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You will create a Spring Boot web application that accepts file uploads. You will also build a simple HTML interface to upload a test file.

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For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the setup for you. This example needs the Spring Web and Thymeleaf dependencies. The following image shows the Initializr set up for this sample project:

The following listing shows the pom.xml file that is created when you choose Maven:

The following listing shows the build.gradle file that is created when you choose Gradle:

To start a Spring Boot MVC application, you first need a starter. In this sample, spring-boot-starter-thymeleaf and spring-boot-starter-web are already added as dependencies. To upload files with Servlet containers, you need to register a MultipartConfigElement class (which would be <multipart-config> in web.xml). Thanks to Spring Boot, everything is auto-configured for you!

All you need to get started with this application is the following UploadingFilesApplication class (from src/main/java/com/example/uploadingfiles/UploadingFilesApplication.java):

As part of auto-configuring Spring MVC, Spring Boot will create a MultipartConfigElement bean and make itself ready for file uploads.

The initial application already contains a few classes to deal with storing and loading the uploaded files on disk. They are all located in the com.example.uploadingfiles.storage package. You will use those in your new FileUploadController. The following listing (from src/main/java/com/example/uploadingfiles/FileUploadController.java) shows the file upload controller:

The FileUploadController class is annotated with @Controller so that Spring MVC can pick it up and look for routes. Each method is tagged with @GetMapping or @PostMapping to tie the path and the HTTP action to a particular controller action.

In this case:

• GET /: Looks up the current list of uploaded files from the StorageService and loads it into a Thymeleaf template. It calculates a link to the actual resource by using MvcUriComponentsBuilder.

• GET /files/{filename}: Loads the resource (if it exists) and sends it to the browser to download by using a Content-Disposition response header.

• POST /: Handles a multi-part message file and gives it to the StorageService for saving.

You will need to provide a StorageService so that the controller can interact with a storage layer (such as a file system). The following listing (from src/main/java/com/example/uploadingfiles/storage/StorageService.java) shows that interface:

The following Thymeleaf template (from src/main/resources/templates/uploadForm.html) shows an example of how to upload files and show what has been uploaded:

This template has three parts:

• An optional message at the top where Spring MVC writes a flash-scoped message.

• A form that lets the user upload files.

• A list of files supplied from the backend.

When configuring file uploads, it is often useful to set limits on the size of files. Imagine trying to handle a 5GB file upload! With Spring Boot, we can tune its auto-configured MultipartConfigElement with some property settings.

Add the following properties to your existing properties settings (in src/main/resources/application.properties):

The multipart settings are constrained as follows:

• spring.http.multipart.max-file-size is set to 128KB, meaning total file size cannot exceed 128KB.

• spring.http.multipart.max-request-size is set to 128KB, meaning total request size for a multipart/form-data cannot exceed 128KB.

You want a target folder to which to upload files, so you need to enhance the basic UploadingFilesApplication class that Spring Initializr created and add a Boot CommandLineRunner to delete and re-create that folder at startup. The following listing (from src/main/java/com/example/uploadingfiles/UploadingFilesApplication.java) shows how to do so:

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That runs the server-side piece that receives file uploads. Logging output is displayed. The service should be up and running within a few seconds.

With the server running, you need to open a browser and visit http://localhost:8080/ to see the upload form. Pick a (small) file and press Upload. You should see the success page from the controller. If you choose a file that is too large, you will get an ugly error page.

You should then see a line resembling the following in your browser window:

“You successfully uploaded <name of your file>!”

There are multiple ways to test this particular feature in our application. The following listing (from src/test/java/com/example/uploadingfiles/FileUploadTests.java) shows one example that uses MockMvc so that it does not require starting the servlet container:

In those tests, you use various mocks to set up the interactions with your controller and the StorageService but also with the Servlet container itself by using MockMultipartFile.

For an example of an integration test, see the FileUploadIntegrationTests class (which is in src/test/java/com/example/uploadingfiles).

Congratulations! You have just written a web application that uses Spring to handle file uploads.

The following guides may also be helpful:

• Serving Web Content with Spring MVC

• Handling Form Submission

• Securing a Web Application

• Building an Application with Spring Boot

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