This guide walks you through the process of creating a "hello world" [RESTful web service][u-rest] 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][u-json] representation of a greeting:

```json
{"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:

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


What you'll need
----------------

 - About 15 minutes
 - A favorite text editor or IDE
 - [JDK 6][jdk] or later
 - [Gradle 1.7+][gradle] or [Maven 3.0+][mvn]
 - You can also import the code from this guide as well as view the web page directly into [Spring Tool Suite (STS)][gs-sts] and work your way through it from there.

[jdk]: http://www.oracle.com/technetwork/java/javase/downloads/index.html
[gradle]: http://www.gradle.org/
[mvn]: http://maven.apache.org/download.cgi
[gs-sts]: /guides/gs/sts


How to complete this guide
--------------------------

Like all Spring's [Getting Started guides](/guides/gs), 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](#scratch).

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-rest-service.git`
 - cd into `gs-rest-service/initial`.
 - Jump ahead to [Create a resource representation class](#initial).

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


<a name="scratch"></a>
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](http://gradle.org) and [Maven](https://maven.apache.org) is included here. If you're not familiar with either, refer to [Building Java Projects with Gradle](/guides/gs/gradle/) or [Building Java Projects with Maven](/guides/gs/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](https://github.com/spring-guides/gs-rest-service/blob/master/initial/build.gradle). But you can also use Maven. The pom.xml file is included [right here](https://github.com/spring-guides/gs-rest-service/blob/master/initial/pom.xml). If you are using [Spring Tool Suite (STS)][gs-sts], you can import the guide directly.

`build.gradle`
```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.M4")
    compile("com.fasterxml.jackson.core:jackson-databind")
    testCompile("junit:junit:4.11")
}

task wrapper(type: Wrapper) {
    gradleVersion = '1.7'
}
```
    
[gs-sts]: /guides/gs/sts    

> **Note:** This guide is using [Spring Boot](/guides/gs/spring-boot/).


<a name="initial"></a>
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:

```json
{
    "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`
```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][jackson] 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 the `GreetingController` below handles `GET` requests for `/greeting` by returning a new instance of the `Greeting` class:

`src/main/java/hello/GreetingController.java`
```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.

> **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 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 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][u-view-templates] 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][jackson] 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][u-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][u-tomcat] servlet container as the HTTP runtime, instead of deploying to an external instance.

### Create an Application class

`src/main/java/hello/Application.java`
```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, 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][u-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][`@EnableAutoConfiguration`] 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](https://github.com/spring-guides/gs-rest-service/blob/master/complete/pom.xml) and build it by typing `mvn clean package`.

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

```groovy
buildscript {
    repositories {
        maven { url "http://repo.spring.io/libs-snapshot" }
        mavenLocal()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:0.5.0.M4")
    }
}
```

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

```groovy
apply plugin: 'spring-boot'
```
You can see the final version of `build.gradle` [right here]((https://github.com/spring-guides/gs-rest-service/blob/master/complete/build.gradle).

The [Spring Boot gradle plugin][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:

```sh
$ ./gradlew build
```

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

```sh
$ java -jar build/libs/gs-rest-service-0.1.0.jar
```

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

```sh
$ java -jar target/gs-rest-service-0.1.0.jar
```

[spring-boot-gradle-plugin]: https://github.com/spring-projects/spring-boot/tree/master/spring-boot-tools/spring-boot-gradle-plugin

> **Note:** The procedure above will create a runnable JAR. You can also opt to [build a classic WAR file](/guides/gs/convert-jar-to-war/) instead.

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

```sh
$ ./gradlew clean build && java -jar build/libs/gs-rest-service-0.1.0.jar
```

> **Note:** If you are using Maven, you can run your service by typing `mvn 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:

```json
{"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!":

```json
{"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
-------

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



[u-rest]: /understanding/REST
[u-json]: /understanding/JSON
[u-view-templates]: /understanding/view-templates
[jackson]: http://wiki.fasterxml.com/JacksonHome
[u-war]: /understanding/WAR
[u-tomcat]: /understanding/Tomcat
[u-application-context]: /understanding/application-context
[`@Controller`]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/stereotype/Controller.html
[`SpringApplication`]: http://docs.spring.io/spring-boot/docs/0.5.0.M3/api/org/springframework/boot/SpringApplication.html
[`@EnableAutoConfiguration`]: http://docs.spring.io/spring-boot/docs/0.5.0.M3/api/org/springframework/boot/autoconfigure/EnableAutoConfiguration.html
[`@Component`]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/stereotype/Component.html
[`@ResponseBody`]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/ResponseBody.html
[`MappingJackson2HttpMessageConverter`]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/http/converter/json/MappingJackson2HttpMessageConverter.html
[`DispatcherServlet`]: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/servlet/DispatcherServlet.html