Building a RESTful Web Service

原文链接：https://spring.io/guides/gs/rest-service/

Building a RESTful Web Service

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 representation of a greeting:

{"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:

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

What you’ll need

About 15 minutes

A favorite text editor or IDE

JDK 1.8 or later

Gradle 2.3+ or Maven
3.0+

You can also import the code straight into your IDE:

Spring Tool Suite (STS)

IntelliJ IDEA

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:

Download and unzip the source repository
for this guide, or clone it using 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.

When you’re finished, you can check your results against the code in 

gs-rest-service/complete

.

Build with Gradle

Build with Maven

Build with your IDE

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:

{
"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

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 get
14efd
Id() {
return id;
}

public String getContent() {
return content;
}
}

As you see in steps below, Spring uses 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. 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

package hello;

import java.util.concurrent.atomic.AtomicLong;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {

private static final String template = "Hello, %s!";
private final AtomicLong counter = new AtomicLong();

@RequestMapping("/greeting")
public Greeting greeting(@RequestParam(value="name", 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.

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 explicitly marked as optional (

required=true

 by
default): 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.

Make the application executable

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

package hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

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

@SpringBootApplication

 is
a convenience annotation that adds all of the following:

@Configuration

 tags
the class as a source of bean definitions for the application context.

@EnableAutoConfiguration

 tells
Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

Normally you would add 

@EnableWebMvc

 for
a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as
setting up a 

DispatcherServlet

.

@ComponentScan

 tells
Spring to look for other components, configurations, and services in the 

hello

 package,
allowing it to find the controllers.

The 

main()

 method
uses Spring Boot’s 

SpringApplication.run()

 method
to launch an application. Did you notice that there wasn’t a single line of XML? No web.xml file either. This web application is 100% pure Java and you didn’t have to deal with configuring
any plumbing or infrastructure.

Build an executable JAR

You can run the application from the command line with Gradle or Maven. Or you can build a single executable JAR file that contains all the necessary dependencies, classes, and
resources, and run that. This makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

If you are using Gradle, you can run the application using 

./gradlew
bootRun

. Or you can build the JAR file using 

./gradlew
build

. Then you can run the JAR file:

java -jar build/libs/gs-rest-service-0.1.0.jar

If you are using Maven, you can run the application using 

./mvnw
spring-boot:run

. Or you can build the JAR file with 

./mvnw
clean package

. Then you can run the JAR file:

java -jar target/gs-rest-service-0.1.0.jar

The procedure above will create a runnable JAR. You can also opt to build a classic WAR file instead.

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:

{"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!":

{"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.

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