ZCW-SpringZero

Getting Started: Building a RESTful Web Service

What you'll build

This guide will walk you through creating a "hello world" RESTful web service with Spring. The service 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'll also be able to customize the greeting by providing an optional name parameter in the query string:

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

this will override the default value of "World" and be reflected in the response:

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

What you'll need

• About 15 minutes
• A favorite text editor or IDE
• JDK 6 or later
• Maven 3.0 or later

[macro:how-to-complete-this-guide]

Set up the project

[macro:build-system-intro]

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure (e.g. with mkdir -p src/main/java/hello on *nix systems):

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

Create a Maven POM

[macro:maven-project-setup-options]

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-service</artifactId>
    <version>1.0</version>

    <parent>
        <groupId>org.springframework.bootstrap</groupId>
        <artifactId>spring-bootstrap-starters</artifactId>
        <version>0.5.0.BUILD-SNAPSHOT</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.bootstrap</groupId>
            <artifactId>spring-bootstrap-web-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
    </dependencies>

    <!-- TODO: remove once bootstrap goes GA -->
    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <url>http://repo.springsource.org/snapshot</url>
            <snapshots><enabled>true</enabled></snapshots>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-snapshots</id>
            <url>http://repo.springsource.org/snapshot</url>
            <snapshots><enabled>true</enabled></snapshots>
        </pluginRepository>
    </pluginRepositories>
</project>

[macro:bootstrap-starter-pom-disclaimer]

Create a resource representation class

With the basics of setting up the project and build system out of the way, it's time to get to the nuts and bolts of creating our service.

It's best to begin this process by thinking about what interacting with the service will look like.

We want to handle GET requests for /greeting, optionally with a name parameter in the query string. In response to such a request, we'd like to send back 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, we'll create a resource representation class. There's nothing fancy about this step—we're just creating 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'll see in a moment, Spring will the use Jackson library to automatically marshal instances of type Greeting into JSON.

Now that we've got our representation class, let's create the resource controller that will serve it.

Create a resource controller

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

[admon:note] We have not explicitly specified GET vs. PUT, POST, etc. above, because @RequestMapping maps all HTTP operations by default. Use @RequestMapping(method=GET) to narrow this down.

@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, so if absent in the request the defaultValue of "World" will be used.

The implementation of the method body is straightforward—create and return a new Greeting object with id and content attributes based on the next value from our counter and formatting the given name using our greeting template.

One key difference between a traditional MVC controller and the RESTful web service controller above is in the way the HTTP response body is created. Rather than relying on a view technology (such as JSP) to perform server-side rendering of our greeting data to HTML, this service controller simply populates and returns a Greeting object, with the goal that its data is written directly to the HTTP response as JSON.

The @ResponseBody annotation helps make this happen. The presence of this 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 only step that remains is converting the Greeting object to JSON. And fortunately, thanks to Spring's HTTP message converter support, we don't need to bother with doing this conversion by hand. Because Jackson 2 is on the classpath, Spring's MappingJackson2HttpMessageConverter is automatically chosen to convert the Greeting instance to JSON.

Make the application executable

While it is possible to package this service up as a traditional web application archive or WAR file for deployment to an external application server, we demonstrate below the simpler approach of creating a standalone application. You'll package everything up a single, executable JAR file, driven by a good old Java main() method. And along the way, we'll use Spring's support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.

Create a main class

src/main/java/hello/ServiceMain.java

package hello;

import org.springframework.bootstrap.SpringApplication;
import org.springframework.bootstrap.context.annotation.EnableAutoConfiguration;
import org.springframework.context.annotation.ComponentScan;

@ComponentScan
@EnableAutoConfiguration
public class ServiceMain {

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

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

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

The @EnableAutoConfiguration annotation has also been added: it switches on a number of 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 we have our ServiceMain class ready to go, we simply need to instruct the build system to create a single, executable jar containing everything. This will make it dead simple to ship and version and deploy the service as an application throughout the development lifecycle, across different environments, etc.

Add the following configuration to your existing Maven POM

pom.xml

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

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

The start-class property tells Maven to create a META-INF/MANIFEST.MF file with a Main-Class: hello.ServiceMain entry. This is the key to being able to run the jar with java -jar.

The Maven Shade plugin extracts classes from all the jars on the classpath and builds a single "über-jar". This makes it much more convenient to execute and transport your service.

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

mvn package

Run the service

That's it! You're ready to run your service with java -jar at the command line:

java -jar target/gs-rest-service-1.0.jar

You'll see a bit of logging output, and the service should be up and running within a second or two.

Test the service

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

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

Try providing 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 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 we're working against the same GreetingController instance across multiple requests, and that its counter field is being incremented on each call as expected.

Summary

Congrats! You've just developed your first RESTful web service using Spring. This of course is just the beginning, and there are many more features to explore and take advantage of. Be sure to check out Spring's support for securing, testing, describing and managing RESTful web services.