Reduce latency by implementing region pinning in G1, so that garbage collection need not be disabled during Java
Native Interface (JNI) critical regions.
In constructors in the Java programming language, allow statements that do not reference the instance being
created to appear before an explicit constructor invocation.
Introduce an API by which Java programs can interoperate with code and data outside of the Java runtime.
Combination of 2 APIs introduced in previous JDKs:
Foreign-Memory Access API (incubator in 14, 15 and 16)
Foreign Linker API (incubator in 16)
The main changes since the 3rd preview are:
Provided a new linker option allowing clients to pass heap segments to downcall method handles;
Introduced the Enable-Native-Access JAR-file manifest attribute, allowing code in executable JAR files to call
restricted methods without having to use the --enable-native-access command-line option;
Enabled clients to build C-language function descriptors programmatically, avoiding platform-specific
constants;
Improved support for variable-length arrays in native memory; and
Added support for arbitrary charsets for native strings.
Support unnamed variables and unnamed patterns. They can be use when variable declarations or nested patterns are required but never used. Both are denoted by the underscore character, _.
String templates complement Java's existing string literals and text blocks by coupling literal text with embedded
expressions and template processors to produce specialized results.
Except for a technical change in the types of template expressions, there are no changes relative to the first
preview
Introduce an API to express vector computations that reliably compile at runtime to optimal vector instructions on
supported CPU architectures, thus achieving performance superior to equivalent scalar computations.
Notable changes since 6th incubator:
Support vector access with heap MemorySegments that are backed by an array of any primitive element type.
Previously access was limited to heap MemorySegments backed by an array of byte.
Enhance the Stream API to support custom intermediate operations. This will allow stream pipelines to transform data in ways that are not easily achievable with the existing built-in intermediate operations.
Simplify concurrent programming by introducing an API for structured concurrency. Structured concurrency treats
groups of related tasks running in different threads as a single unit of work, thereby streamlining error handling
and cancellation, improving reliability, and enhancing observability.
Evolve the Java language so that students can write their first programs without needing to understand language
features designed for large programs. Far from using a separate dialect of Java, students can write streamlined
declarations for single-class programs and then seamlessly expand their programs to use more advanced features as
their skills grow.
changes since first preview in JDK 21:
A source file without an enclosing class declaration is said to implicitly declare a class with a name chosen
by the host system. Such implicitly declared classes behave like normal top-level classes and require no
additional tooling, library, or runtime support.
simplify the selection process for the main method.
Enable the sharing of immutable data within and across threads. They are preferred to thread-local variables,
especially when using large numbers of virtual threads.
Unlike a thread-local variable, a scoped value is written once and is then immutable, and is available only for a
bounded period during execution of the thread.
In effect, a scoped value is an implicit method parameter. It is "as if" every method in a sequence of calls has
an additional, invisible, parameter. None of the methods declare this parameter and only the methods that have
access to the scoped value object can access its value (the data). Scoped values make it possible to pass data
securely from a caller to a faraway callee through a sequence of intermediate methods that do not declare a
parameter for the data and have no access to the data.
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