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Tutorial: Exposing a Jax REST service as an OSGi Remote Service


Introduction

Jax RESTful Web Services (Jax-RS) is a popular standard/specification for exposing services for web-based remote access, with a number of implementations (e.g. Resteasy, Jersey, and CXF and more appearing all the time.

This tutorial will show how to use ECF's implementation of OSGi Remote Services to expose an arbitrary Jax-RS service as an OSGi Service and thereby gain the advantages of using OSGi Services, such as superior handling of service dynamics in an unreliable networking environment, service versioning, and a clear separation of service contract from service implementation.

ECF's implementation of OSGi Remote Services is unique among the Remote Service/Remote Service Admin implementations because it provides open APIs for substituting distribution providers. Distribution providers are responsible for the transmission of remote service calls: marshalling the method arguments, unmarshalling return values, and transporting the call to and from the service using some network protocol. ECF has a number of provider implementations, including a relatively new one for Jax-RS. The git repo containing this provider and the examples from this tutorial may be found here. This provider is based upon the Jax-RS specification, and uses currently uses the Jersey implementation from the Eclipse Foundation Orbit project. This provider is modular and extensible, allowing the easy replacement with other Jax-RS implementations.

This tutorial will guide the reader through

  1. Describing how Jax-RS services are defined
  2. Declaring an example Jax-RS remote service so that it may be used as an OSGi Remote Service
  3. Demonstrating how to use the Remote Service for accessing the underlying Jax-RS service

Jax-RS Services

With Jax-RS, typically a server-side implementation 'resource' class is annotated with Jax-RS specified java annotations. For example:

import javax.ws.rs.*;
 
@Path("/helloservice")
public class HelloResource {
 
    @GET
    @Path("sayhello")
    public String sayHello() {
        return "Hello There";
    }
}

The @Path and @Get annotations are used by the Jax-RS implementation on the server to expose the sayHello method for access by a client via using an URL such as:

curl http://localhost:8080/mycontext/helloservice/sayhello

This would return "Hello There" when accessed.

For this tutorial we will use a service for accessing a simple database of Students. Here is a Jax-RS implementation class. Notice the annotations on the public methods. These annotations signal to the Jax-RS implementation that these methods are to be exposed as remote services. When included in a webapp (war file) with the necessary Jax-RS implementation libraries and dependencies on (e.g.) a Tomcat server, this service would be exposed for web-based remote access.

In order to make this student database service available for clients/consumers as an OSGi Remote Service, it's useful to abstract an interface with the appropriate methods and including the same Jax-RS annotations. Here is such an interface for the StudentResouce

package com.mycorp.examples.student;
 
import java.util.List;
 
import javax.ws.rs.Consumes;
import javax.ws.rs.DELETE;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
 
@Path("/studentservice")
public interface StudentService {
 
	@GET
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students")
	List<Student> getStudents();
 
	@GET
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students/{studentId}")
	Student getStudent(@PathParam("studentId") String id);
 
	@POST
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students/add/{studentName}")
	Student addStudent(@PathParam("studentName") String studentName);
 
	@POST
	@Consumes(MediaType.APPLICATION_XML)
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students/update")
	Student updateStudent(Student student);
 
	@DELETE
	@Path("/students/delete/{studentId}")
	@Produces(MediaType.APPLICATION_XML)
	Student deleteStudent(@PathParam("studentId") String studentId);
}

Please note: the method signatures and annotations are exactly the same as those given on the actual Jax-RS service resource class. A bundle project with this service interface and dependencies is provided here.

Note also that the annotations are only those standardized by the Jax-RS specification (i.e. in javax.ws.rs.* packages), and so are not bound to any Jax-RS implementation.

With this interface and the ECF Remote Service Jax-RS provider bundles and their dependencies, we can create a client/consumer application that uses this remote service. ECF's implementation will dynamically create a proxy implementation of the StudentService interface, and make it available to the consumer.

As an OSGi service, it can be discovered and made available in several ways, but the easiest is to have the proxy instance injected via Declarative Services. Here is the java code for a Declarative Services component that dynamically injects the StudentService

package com.mycorp.examples.student.client;
 
import java.util.List;
 
import com.mycorp.examples.student.Student;
import com.mycorp.examples.student.StudentService;
 
public class StudentServiceClient {
 
        // Called by Service Component Runtime when a StudentService is discovered by RSA
	void bindStudentService(StudentService service) {
		System.out.println("Discovered student service=" + service);
		// Get students
		List<Student> originalStudents = service.getStudents();
		// Print list
		System.out.println("students=" + originalStudents);
		// Get first student
		Student s = originalStudents.get(0);
		System.out.println("Student 0=" + s);
		if (s != null) {
			// Get this student via id
			s = service.getStudent(s.getId());
			System.out.println("Student with id=" + s.getId() + "=" + s);
		}
		// Add a new student
		Student newStudent = service.addStudent("April Snow");
		System.out.println("Created student=" + newStudent);
		// Update with grade
		newStudent.setGrade("First");
		newStudent = service.updateStudent(newStudent);
		System.out.println("Updated student=" + newStudent);
		// Delete student
		System.out.println("Deleted student=" + service.deleteStudent(newStudent.getId()));
	}
}

When called by the DS/SCR runtime, this code in the bindStudentService method invokes all the available StudentService proxy methods. At that time, these proxy calls will be automatically turned into valid Jax-RS client calls by the constructed proxy.

Remote Service Discovery=

There are several ways to discover an OSGi Remote Service, but for the purposes of this tutorialwe will use EDEF (Endpoint Description Extension Format), an OSGi-standardized xml-based file format for describing an OSGi Remote Service. See Static File-based Discovery of Remote Service Endpoints for background on EDEF. For this tutorial here is the EDEF for this service, and here is the project containing this EDEF.

Running a Server to Expose the Jax-RS Student Service

For testing, this project was used to create a Tomcat 7 server (no OSGi) with a webapp that exposes the StudentResource as a Jax-RS service using JBoss' Resteasy implementation. Notice that in this case the server is not using OSGi, and is using the Resteasy Jax-RS implementation, rather than the Jersey implementation on the client.

Running the Jax-RS OSGi Service Client

Using the StudentServiceClient class as above, and triggering the RSA discovery via starting the bundle containing the EDEF file given above, results in the creation of a StudentService proxy, and it's injection via DS into the StudentServiceClient.bindStudentService method, resulting in the following OSGi console output:

osgi> Discovered student service=com.mycorp.examples.student.StudentService.proxy@org.eclipse.ecf.remoteservice.RemoteServiceID[containerID=URIID [uri=http://localhost:8080/studentresource/rs];containerRelativeID=0]
students=[Student [id=94381a53-b2b9-4cb4-ab36-1e8db4643f2f, name=Joe Senior, grade=First, address=Address [street=111 Park Ave, city=New York, state=NY, postalCode=11111]]]
Student 0=Student [id=94381a53-b2b9-4cb4-ab36-1e8db4643f2f, name=Joe Senior, grade=First, address=Address [street=111 Park Ave, city=New York, state=NY, postalCode=11111]]
Student with id=94381a53-b2b9-4cb4-ab36-1e8db4643f2f=Student [id=94381a53-b2b9-4cb4-ab36-1e8db4643f2f, name=Joe Senior, grade=First, address=Address [street=111 Park Ave, city=New York, state=NY, postalCode=11111]]
Created student=Student [id=391d2e3e-de8a-4dea-a9d6-26a408737eee, name=April Snow, grade=null, address=null]
Updated student=Student [id=391d2e3e-de8a-4dea-a9d6-26a408737eee, name=April Snow, grade=First, address=null]
Deleted student=Student [id=391d2e3e-de8a-4dea-a9d6-26a408737eee, name=April Snow, grade=First, address=null]

One major advantage of this approach is that

Background and Related Articles

Getting Started with ECF's OSGi Remote Services Implementation

OSGi Remote Services and ECF

Asynchronous Proxies for Remote Services

Static File-based Discovery of Remote Service Endpoints

Download ECF Remote Services/RSA Implementation

How to Add Remote Services/RSA to Your Target Platform

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