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This guide walks you through the process of creating a server application that can receive HTTP multi-part file uploads.

What You Will Build

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.

Starting with Spring Initializr

You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.

To manually initialize the project:

  1. Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.

  2. Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.

  3. Click Dependencies and select Spring Web and Thymeleaf.

  4. Click Generate.

  5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

Note
 If your IDE has the Spring Initializr integration, you can complete this process from your IDE.
Note
 You can also fork the project from Github and open it in your IDE or other editor.

Create an Application Class

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):

link:initial/src/main/java/com/example/uploadingfiles/UploadingFilesApplication.java[role=include]

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

Create a File Upload Controller

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:

link:complete/src/main/java/com/example/uploadingfiles/FileUploadController.java[role=include]

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.

Note
 In a production scenario, you more likely would store the files in a temporary location, a database, or perhaps a NoSQL store (such as Mongo’s GridFS). It is best to NOT load up the file system of your application with content.

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:

link:complete/src/main/java/com/example/uploadingfiles//storage/StorageService.java[role=include]

You also need four classes to support the storage service:

link:complete/src/main/java/com/example/uploadingfiles//storage/StorageProperties.java[role=include]
link:complete/src/main/java/com/example/uploadingfiles//storage/StorageException.java[role=include]
link:complete/src/main/java/com/example/uploadingfiles//storage/StorageFileNotFoundException.java[role=include]
link:complete/src/main/java/com/example/uploadingfiles//storage/FileSystemStorageService.java[role=include]

Creating an HTML Template

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:

link:complete/src/main/resources/templates/uploadForm.html[role=include]

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.

Tuning File Upload Limits

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):

link:complete/src/main/resources/application.properties[role=include]

The multipart settings are constrained as follows:

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

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

Update the Application

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:

link:complete/src/main/java/com/example/uploadingfiles/UploadingFilesApplication.java[role=include]

Run the Application

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>!”

Testing Your Application

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:

link:complete/src/test/java/com/example/uploadingfiles/FileUploadTests.java[role=include]

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).

Summary

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