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 the use of ECF's 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, service versioning, and a clear separation of service contract from service implementation.

ECF's implementation of OSGi Remote Services is unique because it provides open APIs for customizing or replacing distribution providers. Distribution providers are responsible for the actual transmission of remote service calls: marshalling the method arguments, unmarshalling return values, and transport of the call to and from the service host. ECF has a number of provider implementations, including a relatively new one based upon 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 the Jersey implementation from the Orbit project. This provider is small, modular, and extensible, allowing the easy customization or substitution of alternative Jax-RS implementations.

This tutorial will guide through

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

Jax-RS Remote Services

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

// The Java class will be hosted at the URI path "/helloworld"
@Path("/helloworld")
public class HelloWorldResource implements HelloWorldService {
 
    // The Java method will process HTTP GET requests
    @GET
    // The Java method will produce content identified by the MIME Media
    // type "text/plain"
    @Produces("text/plain")
    public String getMessage() {
        // Return some textual content
        return "Hello World";
    }
}

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/helloworld

This would return "Hello There".

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 StudentResource

@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/{studentName}")
	Student createStudent(@PathParam("studentName") String studentName);
 
	@PUT
	@Consumes(MediaType.APPLICATION_XML)
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students")
	Student updateStudent(Student student);
 
	@DELETE
	@Path("/students/{studentId}")
	@Produces(MediaType.APPLICATION_XML)
	Student deleteStudent(@PathParam("studentId") String studentId);
}

Here is the complete interface class.

Note that the method signatures and annotations in this service interface are exactly the same as those given on the service resource implementation class Jax-RS service resource implementation class. The full example project with this service interface and dependencies is provided here and the entire example is available in these bundles. Note that these projects only include dependencies on standardized Jax-RS classes. They do not contain dependencies on either Jax-RS implementation (Jersey, CXF, etc) code. Further they do not contain dependencies on either ECF code or OSGi code. This makes it very easy to move to alternative implementations of Jax-RS (e.g. CXF, RestEasy), or even to move to some other implementation of OSGi Remote Service.

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. This allows the use of a variety of JAX-RS implementations (Jersey, Apache CXF, RestEasy, others), without having to change the StudentService method signatures or annotations.

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 DS component that injects the StudentService into the client code:

	@Reference(cardinality = ReferenceCardinality.OPTIONAL, policy = ReferencePolicy.DYNAMIC)
	void bindStudentService(StudentService service) throws Exception {
		this.studentService = service;
		System.out.println("Discovered student service=" + this.studentService);
		// Get all students
		List<Student> students = studentService.getStudents();
		// Get first student from list
		Student s0 = students.get(0);
		// Print out first student
		System.out.println("Student0=" + s0);
		// If there is anyone there, then update
		if (s0 != null) {
			s0.setGrade("Eighth");
			// And update
			System.out.println("Updated Student0=" + studentService.updateStudent(s0));
		}
 
		// Create a new student
		Student newstudent = studentService.createStudent("April Snow");
		System.out.println("Created student=" + newstudent);
		// when done change the grade to first
		newstudent.setGrade("First");
		Address addr = new Address();
		addr.setStreet("111 NE 1st");
		addr.setCity("Austin");
		addr.setState("Oregon");
		addr.setPostalCode("97200");
		newstudent.setAddress(addr);
		// update
		Student updatednewstudent = studentService.updateStudent(newstudent);
		System.out.println("Updated student=" + updatednewstudent);
		// Then delete new student
		Student deletedstudent = studentService.deleteStudent(updatednewstudent.getId());
		System.out.println("Deleted student=" + deletedstudent);
	}

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

Jax-RS Server

Here is the StudentService server implementation code

// The jax-rs path annotation for this service
@Path("/studentservice")
// The OSGi DS (declarative services) component annotation. Note that the
// /rsexport.properties file defines this service impl as a remote service
// and configures the usage of the Jersey Jax-RS implementation as the
// desired distribution provider. See /rsexport.jersey.properties
@Component(immediate = true, properties = "rsexport.jersey.properties")
public class StudentServiceImpl implements StudentService {
 
...
	// Provide a map-based storage of students
        private Map<String,Student> students = new HashMap<String,Student();
        // code deleted that initialized a simple Map-based storage/retrieval of Students
        // ...
 
        // Implementation of StudentService with Jax-RS annotations
	@GET
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students")
	public List<Student> getStudents() {
		return new ArrayList<Student>(students.values());
	}
 
	@GET
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students/{studentId}")
	public Student getStudent(@PathParam("studentId") String id) {
		return students.get(id);
	}
 
	@POST
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students/{studentName}")
	public Student createStudent(@PathParam("studentName") String studentName) {
		if (studentName == null)
			return null;
		synchronized (students) {
			Student s = new Student(studentName);
			s.setId(UUID.randomUUID().toString());
			students.put(s.getId(), s);
			return s;
		}
	}
 
	@PUT
	@Consumes(MediaType.APPLICATION_XML)
	@Produces(MediaType.APPLICATION_XML)
	@Path("/students")
	public Student updateStudent(Student student) {
		Student result = null;
		if (student != null) {
			String id = student.getId();
			if (id != null) {
				synchronized (students) {
					result = students.get(student.getId());
					if (result != null) {
						String newName = student.getName();
						if (newName != null)
							result.setName(newName);
						result.setGrade(student.getGrade());
						result.setAddress(student.getAddress());
					}
				}
			}
		}
		return result;
	}
 
	@DELETE
	@Path("/students/{studentId}")
	@Produces(MediaType.APPLICATION_XML)
	public Student deleteStudent(@PathParam("studentId") String studentId) {
		return students.remove(studentId);
	}
}

[ https://github.com/ECF/JaxRSProviders/blob/master/examples/com.mycorp.examples.student.remoteservice.host/src/com/mycorp/examples/student/remoteservice/host/StudentServiceImpl.java Here is the full source code]. Note that Declarative Services is used to create and register the remote service. This DS annotation

@Component(immediate = true, properties = "rsexport.jersey.properties")
public class StudentServiceImpl implements StudentService {
...

specifies that the service properties in rsexport.jersey.properties file are associated with this service. Here is that properties file

# Required OSGi Remote Service Property indicating that the StudentService should
# be exported
service.exported.interfaces=*
# Optional OSGi Remote Service Property indicating that the ecf.jaxrs.jersey.server
# distribution provider should be used to export this service
service.exported.configs=ecf.jaxrs.jersey.server
# Distribution-provider property defining the alias prefix for the Jersey server
ecf.jaxrs.jersey.server.alias=/jersey/is/good
# Other Jersey properties that could be defined
ecf.jaxrs.jersey.server.urlContext=http://localhost:8080

Since these properties include the OSGi-Remote Service standard properties (service.exported.interfaces and service.exported.config), when this component is registered by DS, the ECF Remote Service implementation recognizes that this service is to be exported, and it exports it with the given distribution provider (ecf.jaxrs.jersey.server) and with the given configuration.

As an example of how easy it becomes to use a different distribution provider, here are the contents of an properties file that exports the StudentService via the ECF generic provider

# Required OSGi Remote Service Property indicating that the StudentService should
# be exported
service.exported.interfaces=*
# Use the ecf.generic.server distribution provider instead of the Jersey Jax-RS implementation
service.exported.configs=ecf.generic.server
# Configure the generic server host and port
ecf.generic.server.hostname=localhost
ecf.generic.server.port=4445

Here is the properties file.

By using these these service properties the ECF generic provider will be used to export the StudentService rather than the Jersey provider.

Summary

By using ECF's implementation of OSGi Remote Services and the ECF Jax-RS provider, OSGi Remote Services proxy is automatically created to access the Jax-RS service. Note that with Jax-RS compliant providers, the remote service can also be accessed via non-proxy clients...e.g. via curl, programmer created application code, etc.