Overview

Nowadays, REST API plays a more and more important role in software development. Being able to create REST API is a must for Java developer. Today, we will learn how to create REST APIs using JAX-RS 2.0, and how easy it is :) After reading this post, you will understand:

  • What is JAX-RS?
  • Basic annotations
  • Create a method “ping”
  • Create a JAX-RS application
  • Running JAX-RS application in Jersey

Before getting started, just want to let you know: the source code of this article on GitHub as mincong-h/jaxrs-2.x-demo. You can also clone it using the following command:

git clone https://github.com/mincong-h/jaxrs-2.x-demo.git

What is JAX-RS

According to Wikipedia, JAX-RS: Java API for RESTful Web Services (JAX-RS) is a Java programming language API spec that provides support in creating web services according to the Representational State Transfer (REST) architectural pattern. JAX-RS uses annotations, introduced in Java SE 5, to simplify the development and deployment of web service clients and endpoints. All versions of JAX-RS are part of the Java Specification Requests (JSRs):

Some of the popular JAX-RS implementations available today are:

  • Jersey
  • RESTEasy
  • Apache CXF
  • Restlet

In this article, I’m using the Jersey, the reference implementation of JAX-RS.

Annotations

JAX-RS annotations allow to identify what a resource class or class method will serve requests for. JAX-RS ensures portability of REST API code across all Java EE-compliant application servers. The most common annotations are described in the table below.

Annotation Package Detail
@GET import javax.ws.rs.GET;
@Produces import javax.ws.rs.Produces;
@Path import javax.ws.rs.Path;
@PathParam import javax.ws.rs.PathParam;
@QueryParam import javax.ws.rs.QueryParam;
@POST import javax.ws.rs.POST;
@Consumes import javax.ws.rs.Consumes;
@FormParam import javax.ws.rs.FormParam;
@PUT import javax.ws.rs.PUT;
@DELETE import javax.ws.rs.DELETE;

Since this post is just a quickstart, I’m not going to go further into these annotations. They will be talked in the next articles of the series.

Create Sample Resource: Ping

Now, let’s write some code. In this paragraph, we will try to create the first JAX-RS resource for ping the REST app:

http://localhost:8080/ping

which allows to ensure if the server is running. In our case, we’ll create 3 classes: PingResource for the JAX-RS resource /ping, ShopApplication for the JAX-RS application, and a Jersey server for hosting the application.

REST Server
- REST Application A
  - REST Resource a1
  - REST REsource a2
- REST Application B
  - REST Resource b1
  - REST Resource b2
- ...

You might wonder what is a “resource” class? According to JSR-311, a resource class is a Java class that uses JAX-RS annotations to implement a corresponding Web resource. Resource classes are POJOs that have at least one method annotated with @Path or a request method designator (JSR-311, §3.1 Resource Classes).

The ping resource class:

package io.mincong.shop.rest;

import javax.ws.rs.HEAD;
import javax.ws.rs.Path;

@Path("ping")
public class PingResource {

  @HEAD
  public void ping() {
    // do nothing
  }
}

Create a JAX-RS Application

Once we created the “ping” resource, we need a JAX-RS application to host it. A JAX-RS application consists of one or more resources, and zero or more provider. All REST applications need to extends Application. An application contains two methods: getClasses() and getSingletons(). Both can be used to get a set of root resource, provider and feature classes.

However, these objects have different life-cycles. The default life-cycle for resource class instances is per-request. The default life-cycle for providers (registered directly or via a feature) is singleton. In our case, I choose the per-request for the “ping” resource, which means that it goes to getClasses(). We will talk about singletons in the next articles. So, here’s the related Java code:

package io.mincong.shop.rest;

import java.util.*;
import javax.ws.rs.core.Application;

public class ShopApplication extends Application {

  @Override
  public Set<Class<?>> getClasses() {
    Set<Class<?>> set = new HashSet<>();
    set.add(PingResource.class);
    return set;
  }

  @Override
  public Set<Object> getSingletons() {
    return Collections.emptySet();
  }
}

Running JAX-RS Application in Server

The next step is to create a Jersey server, which hosts the « Shop » application. The configuration for a Jersey server is really simple, you only need to give two things:

  • The URI of the server
  • The JAX-RS applications to be deployed

Here’s the code:

package io.mincong.shop.rest;

import java.io.IOException;
import java.net.URI;
import javax.ws.rs.core.UriBuilder;
import org.glassfish.grizzly.http.server.HttpServer;
import org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpServerFactory;
import org.glassfish.jersey.server.ResourceConfig;

public class Main {

  private static URI getBaseURI() {
    return UriBuilder.fromUri("http://localhost/").port(8080).build();
  }

  static final URI BASE_URI = getBaseURI();

  static HttpServer startServer() {
    ResourceConfig rc = ResourceConfig.forApplication(new ShopApplication());
    return GrizzlyHttpServerFactory.createHttpServer(BASE_URI, rc);
  }

  public static void main(String[] args) throws IOException {
    System.out.println("Starting grizzly...");
    HttpServer httpServer = startServer();
    System.in.read();
    httpServer.shutdownNow();
  }
}

Once created, we can start the server as a JAR:

$ mvn clean install
$ java -jar ./shop-server/target/shop-server-1.0-SNAPSHOT-jar-with-dependencies.jar
Starting grizzly...

Now, you can test the result in your terminal by pinging the resource via curl:

$ curl -I http://localhost:8080/ping
HTTP/1.1 204 No Content

Our resource method “ping” does not return anything, that’s why did not receive any content. However, 204 means the ping is successful. :) Congratulations, you just created the first resource method!

Reproduction

If you want to reproduce the demo of this article, follow the instructions below.

Open one terminal:

~ $ git clone https://github.com/mincong-h/jaxrs-2.x-demo.git
~ $ cd jaxrs-2.x-demo/quickstart
quickstart $ mvn clean install
quickstart $ java -jar target/jaxrs-quickstart-1.0-SNAPSHOT-jar-with-dependencies.jar

Open another terminal:

~ $ curl -I http://localhost:8080/ping
HTTP/1.1 204 No Content

Conclusion

In this article, we learnt the history of JAX-RS and different basic annotations. We also created a simple resource class “ping”, a JAX-RS application “shop”, and a Jersey server for hosting this app. The project hierarchy is:

- JAX-RS Server (Jersey)
  - Application "shop"
    - Resource "ping"

At the end, we use command line tool curl to verify that every works. The entire source code is available on GitHub mincong-h/jaxrs-2.x-demo. Feel free to download it and run the demo on your machine. More articles about JAX-RS are coming in the next weeks. Hope you enjoy this article, see you the next time!

References