This guide walks you through the process of creating a "hello world" RESTful web service with Spring that includes headers for Cross-Origin Resource Sharing (CORS) in the response. You will find more information about Spring CORS support in this blog post.

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!"}
This service differs slightly from the one described in Building a RESTful Web Service in that it will use Spring Framework CORS support to add the relevant CORS response headers.

What you’ll need

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:
When you’re finished, you can check your results against the code in gs-rest-service-cors/complete.

Build with Gradle

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 and Maven is included here. If you’re not familiar with either, refer to Building Java Projects with Gradle or Building Java Projects with 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.
build.gradle
buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.9.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'org.springframework.boot'

jar {
    baseName = 'gs-rest-service-cors'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

dependencies {
    compile("org.springframework.boot:spring-boot-starter-web")
}
The Spring Boot gradle plugin provides many convenient features:
  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.
  • It searches for the public static void main() method to flag as a runnable class.
  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Build with Maven

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 Maven is included here. If you’re not familiar with Maven, refer to Building Java Projects with 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
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-cors</artifactId>
    <version>0.1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.9.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>

    <properties>
        <java.version>1.8</java.version>
    </properties>


    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>
The Spring Boot Maven plugin provides many convenient features:
  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.
  • It searches for the public static void main() method to flag as a runnable class.
  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

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() {
        this.id = -1;
        this.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 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 @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
package hello;

import java.util.concurrent.atomic.AtomicLong;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {

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

    @GetMapping("/greeting")
    public Greeting greeting(@RequestParam(required=false, defaultValue="World") String name) {
        System.out.println("==== in greeting ====");
        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 uses the @GetMapping annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.GET).
@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.
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 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 is on the classpath, Spring’s MappingJackson2HttpMessageConverter is automatically chosen to convert the Greeting instance to JSON.

Enabling CORS

Controller method CORS configuration

So that the RESTful web service will include CORS access control headers in its response, you just have to add a @CrossOrigin annotation to the handler method:
src/main/java/hello/GreetingController.java
    @CrossOrigin(origins = "http://localhost:9000")
    @GetMapping("/greeting")
    public Greeting greeting(@RequestParam(required=false, defaultValue="World") String name) {
        System.out.println("==== in greeting ====");
        return new Greeting(counter.incrementAndGet(), String.format(template, name));
    }
This @CrossOrigin annotation enables cross-origin requests only for this specific method. By default, its allows all origins, all headers, the HTTP methods specified in the @RequestMapping annotation and a maxAge of 30 minutes is used. You can customize this behavior by specifying the value of one of the annotation attributes: origins, methods, allowedHeaders, exposedHeaders, allowCredentials or maxAge. In this example, we only allow http://localhost:9000 to send cross-origin requests.
it is also possible to add this annotation at controller class level as well, in order to enable CORS on all handler methods of this class.

Global CORS configuration

As an alternative to fine-grained annotation-based configuration, you can also define some global CORS configuration as well. This is similar to using a Filter based solution, but can be declared within Spring MVC and combined with fine-grained @CrossOrigin configuration. By default all origins and GET, HEAD and POST methods are allowed.
src/main/java/hello/GreetingController.java
    @GetMapping("/greeting-javaconfig")
    public Greeting greetingWithJavaconfig(@RequestParam(required=false, defaultValue="World") String name) {
        System.out.println("==== in greeting ====");
        return new Greeting(counter.incrementAndGet(), String.format(template, name));
    }
src/main/java/hello/Application.java
    @Bean
    public WebMvcConfigurer corsConfigurer() {
        return new WebMvcConfigurerAdapter() {
            @Override
            public void addCorsMappings(CorsRegistry registry) {
                registry.addMapping("/greeting-javaconfig").allowedOrigins("http://localhost:9000");
            }
        };
    }
You can easily change any properties (like the allowedOrigins one in the example), as well as only apply this CORS configuration to a specific path pattern. Global and controller level CORS configurations can also be combined.

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;
import org.springframework.context.annotation.Bean;
import org.springframework.web.servlet.config.annotation.CorsRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

@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-cors-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-cors-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.
Now to test that the CORS headers are in place and allowing a Javascript client from another origin to access the service, you’ll need to create a Javascript client to consume the service.
First, create a simple Javascript file named hello.js with the following content:
public/hello.js
$(document).ready(function() {
    $.ajax({
        url: "http://localhost:8080/greeting"
    }).then(function(data, status, jqxhr) {
       $('.greeting-id').append(data.id);
       $('.greeting-content').append(data.content);
       console.log(jqxhr);
    });
});
This script uses jQuery to consume the REST service at http://localhost:8080/greeting. It is loaded by index.html as shown here:
public/index.html
<!DOCTYPE html>
<html>
    <head>
        <title>Hello CORS</title>
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
        <script src="hello.js"></script>
    </head>

    <body>
        <div>
            <p class="greeting-id">The ID is </p>
            <p class="greeting-content">The content is </p>
        </div>
    </body>
</html>
This is essentially the REST client created in Consuming a RESTful Web Service with jQuery, modified slightly to consume the service running on localhost, port 8080. See that guide for more details on how this client was developed.
Because the REST service is already running on localhost, port 8080, you’ll need to be sure to start the client from another server and/or port. This will not only avoid a collision between the two applications, but will also ensure that the client code is served from a different origin than the service. To start the client running on localhost, port 9000:
mvn spring-boot:run -Dserver.port=9000
Once the client starts, open http://localhost:9000 in your browser, where you should see:
Model data retrieved from the REST service is rendered into the DOM if the proper CORS headers are in the response.
If the service response includes the CORS headers, then the ID and content will be rendered into the page. But if the CORS headers are missing (or insufficiently defined for the client), then the browser will fail the request and the values will not be rendered into the DOM:
The browser will fail the request if the CORS headers are missing from the response. No data will be rendered into the DOM.

Summary

Congratulations! You’ve just developed a RESTful web service including Cross-Origin Resource Sharing with Spring.

Source : https://spring.io/guides/gs/rest-service-cors/