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This guide walks you through the process of creating a "hello world" RESTful web service with Spring.
You’ll build a service that will accept HTTP GET requests at:
https://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:
https://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!"}
https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/create_both_builds.adoc
build.gradle
link:initial/build.gradle[role=include]
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
link:complete/src/main/java/hello/Greeting.java[role=include]
Note
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As you see in steps below, Spring uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON.
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Next you create the resource controller that will serve these greetings.
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
link:complete/src/main/java/hello/GreetingController.java[role=include]
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
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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.
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@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 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.
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.
src/main/java/hello/Application.java
link:complete/src/main/java/hello/Application.java[role=include]
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 @RestController
, 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.
Logging output is displayed. The service should be up and running within a few seconds.
Now that the service is up, visit https://localhost:8080/greeting, where you see:
{"id":1,"content":"Hello, World!"}
Provide a name
query string parameter with https://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.