tags	projects
	spring-framework

Table of Contents

• What you’ll build
• What you’ll need
• Create a configuration class
• Create a file upload controller
• Tuning file upload limits
• Create an HTML form to upload a file
• Make the application executable
• Summary

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

What you’ll build

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

What you’ll need

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Create a configuration class

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

link:initial/src/main/java/hello/Application.java[role=include]

This class is used to configure the server application that will receive file uploads, thanks to the @Configuration annotation.

You will soon add a Spring MVC controller, which is why you need @SpringBootApplication and @ComponentScan. Normally, you would use @EnableWebMvc for a Spring MVC application, but Spring Boot automatically adds this annotation when it detects spring-webmvc on your classpath. @ComponentScan makes it possible to automatically find @Controller-marked classes.

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

Note	MultipartConfigElement is a Servlet 3.0 standard element that defines the limits on uploading files. This component is supported by all compliant containers like Tomcat and Jetty. Later on in this guide, we’ll see how to configure its limits.

Create a file upload controller

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

link:complete/src/main/java/hello/FileUploadController.java[role=include]

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 returns a very simple message indicating the POST operation is available.

The handleFileUpload method is geared to handle a two-part message: name and file. It checks to make sure the file is not empty, and if it is empty, the method grabs the bytes. Next, it writes them out through a BufferedOutputStream.

Note

In a production scenario, you more likely would store the files in a temporary location, a database, or perhaps a NoSQL store like Mongo’s GridFS. You also need controls in place to avoid filling up the filesystem while also protecting yourself from vulnerabilities such as uploading executables and overwriting existing files.

Tuning file upload limits

When configuring file uploads, its often useful to set limits on the size of files. Imagine trying to handle a 5GB file upload! With Spring Boot, we can configure 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

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

The multipart settings are constrained as follows:

• multipart.maxFileSize is set to 128KB, meaning total file size cannot exceed 128KB.

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

Create an HTML form to upload a file

Create an "index.html" file in src/main/resources/static (so it ends up in the classpath under /static):

src/main/resources/static/index.html

link:complete/src/main/resources/static/index.html[role=include]

Make the application executable

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

link:complete/src/main/java/hello/Application.java[role=include]

@SpringBootApplication is a convenience annotation that adds all of the following:

• @Configuration tags the class as a source of bean definitions for the application context.

• @EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

• Normally you would add @EnableWebMvc for a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as setting up a DispatcherServlet.

• @ComponentScan tells Spring to look for other components, configurations, and services in the the hello package, allowing it to find the HelloController.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there wasn’t a single line of XML? No web.xml file either. This web application is 100% pure Java and you didn’t have to deal with configuring any plumbing or infrastructure.

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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:

You successfully uploaded <name of your file>!

The controller itself doesn’t print anything out, but instead returns the message posted to the browser.

Summary

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