Difference between revisions of "Tutorial: Creating a RESTful Remote Service Provider"

Revision as of 20:49, 10 December 2013

Introduction

In previous tutorials we've focused on how to use OSGi Remote Services to export a simple ITimeService and have a consumer use this service.

This tutorial will focus on customizing ECF's implementation of OSGi Remote Services to use a custom distribution provider...aka a Remote Service provider. Creating a custom Remote Service provider is tantamount to creating your own implementation of the distribution function of OSGi Remote Services.

Why would one want to do this? One reason is interoperability and integration. There are many existing services on the Internet currently...exposed through many different protocols (e.g. http+json, http+xml, JMS+json, etc., etc.). By creating a new remote service provider, it's quite possible to take an existing service, implemented via an existing transport+serialization approach, and easily expose it as an OSGi Remote Service...while allowing existing services to run unmodified.

Another reason is that in the remoting of services, there are frequently non-functional requirements...e.g. for a certain kind of transport-level security, or using an existing protocol (e.g. http), or using a certain kind of serialization (e.g. json, or xml, or protocol buffers). By creating a new distribution/remote service provider, these requirements can be met...simply by using the ECF provider architecture...rather than being required to reimplement all aspects of OSGi Remote Services/RSA.

Exporting Using a Config Type

As shown in Tutorial:_Building_your_first_OSGi_Remote_Service, here is the remote service metadata needed to export a remote service using the ECF generic server distribution provider

Dictionary<String, String> props = new Hashtable<String, String>();
// OSGi Standard Property - indicates which of the interfaces of the service will be exported.  '*' means 'all'.
props.put("service.exported.interfaces", "*");
// OSGi Standard Property (optional) - indicates which provider config(s) will be used to export the service
// (If not explicitly given here, the provider is free to choose a default configuration for the service)
props.put("service.exported.configs","ecf.generic.server");
// Register a new TimeServiceImpl with the above props
bundleContext.registerService(ITimeService.class, new TimeServiceImpl(), props);

Note the line:

props.put("service.exported.configs","ecf.generic.server");

This is one of the ECF-provided distribution providers, identified by as the config type "ecf.generic.server". This provider is a general provider, capable of supporting the export of any service.

It's quite possible, however, to create a replacement provider, based upon our own http+rest+json transport protocol. The remainder of this tutorial will step through how to create and run your own provider.

Provider Step 1: Implement New Provider Namespace

ECF allows providers to create their own namespaces, so that the endpoint id of the remote service can be properly interpreted. Here is a TimeServiceRestNamespace class

public class TimeServiceRestNamespace extends RestNamespace {
 
	public static final String NAME = "com.mycorp.examples.timeservice.provider.rest.namespace";
 
	@Override
	public ID createInstance(Object[] parameters) throws IDCreateException {
		return new TimeServiceRestID(this, URI.create((String) parameters[0]));
	}
 
	public static class TimeServiceRestID extends RestID {
 
		public TimeServiceRestID(Namespace namespace, URI uri) {
			super(namespace, uri);
		}
	}
}

As can be seen above, when the createInstance call is made a new instance of TimeServiceRestID is created from the single String parameter[0] passed into the createInstance call. Along with this class, we can define an extension in the plugin.xml of this bundle to allow this namespace to be used at the appropriate point.

   <extension
         point="org.eclipse.ecf.identity.namespace">
      <namespace
            class="com.mycorp.examples.timeservice.provider.rest.common.TimeServiceRestNamespace"
            description="Timeservice Rest Namespace"
            name="com.mycorp.examples.timeservice.provider.rest.namespace">
      </namespace>
   </extension>

Since this Namespace/ID is needed for both the host and the consumer providers, it should be placed in a separate bundle. The complete bundle is available via the ECF git repo with project (and symbolic) name com.mycorp.examples.timeservice.provider.rest.common.

Note that the package with this new Namespace/ID class must be exported by having this in the manifest.mf:

Export-Package: com.mycorp.examples.timeservice.provider.rest.common

Provider Step 2: Creating the Host Implementation

Since we would like to interact with this via http+rest, we will create a Servlet to actually handle the request...and register it dynamically to the OSGi standard HttpService. That way we can implement the ITimeService.getCurrentTime() method by implementing the HttpServlet.doGet method. Here is the complete implementation of the TimeRemoteServiceHttpServlet as well as the necessary ECF container code

package com.mycorp.examples.timeservice.internal.provider.rest.host;
 
import java.io.IOException;
 
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
import org.eclipse.ecf.core.ContainerCreateException;
import org.eclipse.ecf.core.identity.IDFactory;
import org.eclipse.ecf.core.identity.Namespace;
import org.eclipse.ecf.remoteservice.servlet.HttpServiceComponent;
import org.eclipse.ecf.remoteservice.servlet.RemoteServiceHttpServlet;
import org.eclipse.ecf.remoteservice.servlet.ServletServerContainer;
import org.json.JSONException;
import org.json.JSONObject;
import org.osgi.service.http.HttpService;
 
import com.mycorp.examples.timeservice.ITimeService;
import com.mycorp.examples.timeservice.provider.rest.common.TimeServiceRestNamespace;
 
public class TimeServiceServerContainer extends ServletServerContainer {
 
	public static final String TIMESERVICE_HOST_CONFIG_NAME = "com.mycorp.examples.timeservice.rest.host";
	public static final String TIMESERVICE_SERVLET_NAME = "/" + ITimeService.class.getName();
 
	private final HttpService httpService;
 
	TimeServiceServerContainer(String id, HttpService httpService) throws ContainerCreateException {
		super(IDFactory.getDefault()
				.createID(TimeServiceRestNamespace.NAME, id));
		this.httpService = httpService;
		// Register our servlet with the given httpService with the TIMESERVICE_SERVLET_NAME
		// which is "/com.mycorp.examples.timeservice.ITimeService"
		try {
			this.httpService.registerServlet(TIMESERVICE_SERVLET_NAME,
					new TimeRemoteServiceHttpServlet(), null, null);
		} catch (Exception e) {
			throw new ContainerCreateException("Could not create Time Service Server Container",e);
		}
	}
 
	@Override
	public void dispose() {
		httpService.unregister(TIMESERVICE_SERVLET_NAME);
		super.dispose();
	}
 
	@Override
	public Namespace getConnectNamespace() {
		return IDFactory.getDefault().getNamespaceByName(
				TimeServiceRestNamespace.NAME);
	}
 
	class TimeRemoteServiceHttpServlet extends RemoteServiceHttpServlet {
 
		private static final long serialVersionUID = 3906126401901826462L;
 
		// Handle get call right here.
		@Override
		protected void doGet(HttpServletRequest req, HttpServletResponse resp)
				throws ServletException, IOException {
 
			// No arguments to getCurrentTime() method, so
			// nothing to deserialize from request
 
			// Get local OSGi ITimeService
			ITimeService timeService = HttpServiceComponent.getDefault()
					.getService(ITimeService.class);
 
			// Call local service to get the time
			Long currentTime = timeService.getCurrentTime();
 
			// Serialize response
		    try {
				resp.getOutputStream().print(new JSONObject().put("time", currentTime).toString());
			} catch (JSONException e) {
				throw new ServletException("json response object could not be created for time service", e);
			}
		}
	}
 
}

In the TimeServiceServerContainer constructor a new instance of the TimeRemoteServiceHttpServlet is created and registered as a servlet with the HttpService

this.httpService.registerServlet(TIMESERVICE_SERVLET_NAME,
				new TimeRemoteServiceHttpServlet(), null, null);
<source>
 
Note that the TIMESERVICE_SERVLET_NAME is defined to be "/com.mycorp.examples.timeservice.ITimeService".  Since this is used to register the servlet this means that this service will have an URL that is (for example):
 
<source lang="java">
http://localhost:8080/com.mycorp.examples.timeservice.ITimeService

Other paths can obviously be specified as desired.

Also present are overrides of two ECF container lifecycle methods

public Namespace getConnectNamespace() {
	return IDFactory.getDefault().getNamespaceByName(
			TimeServiceRestNamespace.NAME);
}

which reference the TimeServiceRestNamespace, as defined above, and

public void dispose() {
	httpService.unregister(TIMESERVICE_SERVLET_NAME);
	super.dispose();
}

which dynamically unregisters the servlet when the container instance is disposed.

All of the actual behavior is given by the doGet method implementation in the TimeRemoteServiceHttpServlet

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
		throws ServletException, IOException {
 
	// 1. Get local OSGi ITimeService
	ITimeService timeService = HttpServiceComponent.getDefault()
			.getService(ITimeService.class);
 
	// 2. Call local service to get the time
	Long currentTime = timeService.getCurrentTime();
 
	// 3. Serialize response
        try {
		resp.getOutputStream().print(new JSONObject().put("time", currentTime).toString());
	} catch (JSONException e) {
		throw new ServletException("json response object could not be created for time service", e);
	}
}

As can be seen by the comments, all that's happening here is that

1 The local ITimeService implementation is retrieved
2 The ITimeService.getCurrentTime() method is called to get the local time (Long)
3 The result is serialized to json (in this case using the json.org json implementation) and printed to the HttpServletResponse output stream.

Notice: this Wiki will be going read only early in 2024 and edits will no longer be possible. Please see: https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/wikis/Wiki-shutdown-plan for the plan.

Difference between revisions of "Tutorial: Creating a RESTful Remote Service Provider"

Revision as of 20:49, 10 December 2013

Contents

Introduction

Exporting Using a Config Type

Provider Step 1: Implement New Provider Namespace

Provider Step 2: Creating the Host Implementation

Breadcrumbs

Notice: this Wiki will be going read only early in 2024 and edits will no longer be possible. Please see: https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/wikis/Wiki-shutdown-plan for the plan.

Difference between revisions of "Tutorial: Creating a RESTful Remote Service Provider"

Revision as of 20:49, 10 December 2013

Contents

Introduction

Exporting Using a Config Type

Provider Step 1: Implement New Provider Namespace

Provider Step 2: Creating the Host Implementation