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Example 4 - Robust Dictionary Client Bundle

In Example 3, we create a simple client bundle for our dictionary service. The problem with that client was that it did not monitor the dynamic availability of the dictionary service, thus an error would occur if the dictionary service disappeared while the client was using it. In this example we create a client for the dictionary service that monitors the dynamic availability of the dictionary service. The result is a more robust client.

The functionality of the new dictionary client is essentially the same as the old client, it reads words from standard input and checks for their existence in the dictionary service. Our bundle uses its bundle context to register itself as a service event listener; monitoring service events allows the bundle to monitor the dynamic availability of the dictionary service. Our client uses the first dictionary service it finds. The source code for our bundle is as follows in a file called Activator.java:

/*
 * Apache Felix OSGi tutorial.
**/

package tutorial.example4;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.InvalidSyntaxException;
import org.osgi.framework.ServiceReference;
import org.osgi.framework.ServiceListener;
import org.osgi.framework.ServiceEvent;

import tutorial.example2.service.DictionaryService;

/**
 * This class implements a bundle that uses a dictionary
 * service to check for the proper spelling of a word by
 * checking for its existence in the dictionary. This bundle
 * is more complex than the bundle in Example 3 because it
 * monitors the dynamic availability of the dictionary
 * services. In other words, if the service it is using
 * departs, then it stops using it gracefully, or if it needs
 * a service and one arrives, then it starts using it
 * automatically. As before, the bundle uses the first service
 * that it finds and uses the calling thread of the
 * start() method to read words from standard input.
 * You can stop checking words by entering an empty line, but
 * to start checking words again you must stop and then restart
 * the bundle.
**/
public class Activator implements BundleActivator, ServiceListener
{
    // Bundle's context.
    private BundleContext m_context = null;
    // The service reference being used.
    private ServiceReference m_ref = null;
    // The service object being used.
    private DictionaryService m_dictionary = null;

    /**
     * Implements BundleActivator.start(). Adds itself
     * as a listener for service events, then queries for 
     * available dictionary services. If any dictionaries are
     * found it gets a reference to the first one available and
     * then starts its "word checking loop". If no dictionaries
     * are found, then it just goes directly into its "word checking
     * loop", but it will not be able to check any words until a
     * dictionary service arrives; any arriving dictionary service
     * will be automatically used by the client if a dictionary is
     * not already in use. Once it has dictionary, it reads words
     * from standard input and checks for their existence in the
     * dictionary that it is using.
     * (NOTE: It is very bad practice to use the calling thread
     * to perform a lengthy process like this; this is only done
     * for the purpose of the tutorial.)
     * @param context the framework context for the bundle.
    **/
    public void start(BundleContext context) throws Exception
    {
        m_context = context;

        // We synchronize while registering the service listener and
        // performing our initial dictionary service lookup since we
        // don't want to receive service events when looking up the
        // dictionary service, if one exists.
        synchronized (this)
        {
            // Listen for events pertaining to dictionary services.
            m_context.addServiceListener(this,
                "(&(objectClass=" + DictionaryService.class.getName() + ")" +
                "(Language=*))");

            // Query for any service references matching any language.
            ServiceReference[] refs = m_context.getServiceReferences(
                DictionaryService.class.getName(), "(Language=*)");

            // If we found any dictionary services, then just get
            // a reference to the first one so we can use it.
            if (refs != null)
            {
                m_ref = refs[0];
                m_dictionary = (DictionaryService) m_context.getService(m_ref);
            }
        }

        try
        {
            System.out.println("Enter a blank line to exit.");
            String word = "";
            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

            // Loop endlessly.
            while (true)
            {
                // Ask the user to enter a word.
                System.out.print("Enter word: ");
                word = in.readLine();

                // If the user entered a blank line, then
                // exit the loop.
                if (word.length() == 0)
                {
                    break;
                }
                // If there is no dictionary, then say so.
                else if (m_dictionary == null)
                {
                    System.out.println("No dictionary available.");
                }
                // Otherwise print whether the word is correct or not.
                else if (m_dictionary.checkWord(word))
                {
                    System.out.println("Correct.");
                }
                else
                {
                    System.out.println("Incorrect.");
                }
            }
        } catch (Exception ex) { }
    }

    /**
     * Implements BundleActivator.stop(). Does nothing since
     * the framework will automatically unget any used services.
     * @param context the framework context for the bundle.
    **/
    public void stop(BundleContext context)
    {
        // NOTE: The service is automatically released.
    }

    /**
     * Implements ServiceListener.serviceChanged(). Checks
     * to see if the service we are using is leaving or tries to get
     * a service if we need one.
     * @param event the fired service event.
    **/
    public synchronized void serviceChanged(ServiceEvent event)
    {
        String[] objectClass =
            (String[]) event.getServiceReference().getProperty("objectClass");

        // If a dictionary service was registered, see if we
        // need one. If so, get a reference to it.
        if (event.getType() == ServiceEvent.REGISTERED)
        {
            if (m_ref == null)
            {
                // Get a reference to the service object.
                m_ref = event.getServiceReference();
                m_dictionary = (DictionaryService) m_context.getService(m_ref);
            }
        }
        // If a dictionary service was unregistered, see if it
        // was the one we were using. If so, unget the service
        // and try to query to get another one.
        else if (event.getType() == ServiceEvent.UNREGISTERING)
        {
            if (event.getServiceReference() == m_ref)
            {
                // Unget service object and null references.
                m_context.ungetService(m_ref);
                m_ref = null;
                m_dictionary = null;

                // Query to see if we can get another service.
                ServiceReference[] refs = null;
                try
                {
                    refs = m_context.getServiceReferences(
                        DictionaryService.class.getName(), "(Language=*)");
                }
                catch (InvalidSyntaxException ex)
                {
                    // This will never happen.
                }
                if (refs != null)
                {
                    // Get a reference to the first service object.
                    m_ref = refs[0];
                    m_dictionary = (DictionaryService) m_context.getService(m_ref);
                }
            }
        }
    }
}

The client listens for service events indicating the arrival or departure of dictionary services. If a new dictionary service arrives, the bundle will start using that service if and only if it currently does not have a dictionary service. If an existing dictionary service disappears, the bundle will check to see if the disappearing service is the one it is using; if it is it stops using it and tries to query for another dictionary service, otherwise it ignores the event.

Like normal, we must create a manifest.mf file that contains the meta-data for our bundle:

Bundle-Name: Dynamic dictionary client
Bundle-Description: A bundle that uses the dictionary service whenever it becomes available
Bundle-Vendor: Apache Felix
Bundle-Version: 1.0.0
Bundle-Activator: tutorial.example4.Activator
Import-Package: org.osgi.framework,
 tutorial.example2.service

We specify the class to activate our bundle via the Bundle-Activator attribute and also specify that our bundle imports the core OSGi framework package and the dictionary service interface package using the Import-Package attribute. The OSGi framework will automatically handle the details of resolving import packages. (Note: Make sure your manifest file ends in a trailing carriage return or else the last line will be ignored.)

To compile our source code, we need to have the felix.jar file (found in Felix' bin directory) and the example2.jar file in our class path. We compile the source file using a command like:

javac -d c:\classes *.java

This command compiles all source files and outputs the generated classes into a subdirectory of the c:\classes directory; this subdirectory is tutorial\example4, named after the package we specified in the source file. For the above command to work, the c:\classes directory must exist. After compiling, we need to create a JAR file containing the generated package directories. We will also add our manifest file that contains the bundle's meta-data to the JAR file. To create the JAR file, we issue the command:

jar cfm example4.jar manifest.mf -C c:\classes tutorial\example4

This command creates a JAR file using the manifest file we created and includes all of the classes in the tutorial\example4 directory inside of the c:\classes directory. Once the JAR file is created, we are ready to install and start the bundle.

To run Felix, we follow the instructions described in usage.html. When we start Felix, it asks for a profile name, we will put all of our bundles in a profile named tutorial. After running Felix, we should check that all tutorial bundles are stopped, except for the English dictionary service bundle from Example 2. We can use the Felix lb shell command to get a list of all bundles, their state, and their bundle identifier number. If the Example 2 bundle is not active, we should start the bundle using the start command along with the bundle's identifier number displayed by the lb command and stop any other unneeded tutorial bundles using the stop command. (Note: Felix uses some bundles to provide its command shell, so do not stop these bundles.) Now we can install and start our dictionary client bundle. Assuming that we created our bundle in the directory c:\tutorial, we can install and start it in Felix' shell using the following command:

start file:/c:/tutorial/example4.jar

The above command installs and starts the bundle in a single step; it is also possible to install and start the bundle in two steps by using the Felix install and start shell commands. When we start the bundle, it will use the shell thread to prompt us for words. Enter one word at a time to check the words and enter a blank line to stop checking words. To restart the bundle, we must use the Felix shell lb command to get the bundle identifier number for the bundle and first use the stop command to stop the bundle, then the start command to restart it. To test the dictionary service, enter any of the words in the dictionary (e.g., "welcome", "to", "the", "OSGi", "tutorial") or any word not in the dictionary.

Since this client monitors the dynamic availability of the dictionary service, it is robust in the face of sudden departures of the the dictionary service. Further, when a dictionary service arrives, it automatically gets the service if it needs it and continues to function. These capabilities are a little difficult to demonstrate since we are using a simple single-threaded approach, but in a multi-threaded or GUI-oriented application this robustness is very useful.

Last modified by heavy@ungoverned.org on 2011-03-17 10:47:02.0
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