Using Equinox CommandProvider to make OSGi console interactive.

After fiddling with the First Bundles that "Hello World"-ed upon Activation, You want to see more interactivity in OSGi. Although Using OSGi for an interactive Command Line Application would be like this one would be, well, a callable over-kill, I am going to start with an example and Expand it in later posts. So, please Welcome CommandProvider.

CommandProvider is an EQUINOX specific API for extending the Console. This basic Example illustrates how to get a command from console and do something in java and also gets your feet wet on Service Registry

package com.so.examples.commandconsole;

import org.eclipse.osgi.framework.console.CommandInterpreter;
import org.eclipse.osgi.framework.console.CommandProvider;

public class DisplayMessageCommand implements CommandProvider {

 public void _say(CommandInterpreter ci) {
  ci.print("You said:" + ci.nextArgument());
 }

 @Override
 public String getHelp() {
  return "\tsay - repeats what you say\n";
 }

}

This provider class implements CommandProvider, which is actually just a marker interface, except that getHelp is a helper to print out additional help data when user types in help. The actual commands follow the java reflection pattern as a method that starts with an underscore(_), takes CommandInterpeter as parameter and returns void. So there can be more than one command provided by one CommandProvider Class. Important thing here is, this could have been easily(, better) achieved with annotations, but OSGi inherently is for ALL java platforms, so its implemented this way.

Next step is to tell the framework to commands from this CommandProvider. To do that, We have to "bind" this to framework as a service. You could do it anytime you have access to the bundle context, but the easiest place is when starting the bundle. An Activator like the one below should do.

public class CommandBundleActivator implements BundleActivator {

 @Override
 public void start(BundleContext context) throws Exception {
  context.registerService(CommandProvider.class.getName(),
    new DisplayMessageCommand(), null);

 }
//stripped ...

Notice that we are using equinox specific packages, so in our manifest, we add, along with the Activator

Bundle-Activator: com.so.examples.commandconsole.CommandBundleActivator
Import-Package: org.osgi.framework,
 org.eclipse.osgi.framework.console

What happens under the hood: Equinox when started with -console parameter, starts the framework also initiating a default CommandProvider Service Implementation (ignoring -console will not start the console, hence exitting back to shell). Which is the one that accepts the commands and performs actions. And EQUINOX, provides a way to extend it by adding other Implementations of CommandProvider to the ServiceRegistry.

Now start up the framework, install the new bundle, and start it,

osgi> help
 say - repeats what you say
---Eclipse Runtime commands---
 diag - Displays unsatisfied constraints for the specified bundle(s).
 enableBundle - enable the specified bundle(s)
#stripped off..
osgi> say Sarath
You said:Sarath

This brings to the next topic, which I will discuss in the next post, The Service Registry. The example project is available here.