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

• A favorite text editor or IDE.
• JDK 1.8 or later.
• Gradle 2.3+ or Maven 3.0+

How to complete this guide

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.

• Building Java Projects with Gradle
• Building Java Projects with Maven

To complete this tutorial, we can use Gradle or Maven for build this project. So this project we Build with Gradle

Build with Gradle

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Create the directory structure

• Create a project root directory named spring-restful-web-service
• 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.

build.gradle

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.3.0.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'spring-boot'

jar {
    baseName = 'spring-restful-web-service'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

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

task wrapper(type: Wrapper) {
    gradleVersion = '2.8'
}

The Spring Boot gradle plugin provides many convenient features:

• It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.
• It searches for the public static void main() method to flag as a runnable class.
• It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Create a resource representation class

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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;
    }
}

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

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

Create a resource controller

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In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are easily identified by the @RestController annotation, and the GreetingController 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.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {

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

    @RequestMapping("/greeting")
    public Greeting greeting(@RequestParam(value="name", 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.

Note: 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 name parameter of the greeting() method. This query string parameter is optional (required=false by default): 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 with id 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.

This code uses Spring 4’s new @RestController annotation, which marks the class as a controller where every method returns a domain object instead of a view. It’s shorthand for @Controller and @ResponseBody rolled together.

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

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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. 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 new Class Named: src/main/java/hello/Application.java

package hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

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

@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 hello package, allowing it to find the GreetingController.

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.

Build an executable JAR

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If you are using Gradle, you can run the application using gradlew bootRun or ./gradlew bootRun.

You can build a single executable JAR file that contains all the necessary dependencies, classes, and resources. This makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

./gradlew build

Then you can run the JAR file: java -jar build/libs/gs-rest-service-0.1.0.jar

If you are using Maven, you can run the application using mvn spring-boot:run. Or you can build the JAR file with mvn clean package and run the JAR by typing:

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

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

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 its counter field is being incremented on each call as expected.

Summary

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