Building a RESTful Web Service

This guide walks you through the process of creating a "hello world" RESTful web service with Spring.

What you’ll build

You’ll build a service that will accept HTTP GET requests at:

http://localhost:8080/greeting

and respond with a JSON representation of a greeting:

{"id":1,"content":"Hello, World!"}

You can customize the greeting with an optional name parameter in the query string:

http://localhost:8080/greeting?name=User

The name parameter value overrides the default value of "World" and is reflected in the response:

{"id":1,"content":"Hello, User!"}

What you’ll need

• About 15 minutes
• A favorite text editor or IDE
• JDK 1.6 or later
• Gradle 1.8+ or Maven 3.0+
• You can also import the code from this guide as well as view the web page directly intoSpring Tool Suite (STS) and work your way through it from there.

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 and unzip the source repository for this guide, or clone it using Git:git clone https://github.com/spring-guides/gs-rest-service.git
• cd into gs-rest-service/initial
• Jump ahead to Create a resource representation class.

When you’re finished, you can check your results against the code ings-rest-service/complete.

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 includedright here. If you are using Spring Tool Suite (STS), you can import the guide directly.

build.gradle

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

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

jar {
    baseName = 'gs-rest-service'
    version =  '0.1.0'
}

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

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web:0.5.0.M6")
    compile("com.fasterxml.jackson.core:jackson-databind")
    testCompile("junit:junit:4.11")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.8'
}

This guide is using Spring Boot.

Create a resource representation class

Now that you’ve set up the project and build system, you can create your web service.

Begin the process by thinking about service interactions.

The service will handle GET requests for /greeting, optionally with a name parameter in the query string. The GET request should return a 200 OK response with JSON in the body that represents a greeting. It should look something like this:

{
    "id": 1,
    "content": "Hello, World!"
}

The id field is a unique identifier for the greeting, and content is the textual representation of the greeting.

To model the greeting representation, you create a resource representation class. Provide a plain old java object with fields, constructors, and accessors for the id and content data:

src/main/java/hello/Greeting.java

package hello;

public class Greeting {

    private final long id;
    private final String content;

    public Greeting(long id, String content) {
        this.id = id;
        this.content = content;
    }

    public long getId() {
        return id;
    }

    public String getContent() {
        return content;
    }
}

As you see in steps below, Spring uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON.

Next you create the resource controller that will serve these greetings.

Create a resource controller

In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are easily identified by the @Controller annotation, and theGreetingController below handles GET requests for /greeting by returning a new instance of the Greeting class:

src/main/java/hello/GreetingController.java

package hello;

import java.util.concurrent.atomic.AtomicLong;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/greeting")
    public @ResponseBody Greeting greeting(
            @RequestParam(value="name", required=false, defaultValue="World") String name) {
        return new Greeting(counter.incrementAndGet(),
                            String.format(template, name));
    }
}

This controller is concise and simple, but there’s plenty going on under the hood. Let’s break it down step by step.

The @RequestMapping annotation ensures that HTTP requests to /greeting are mapped to the greeting() method.

The above example does not specify GET vs. PUT, POST, and so forth, because@RequestMapping maps all HTTP operations by default. Use@RequestMapping(method=GET) to narrow this mapping.

@RequestParam binds the value of the query string parameter name into the nameparameter of the greeting() method. This query string parameter is not required; if it is absent in the request, the defaultValue of "World" is used.

The implementation of the method body creates and returns a new Greeting object withid and content attributes based on the next value from the counter, and formats the given name by using the greeting template.

A key difference between a traditional MVC controller and the RESTful web service controller above is the way that the HTTP response body is created. Rather than relying on a view technology to perform server-side rendering of the greeting data to HTML, this RESTful web service controller simply populates and returns a Greeting object. The object data will be written directly to the HTTP response as JSON.

To accomplish this, the @ResponseBody annotation on the greeting() method tells Spring MVC that it does not need to render the greeting object through a server-side view layer, but that instead that the greeting object returned is the response body, and should be written out directly.

The Greeting object must be converted to JSON. Thanks to Spring’s HTTP message converter support, you don’t need to do this conversion manually. Because Jackson 2 is on the classpath, Spring’s MappingJackson2HttpMessageConverter is automatically chosen to convert the Greeting instance to JSON.

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 Javamain() method. 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

package hello;

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

@ComponentScan
@EnableAutoConfiguration
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

The main() method defers to the SpringApplication helper class, providingApplication.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 hellopackage and its children for classes marked directly or indirectly with Spring’s @Componentannotation. This directive ensures that Spring finds and registers the GreetingController, 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! 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.xmlright 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.spring.io/libs-snapshot" }
        mavenLocal()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:0.5.0.M6")
    }
}

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.

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-rest-service-0.1.0.jar

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

java -jar target/gs-rest-service-0.1.0.jar

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-rest-service-0.1.0.jar

If you are using Maven, you can run your service by typingmvn clean package && java -jar target/gs-rest-service-0.1.0.jar.

Logging output is displayed. The service should be up and running within a few seconds.

Test the service

Now that the service is up, visit http://localhost:8080/greeting, where you see:

{"id":1,"content":"Hello, World!"}

Provide a name query string parameter with http://localhost:8080/greeting?name=User. Notice how the value of the content attribute changes from "Hello, World!" to "Hello User!":

{"id":2,"content":"Hello, User!"}

This change demonstrates that the @RequestParam arrangement in GreetingController is working as expected. The name parameter has been given a default value of "World", but can always be explicitly overridden through the query string.

Notice also how the id attribute has changed from 1 to 2. This proves that you are working against the same GreetingController instance across multiple requests, and that itscounter field is being incremented on each call as expected.

Summary

Congratulations! You’ve just developed a RESTful web service with Spring.