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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

How to complete this guide

Like all Spring's Getting Started guides, you can start from scratch and complete each step, or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Set up the project.

To skip the basics, do the following:

  • [Download][zip] and unzip the source repository for this guide, or clone it using [Git][u-git]: git clone https://github.com/spring-guides/gs-uploading-files.git
  • cd into gs-uploading-files/initial.
  • Jump ahead to Create a configuration class.

When you're finished, you can check your results against the code in gs-uploading-files/complete. [zip]: https://github.com/spring-guides/gs-uploading-files/archive/master.zip [u-git]: /understanding/Git

Set up the project

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Gradle and Maven is included here. If you're not familiar with either, refer to Building Java Projects with Gradle or Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

Create a Gradle build file

Below is the initial Gradle build file. But you can also use Maven. The pom.xml file is included right here. If you are using Spring Tool Suite (STS), you can import the guide directly.

build.gradle

buildscript {
    repositories {
        maven { url "http://repo.springsource.org/libs-snapshot" }
        mavenLocal()
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'

jar {
    baseName = 'gs-uploading-files'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
    maven { url "http://repo.springsource.org/libs-snapshot" }
}

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web:0.5.0.M2")
    testCompile("junit:junit:4.11")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.7'
}

Note: This guide is using Spring Boot.

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).

src/main/java/hello/Application.java

package hello;

import javax.servlet.MultipartConfigElement;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan
@EnableAutoConfiguration
public class Application {

    @Bean
    MultipartConfigElement multipartConfigElement() {
        return new MultipartConfigElement("");
    }

}

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 both @EnableAutoConfiguration 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.

Using @EnableAutoConfiguration, the application will also detect the 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. Here it's configured to upload to the folder the application runs in with no limits, but you can override these settings if you wish.

Create a file upload controller

In Spring, REST endpoints are just Spring MVC controllers. The following code provides the web app with the ability to upload files.

src/main/java/hello/FileUploadController.java

package hello;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.multipart.MultipartFile;

@Controller
public class FileUploadController {
    
    @RequestMapping(value="/upload", method=RequestMethod.GET)
    public @ResponseBody String provideUploadInfo() {
        return "You can upload a file by posting to this same URL.";
    }
    
    @RequestMapping(value="/upload", method=RequestMethod.POST)
    public @ResponseBody String handleFileUpload(@RequestParam("name") String name, 
            @RequestParam("file") MultipartFile file){
        if (!file.isEmpty()) {
            try {
                byte[] bytes = file.getBytes();
                BufferedOutputStream stream = 
                        new BufferedOutputStream(new FileOutputStream(new File(name + "-uploaded")));
                stream.write(bytes);
                stream.close();
                return "You successfully uploaded " + name + " into " + name + "-uploaded !";
            } catch (Exception e) {
                return "You failed to upload " + name + " => " + e.getMessage();
            }
        } else {
            return "You failed to upload " + name + " because the file was empty.";
        }
    }
    
}

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 REST 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. Finally, it appends -uploaded to the target filename to clearly show when a file has been uploaded.

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.

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.

Create an Application class

src/main/java/hello/Application.java

package hello;

import javax.servlet.MultipartConfigElement;

import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.SpringApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan
@EnableAutoConfiguration
public class Application {

    @Bean
    MultipartConfigElement multipartConfigElement() {
        return new MultipartConfigElement("");
    }
    
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

The main() method defers to the SpringApplication helper class, providing Application.class as an argument to its run() method. This tells Spring to read the annotation metadata from Application and to manage it as a component in the Spring application context.

The @ComponentScan annotation tells Spring to search recursively through the hello package and its children for classes marked directly or indirectly with Spring's @Component annotation. This directive ensures that Spring finds and registers the FileUploadController, because it is marked with @Controller, which in turn is a kind of @Component annotation.

The @EnableAutoConfiguration annotation switches on reasonable default behaviors based on the content of your classpath. For example, because the application depends on the embeddable version of Tomcat (tomcat-embed-core.jar), a Tomcat server is set up and configured with reasonable defaults on your behalf. And because the application also depends on Spring MVC (spring-webmvc.jar), a Spring MVC DispatcherServlet is configured and registered for you — no web.xml necessary! Because there is a MultipartConfigElement, it configured the DispatcherServlet with multipart file upload functionality. Auto-configuration is a powerful, flexible mechanism. See the API documentation for further details.

Build an executable JAR

Now that your Application class is ready, you simply instruct the build system to create a single, executable jar containing everything. This makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

Below are the Gradle steps, but if you are using Maven, you can find the updated pom.xml right here and build it by typing mvn clean package.

Update your Gradle build.gradle file's buildscript section, so that it looks like this:

buildscript {
    repositories {
        maven { url "http://repo.springsource.org/libs-snapshot" }
        mavenLocal()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:0.5.0.M2")
    }
}

Further down inside build.gradle, add the following to the list of applied plugins:

apply plugin: 'spring-boot'

You can see the final version of build.gradle [right here]((https://github.com/spring-guides/gs-uploading-files/blob/master/complete/build.gradle).

The Spring Boot gradle plugin collects all the jars on the classpath and builds a single "über-jar", which makes it more convenient to execute and transport your service. It also searches for the public static void main() method to flag as a runnable class.

Now run the following command to produce a single executable JAR file containing all necessary dependency classes and resources:

$ ./gradlew build

If you are using Gradle, you can run the JAR by typing:

$ java -jar build/libs/gs-uploading-files-0.1.0.jar

If you are using Maven, you can run the JAR by typing:

$ java -jar target/gs-uploading-files-0.1.0.jar

Note: The procedure above will create a runnable JAR. You can also opt to build a classic WAR file instead.

Run the service

If you are using Gradle, you can run your service at the command line this way:

$ ./gradlew clean build && java -jar build/libs/gs-uploading-files-0.1.0.jar

Note: If you are using Maven, you can run your service by typing mvn clean package && java -jar target/gs-uploading-files-0.1.0.jar.

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.

Create a client and upload a file

So far, you have built a server application capable of receiving file uploads. It would not be of much use unless you also build a client application to upload a file. The easiest way to do that is by using Spring MVC's RestTemplate.

src/main/java/hello/FileUploader.java

package hello;

import java.io.FileNotFoundException;

import org.springframework.core.io.FileSystemResource;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.web.client.RestTemplate;

public class FileUploader {
    
    public static void main(String[] args) throws FileNotFoundException {
        if (args.length == 0) {
            System.out.println("Usage: Requires the name of a file to upload.");
            System.exit(1);
        }
        
        RestTemplate template = new RestTemplate();
        MultiValueMap<String, Object> parts = new LinkedMultiValueMap<String, Object>();
        parts.add("name", args[0]);
        parts.add("file", new FileSystemResource(args[0]));
        String response = template.postForObject("http://localhost:8080/upload", parts, String.class);
        System.out.println(response);
    }

}

This client application creates a RestTemplate and then loads up a MultiValueMap with the name and the file. This leverages Spring's FileSystemResource class to properly load the bytes for the file. Then the template uses its postForObject method to POST the file to the server. Because the server was coded to write a textual message straight into the HTTP response, the client application prints that message out to the console.

Note: In more sophisticated applications, you probably want to use real HTML and some type of file chooser component to pick the file for upload.

You just coded some client code to upload a sample file. To run the code, add this to your Gradle build file:

apply plugin: 'application'
mainClassName = "hello.FileUploader"
run {
    args 'sample.txt'
}

Note: If you clicked on the link up above to view the final build.gradle file, you will have already seen this. There is similar material added to the pom.xml file.

With the server running in one window, you need to open another window to run the client.

$ ./gradlew run

Note: If you are using Maven, you can run the client by typing mvn package exec:java.

It should produce some output like this in the client window:

You successfully uploaded sample.txt into sample.txt-uploaded !

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

Summary

Congratulations! You have just written a client and server that use Spring to handle file uploads.