required a single SELECT

Entity Inside Spring Projection

Description: If, for any reason, you need an entity in your Spring projection (DTO), then this application shows you how to do it via an example. In this case, there are two entities, Author and Book, involved in a lazy bidirectional one-to-many association (it can be other association as well, or even no materialized association). And, we want to fetch in a Spring projection the authors as entities, Author, and the title of the books.

Key points:
- define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
- define the proper Spring projection having public Author getAuthor() and public String getTitle()
- write a JPQL to fetch data

Entity Inside Spring Projection (no association)

Description: If, for any reason, you need an entity in your Spring projection (DTO), then this application shows you how to do it via an example. In this case, there are two entities, Author and Book, that have no materialized association between them, but, they share the genre attribute. We use this attribute to join authors with books via JPQL. And, we want to fetch in a Spring projection the authors as entities, Author, and the title of the books.

Key points:
- define two unrelated entities (e.g., Author and Book)
- define the proper Spring projection having public Author getAuthor() and public String getTitle()
- write a JPQL to fetch data

Avoid Entity In DTO Via Constructor Expression (no association)

Description: Let's assume that we have two entities, Author and Book. There is no materialized association between them, but, both entities shares an attribute named, genre. We want to use this attribute to join the tables corresponding to Author and Book, and fetch the result in a DTO. The result should contain the Author entity and only the title attribute from Book. Well, when you are in a scenario as here, it is strongly advisable to avoid fetching the DTO via constructor expression. This approach cannot fetch the data in a single SELECT, and is prone to N+1. Way better than this consists of using Spring projections, JPA Tuple or even Hibernate ResultTransformer. These approaches will fetch the data in a single SELECT. This application is a DON'T DO THIS example. Check the number of queries needed for fetching the data. In place, do it as here: Entity Inside Spring Projection (no association).

How To DTO an @ElementCollection

Description: This application is an example of fetching a DTO that includes attributes from an @ElementCollection.

Key points:
- by default, @ElementCollection is loaded lazy, keep it lazy
- use a Spring projection and JOIN in the repository

Ordering The Set Of Associated Entities In @ManyToMany Via @OrderBy

Description: In case of @ManyToMany association, we always should rely on Set (not on List) for mapping the collection of associated entities (entities of the other parent-side). Why? Well, please see Prefer Set Instead of List in @ManyToMany Relationships. But, is well-known that HashSet doesn't have a predefined entry order of elements. If this is an issue then this application relies on @OrderBy which adds an ORDER BY clause in the SQL statement. The database will handle the ordering. Further, Hibernate will preserve the order via a LinkedHashSet.

This application uses two entities, Author and Book, involved in a lazy bidirectional many-to-many relationship. First, we fetch a Book by title. Further, we call getAuthors() to fetch the authors of this book. The fetched authors are ordered descending by name. The ordering is done by the database as a result of adding @OrderBy("name DESC"), and is preserved by Hibernate.

Key points:
- ask the database to handle ordering and Hibernate to preserve this order via @OrderBy
- this works with HashSet, but doesn't provide consistency across all transition states (e.g., over transient state)
- for consistency across transient state as well, consider using explicitly LinkedHashSet instead of HashSet

Note: Alternatively, we can use @OrderColumn. This gets materialized in an additional column in the junction table. This is needed for maintaining a permanent ordering of the related data.

Versioned Optimistic Locking And Detached Entities Sample

Description: This is a sample application that shows how versioned (@Version) optimistic locking and detached entity works. Running the application will result in an optimistic locking specific exception (e.g., the Spring Boot specific, OptimisticLockingFailureException).

Key points:
- in a transaction, fetch an entity via findById(1L); commit transaction and close the persistence context
- in a second transaction, fetch another entity via findById(1L) and update it; commit the transaction and close the persistence context
- outside transactional context, update the detached entity (fetched in the first transaction)
- in a third transaction, call save() and pass to it the detached entity; trying to re-attach (EntityManager.merge()) the entity will end up in an optimistic locking exception since the version of the detached and just loaded entity don't match

How To Simulate OptimisticLockException Shaped Via @Version

Note: Optimistic locking via @Version works for detached entities as well.

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic lock exception. So, running the application should end up with a Spring specific ObjectOptimisticLockingFailureException exception.

Key points:
- set up versioned optimistic locking mechanism
- rely on two concurrent threads that call the same a @Transactional method used for updating data

How To Retry Transaction Via TransactionTemplate After OptimisticLockException Shaped Via @Version

Note: Optimistic locking via @Version works for detached entities as well.

Key points:
- in pom.xml, add the db-util dependency
- configure the OptimisticConcurrencyControlAspect bean
- rely on TransactionTemplate

How To Simulate Version-less OptimisticLockException

Note: Version-less optimistic locking doesn't work for detached entities (do not close the persistence context).

Key points:
- set up version-less optimistic locking mechanism
- rely on two concurrent threads that call the same a @Transactional method used for updating data

How To Retry Transaction Via TransactionTemplate After OptimisticLockException Shaped Via Hibernate Version-less Optimistic Locking Mechanism

Note: Version-less optimistic locking doesn't work for detached entities (do not close the persistence context).

Key points:
- in pom.xml, add the db-util dependency
- configure the OptimisticConcurrencyControlAspect bean
- rely on TransactionTemplate

HTTP Long Conversation Via Versioned Optimistic Locking And Detached Entities In Session

Description: This is a sample application that shows how to take advantage of versioned optimistic locking and detached entities in HTTP long conversations. The climax consists of storing the detached entities across multiple HTTP requests. Commonly, this can be accomplished via HTTP session.

Key points:
- prepare the entity via @Version
- rely on @SessionAttributes for storing the detached entities

Sample output (check the message caused by optimistic locking exception):

Filter Association Via Hibernate @Where

Note: Rely on this approach only if you simply cannot use JOIN FETCH WHERE or @NamedEntityGraph.

Description: This application is a sample of using Hibernate @Where for filtering associations.

Key points:
- use @Where(clause = "condition to be met") in entity (check the Author entity)

Batch Inserts In Spring Boot Style

Description: Batch inserts (in MySQL) in Spring Boot style.

Output example:

Offset Pagination - Trigger COUNT(*) OVER And Return Page<entity> Via Extra Column

Key points:
- write a repository that extends PagingAndSortingRepository
- in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
- fetch data via a native query (that includes counting) into a List<entity>, and a Pageable
- use the fetched List<entity> and Pageable to create a Page<entity>

Offset Pagination - Trigger SELECT COUNT Subquery And Return List<entity> Via Extra Column

Description: This application fetches data as List<entity> via Spring Boot offset pagination. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, one query is needed for fetching the data and one for counting the total number of records).

Key points:
- write a repository that extends PagingAndSortingRepository
- in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
- fetch data via a native query (that includes SELECT COUNT subquery) into a List<entity>

Offset Pagination - Trigger SELECT COUNT Subquery And Return List<projection> That Maps Entities And The Total Number Of Records Via Projection

Description: This application fetches data as List<projection> via Spring Boot offset pagination. The projection maps the entity and the total number of records. This information is fetched in a single database rountrip because the SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records). Use this approch only if the fetched data is not read-only. Otherwise, prefer List<dto> as here.

Key points:
- write a Spring projection that maps the entity and the total number of records
- write a repository that extends PagingAndSortingRepository
- fetch data via a JPQL query (that includes SELECT COUNT subquery) into a List<projection>

Offset Pagination - Trigger COUNT(*) OVER And Return List<entity> Via Extra Column

Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<entity> Via Extra Column

Description: This application fetches data as Page<entity> via Spring Boot offset pagination. Use this only if the fetched data will be modified. Otherwise, fetch Page<dto> as here. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<projection> That Maps Entities And The Total Number Of Records Via Projection

Description: This application fetches data as Page<projection> via Spring Boot offset pagination. The projection maps the entity and the total number of records. This information is fetched in a single database rountrip because the SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records). Use this approch only if the fetched data is not read-only. Otherwise, prefer Page<dto> as here.

Key points:
- define a Spring projection that maps the entity and the total number of records
- write a repository that extends PagingAndSortingRepository
- fetch data via a JPQL query (that includes counting) into a List<projection>, and a Pageable
- use the fetched List<projection> and Pageable to create a Page<projection>

Offset Pagination - Trigger COUNT(*) OVER And Return Page<dto>

Key points:
- create a Spring projection (DTO) to contains getters only for the data that should be fetched
- write a repository that extends PagingAndSortingRepository
- fetch data via a native query (that includes counting) into a List<dto>, and a Pageable
- use the fetched List<dto> and Pageable to create a Page<dto>

Example:

How To Fetch Slice<entity>/Slice<dto> Via fetchAll/fetchAllDto

Story: Spring Boot provides an offset based built-in paging mechanism that returns a Page or Slice. Each of these APIs represents a page of data and some metadata. The main difference is that Page contains the total number of records, while Slice can only tell if there is another page available. For Page, Spring Boot provides a findAll() method capable to take as arguments a Pageable and/or a Specification. In order to populate a Page containing the total number of records, this method triggers an SELECT COUNT extra-query next to the query used to fetch the current page . This can be a performance penalty since the SELECT COUNT query is triggered every time we request a page. In order to avoid this extra-query, Spring Boot provides a more relaxed API, the Slice API. Using Slice instead of Page removes the need of this extra SELECT COUNT query and returns the page (records) and some metadata without the total number of records. So, while Slice doesn't know the total number of records, it still can tell if there is another page available after the current one or this is the last page. The problem is that Slice work fine for queries containing the SQL, WHERE clause (including those that uses the query builder mechanism built into Spring Data), but it doesn't work for findAll(). This method will still return a Page instead of Slice therefore the SELECT COUNT query is triggered for Slice<T> findAll(...);.

Workaround: The trick is to simply define a method named fetchAll() that uses JPQL and Pageable to return Slice<entity>, and a method named fetchAllDto() that uses JPQL and Pageable as well to return Slice<dto>. So, avoid naming the method findAll().

Usage example:
public Slice<Author> fetchNextSlice(int page, int size) {
return authorRepository.fetchAll(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "age")));
}

public Slice<AuthorDto> fetchNextSliceDto(int page, int size) {
return authorRepository.fetchAllDto(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "age")));
}

How To Use Spring Projections(DTOs) And Inclusive Full Joins (MySQL)

Description: This application is a proof of concept for using Spring Projections(DTOs) and inclusive full joins written in native SQL (for MySQL).

How To Declare Immutable Entities And Store Them In Second Level Cache (e.g., EhCache)

Description: This application is a sample of declaring an immutable entity. Moreover, the immutable entity will be stored in Second Level Cache via EhCache implementation.

Key points of declaring an immutable entity:
- annotate the entity with @Immutable (org.hibernate.annotations.Immutable)
- avoid any kind of associations
- set hibernate.cache.use_reference_entries configuration to true

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