Apache Felix Tutorial Example 4

/*
 * 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);
                }
            }
        }
    }
}