tags	projects
files upload hateoas mvc	spring-framework spring-hateoas

Table of Contents

• What you’ll build
• What you’ll need
• Create a configuration class
• Create a file upload controller
• Creating a barebones template
• Tuning file upload limits
• 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]

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.

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 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).

Note	This example uses Java 8’s stream support combined with Java NIO operations to build paths, walk the directory of files, and generate Spring HATEOAS links.

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.

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. It’s is best to NOT load up the file system of your application with content.

Creating a barebones template

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

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 messages.

• A form allowing the user to 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 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.

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 FileUploadController.

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.

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:

You successfully uploaded <name of your file>!

Summary

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

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