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

Hypermedia is an important aspect of REST. It allows you to build services that decouple client and server to a large extent and allow them to evolve independently. The representations returned for REST resources contain links that indicate which further resources the client should look at and interact with. Thus the design of the representations is crucial to the design of the overall service.

What you’ll build

You’ll build a hypermedia-driven REST service with Spring HATEOAS, a library of APIs that you can use to easily create links pointing to Spring MVC controllers, build up resource representations, and control how they’re rendered into supported hypermedia formats such as HAL.

The service will accept HTTP GET requests at:

http://localhost:8080/greeting

and respond with a JSON representation of a greeting enriched with the simplest possible hypermedia element, a link pointing to the resource itself:

{
  "content":"Hello, World!",
  "_links":{
    "self":{
      "href":"http://localhost:8080/greeting?name=World"
    }
  }
}

The response already indicates 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:

{
  "content":"Hello, User!",
  "_links":{
    "self":{
      "href":"http://localhost:8080/greeting?name=User"
    }
  }
}

What you’ll need

How to complete this guide

Like most Spring 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 Build with Gradle.

To skip the basics, do the following:

When you’re finished, you can check your results against the code in gs-rest-hateoas/complete.

Build with Gradle

Build with Gradle

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

build.gradle

buildscript {
    repositories {
        maven { url "http://repo.spring.io/libs-release" }
        mavenLocal()
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.1.6.RELEASE")
    }
}

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

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

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

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web")
    compile("com.fasterxml.jackson.core:jackson-databind")
    compile("org.springframework.hateoas:spring-hateoas")
    compile("org.springframework.plugin:spring-plugin-core:1.1.0.RELEASE")
    compile("com.jayway.jsonpath:json-path:0.9.1")
    testCompile("org.springframework.boot:spring-boot-starter-test")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.11'
}

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.

Build with Maven

Build with Maven

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 Maven is included here. If you’re not familiar with Maven, refer to 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

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.springframework</groupId>
    <artifactId>gs-rest-hateoas</artifactId>
    <version>0.1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.1.6.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.plugin</groupId>
            <artifactId>spring-plugin-core</artifactId>
            <version>1.1.0.RELEASE</version>
        </dependency>
        <dependency>
        	<groupId>com.jayway.jsonpath</groupId>
        	<artifactId>json-path</artifactId>
        	<version>0.9.1</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.hateoas</groupId>
            <artifactId>spring-hateoas</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <properties>
        <start-class>hello.Application</start-class>
    </properties>

    <build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
            </plugin>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

    <repositories>
        <repository>
            <id>spring-releases</id>
            <url>http://repo.spring.io/libs-release</url>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-releases</id>
            <url>http://repo.spring.io/libs-release</url>
        </pluginRepository>
    </pluginRepositories>
</project>
You might notice a specific version of maven-compiler-plugin. This is NOT recommended in general. Instead, it’s meant to solve an issue with our CI system that defaulted to a very old (pre-Java5) version of this plugin.

The Spring Boot Maven 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.

Build with Spring Tool Suite

Build with Spring Tool Suite

If you have Spring Tool Suite, then you can simply import this guide directly.

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 expose a resource at /greeting to handle GET requests, 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.

Beyond that, the JSON representation of the resource will be enriched with a list of hypermedia elements in a _links property. The most rudimentary form of this is a link pointing to the resource itself. So the representation should look something like this:

{
  "content":"Hello, World!",
  "_links":{
    "self":{
      "href":"http://localhost:8080/greeting?name=World"
    }
  }
}

The content is the textual representation of the greeting. The _links element contains a list of links, in this case exactly one with the relation type of rel and the href attribute pointing to the resource just accessed.

To model the greeting representation, you create a resource representation class. As the _links property is a fundamental property of the representation model, Spring HATEOAS ships with a base class ResourceSupport that allows you to add instances of Link and ensures that they are rendered as shown above.

So you simply create a plain old java object extending ResourceSupport and add the field and accessor for the content as well as a constructor:

src/main/java/hello/Greeting.java

package hello;

import org.springframework.hateoas.ResourceSupport;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

public class Greeting extends ResourceSupport {

    private final String content;

    @JsonCreator
    public Greeting(@JsonProperty("content") String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }
}
  • @JsonCreator - signal on how Jackson can create an instance of this POJO

  • @JsonProperty - clearly marks what field Jackson should put this constructor argument into

As you’ll see in steps below, Spring will use 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. The components are easily identified by the @Controller 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 static org.springframework.hateoas.mvc.ControllerLinkBuilder.*;

import org.springframework.http.HttpEntity;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
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!";

    @RequestMapping("/greeting")
    @ResponseBody
    public HttpEntity<Greeting> greeting(
            @RequestParam(value = "name", required = false, defaultValue = "World") String name) {

        Greeting greeting = new Greeting(String.format(TEMPLATE, name));
        greeting.add(linkTo(methodOn(GreetingController.class).greeting(name)).withSelfRel());

        return new ResponseEntity<Greeting>(greeting, HttpStatus.OK);
    }
}

This controller is concise and simple, but there’s plenty going on. 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 name parameter 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 @ResponseBody annotation on the greeting method will cause Spring MVC to render the returned HttpEntity and its payload, the Greeting, directly to the response.

The most interesting part of the method implementation is how you create the link pointing to the controller method and how you add it to the representation model. Both linkTo(…) and methodOn(…) are static methods on ControllerLinkBuilder that allow you to fake a method invocation on the controller. The LinkBuilder returned will have inspected the controller method’s mapping annotation to build up exactly the URI the method is mapped to.

The call to withSelfRel() creates a Link instance that you add to the Greeting representation model.

Make the application executable

Although it is possible to package this service as a traditional web application archive or 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

package hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
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, 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 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

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

You can alternatively run the app directly from Gradle like this:

./gradlew bootRun
With mvn, you can run mvn spring-boot:run.

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:

{
  "content":"Hello, World!",
  "_links":{
    "self":{
      "href":"http://localhost:8080/greeting?name=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!" and the href attribute of the self link reflects that change as well:

{
  "content":"Hello, User!",
  "_links":{
    "self":{
      "href":"http://localhost:8080/greeting?name=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.

Summary

Congratulations! You’ve just developed a hypermedia-driven RESTful web service with Spring HATEOAS.