dattnguyen82 / gs-rest-service-cors Goto Github PK

You’ll build a service that will accept HTTP GET requests at:

and respond with a JSON representation of a greeting:

You can customize the greeting with an optional name parameter in the query string:

The name parameter value overrides the default value of "World" and is reflected in the response:

This service differs slightly from the one described in Building a RESTful Web Service in that it will have a filter that adds CORS headers to the response.

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/prereq_editor_jdk_buildtools.adoc

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/how_to_complete_this_guide.adoc

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/hide-show-gradle.adoc

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/hide-show-maven.adoc

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/hide-show-sts.adoc

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:

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

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 @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

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.

@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 that 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 2 is on the classpath, Spring’s MappingJackson2HttpMessageConverter is automatically chosen to convert the Greeting instance to JSON.

So that the RESTful web service will include CORS access control headers in its response, you’ll need to write a filter that adds those headers to the response. This SimpleCORSFilter class provides a simple implementation of such a filter:

src/main/java/hello/SimpleCORSFilter.java

As it is written, SimpleCORSFilter responds to all requests with certain Access-Control-* headers. In this case, the headers are set to allow POST, GET, OPTIONS, or DELETE requests from clients originated from any host. The results of a preflight request may be cached for up to 3,600 seconds (1 hour). And the request may include an x-requested-with header.

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

@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 the hello package, allowing it to find the HelloController.

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.

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/build_an_executable_jar_subhead.adoc

https://raw.githubusercontent.com/spring-guides/getting-started-macros/master/build_an_executable_jar_with_both.adoc

Logging output is displayed. The service should be up and running within a few seconds.

Now that the service is up, visit http://localhost:8080/greeting, where you see:

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!":

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

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

You can now run the client using the Spring Boot CLI (Command Line Interface). Spring Boot includes an embedded Tomcat server, which offers a simple approach to serving web content. See Building an Application with Spring Boot for more information about installing and using the CLI.

In order to serve static content from Spring Boot’s embedded Tomcat server, you’ll also need to create a minimal amount of web application code so that Spring Boot knows to start Tomcat. The following app.groovy script is sufficient for letting Spring Boot know that you want to run Tomcat:

app.groovy

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:

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:

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