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You will create a Spring MVC application that accepts file uploads. You will also build a simple client to upload a test file.

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To upload files with Servlet 3.0 containers, you need to register a MultipartConfigElement class (which would be <multipart-config> in web.xml). Thanks to Spring Boot, that bean is already registered and available! All you need to get started with this application is the following, empty configuration setup.

src/main/java/hello/Application.java

You will soon add a Spring MVC controller, which is why you need @SpringBootApplication. Spring Boot automatically finds @Controller-marked classes and registers them with the application context.

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

In Spring MVC, a controller is used to handle file upload requests. The following code provides the web app with the ability to upload files.

src/main/java/hello/FileUploadController.java

The entire class is marked up with @Controller so Spring MVC can pick it up and look for routes.

Each method is tagged with @RequestMapping to flag the path and the HTTP action. In this case, GET looks up the current list of uploaded files (stored in Application.ROOT folder) and loads it into a Thymeleaf template. It provides a link to not only see, but "surf" to the file (letting the browser decide how to render).

The handleFileUpload method is geared to handle a multi-part message: file. After verifying the file isn’t empty, it uses Java NIO to copy the input stream to a local file.

To build something of interest, the following Thymeleaf template is a nice example of uploading files as well as showing what’s been uploaded.

src/main/resources/templates/uploadForm.html

This template has three parts:

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

• A form allowing the user to 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.

Create src/main/resources/application.properties and make it look like this:

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.

Although it is possible to package this service as a traditional WAR file for deployment to an external application server, the simpler approach demonstrated below creates a standalone application. You package everything in a single, executable JAR file, driven by a good old Java main() method. And along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.

src/main/java/hello/Application.java

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You also want a target folder to upload files to, so there is a Java 8 lambda used to create a Boot CommandLineRunner at startup which creates that folder.

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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" and you should see the success page from the controller. Choose a file that is too large and you will get an ugly error page.

You should then see something like this in your browser window:

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

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