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

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==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. [https://jersey.java.net/ Jersey], and [http://cxf.apache.org/ CXF]) and others 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 [https://github.com/ECF/JaxRSProviders found here].  This provider is based upon the [https://jcp.org/aboutJava/communityprocess/final/jsr339/index.html Jax-RS specification], and uses the [https://jersey.java.net Jersey implementation] from the Eclipse 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 remote services are implemented
#Declaring an example Jax-RS remote CRUD service so that it may be used as an OSGi Remote Service
#Showing how to implement the Remote 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:

<source lang="java">
// 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";
    }
}
</source>

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:   

<pre>
curl http://localhost:8080/helloworld
</pre>

This would return "Hello World".

For this tutorial we will use a service for accessing a simple database of Students.   [https://github.com/ECF/JaxRSProviders/blob/master/examples/com.mycorp.examples.student.webapp.host/src/com/mycorp/examples/student/webapp/host/StudentResource.java 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.  

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

<source lang="java">
@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);
}
</source>

[https://github.com/ECF/JaxRSProviders/blob/master/examples/com.mycorp.examples.student/src/com/mycorp/examples/student/StudentService.java 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 [https://github.com/ECF/JaxRSProviders/blob/master/examples/com.mycorp.examples.student.webapp.host/src/com/mycorp/examples/student/webapp/host/StudentResource.java Jax-RS service resource implementation class].  The full example project with this service interface and dependencies is [https://github.com/ECF/JaxRSProviders/tree/master/examples/com.mycorp.examples.student provided here] and the entire example is available [https://github.com/ECF/JaxRSProviders/tree/master/examples 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 [https://github.com/ECF/JaxRSProviders/tree/master/bundles 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:

<source lang="java">
	@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);
	}
</source>

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.

==Implementing the Jax-RS Server==

Here is the StudentService server implementation code

<source lang="java">
// 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);
	}
}
</source>

[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

<source lang="java">
@Component(immediate = true, properties = "rsexport.jersey.properties")
public class StudentServiceImpl implements StudentService {
...
</source>

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

<source lang="java">
# 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
</source>

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 [https://github.com/ECF/JaxRSProviders/blob/master/examples/com.mycorp.examples.student.remoteservice.host/rsexport.generic.properties properties file that exports the StudentService via the ECF generic provider]

<source lang="java">
# 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
</source>

With these service properties and absolutely no code changes, the ECF generic provider will be used at service registration time to export the StudentService.

==Summary==

By using ECF's implementation of OSGi Remote Services and the [https://github.com/ECF/JaxRSProviders/tree/master/bundles 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 easily accessed via non-proxy or non-Java clients...e.g. via curl, application code, javascript, other languages, etc.

==Background and Related Articles==

[[Tutorial: Using REST and OSGi Standards for Micro Services]]

[[Getting Started with ECF's OSGi Remote Services Implementation]]

[[OSGi Remote Services and ECF]]

[[Asynchronous Proxies for Remote Services]]

[[File-based Discovery | Static File-based Discovery of Remote Service Endpoints]]

[[EIG:Download|Download ECF Remote Services/RSA Implementation]]

[[EIG:Add to Target Platform|How to Add Remote Services/RSA to Your Target Platform]]