/complete

Primary LanguageJava

Table of Contents

This guide walks you through the process of creating a “Hello, World” RESTful web service with Spring.

What You Will Build

You will build a service that will accept HTTP GET requests at http://localhost:8080/greeting.

It will respond with a JSON representation of a greeting, as the following listing shows:

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

You can customize the greeting with an optional name parameter in the query string, as the following listing shows:

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

The name parameter value overrides the default value of World and is reflected in the response, as the following listing shows:

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

What You Need

Starting with Spring Initializr

For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the setup for you. This example needs only the Spring Web dependency. The following image shows the Initializr set up for this sample project:

initializr
Note
 The preceding image shows the Initializr with Maven chosen as the build tool. You can also use Gradle. It also shows values of com.example and rest-service as the Group and Artifact, respectively. You will use those values throughout the rest of this sample.

The following listing shows the pom.xml file that is created when you choose Maven:

link:initial/pom.xml[role=include]

The following listing shows the build.gradle file that is created when you choose Gradle:

link:initial/build.gradle[role=include]

Create a Resource Representation Class

Now that you have 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 resemble the following output:

{
    "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, create a resource representation class. To do so, provide a plain old Java object with fields, constructors, and accessors for the id and content data, as the following listing (from src/main/java/com/example/restservice/Greeting.java) shows:

link:complete/src/main/java/com/example/restservice/Greeting.java[role=include]
Note
 This application uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON. Jackson is included by default by the web starter.

Create a Resource Controller

In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are identified by the @RestController annotation, and the GreetingController shown in the following listing (from src/main/java/com/example/restservice/GreetingController.java) handles GET requests for /greeting by returning a new instance of the Greeting class:

link:complete/src/main/java/com/example/restservice/GreetingController.java[role=include]

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

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

Note
 There are companion annotations for other HTTP verbs (e.g. @PostMapping for POST). There is also a @RequestMapping annotation that they all derive from, and can serve as a synonym (e.g. @RequestMapping(method=GET)).

@RequestParam binds the value of the query string parameter name into the name parameter of the greeting() method. If the name parameter 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 shown earlier 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 populates and returns a Greeting object. The object data will be written directly to the HTTP response as JSON.

This code uses Spring @RestController annotation, which marks the class as a controller where every method returns a domain object instead of a view. It is shorthand for including both @Controller and @ResponseBody.

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

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 should see:

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

Provide a name query string parameter by visiting http://localhost:8080/greeting?name=User. Notice how the value of the content attribute changes from Hello, World! to Hello, User!, as the following listing shows:

{"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 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 have just developed a RESTful web service with Spring.

See Also

The following guides may also be helpful: