Enabling Cross Origin Requests for a RESTful Web Service
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
- 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:
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-cors.git
- cd into
gs-rest-service-cors/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-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.gradlebuildscript {
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
- Read how to import this guide straight into Spring Tool Suite.
- Read how to work with this guide in IntelliJ IDEA.
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.javapackage 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.javapackage 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.javapackage 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:-
@Configurationtags the class as a source of bean definitions for the application context.
-
@EnableAutoConfigurationtells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.
- Normally you would add
@EnableWebMvcfor 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 aDispatcherServlet.
-
@ComponentScantells Spring to look for other components, configurations, and services in thehellopackage, 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:
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:| 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:
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:
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/
Source : https://spring.io/guides/gs/rest-service-cors/
