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Smack is an Open Source XMPP (Jabber) client library for instant messaging and presence. This library provides the client side functionality as specified in the extensions to the XMPP specifications as related to the client side of said specifications.
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package org.jivesoftware.smack;

import org.jivesoftware.smack.packet.StreamError;
import java.util.Random;
/**
 * Handles the automatic reconnection process. Every time a connection is dropped without
 * the application explictly closing it, the manager automatically tries to reconnect to
 * the server.
 *
 * The reconnection mechanism will try to reconnect periodically:
 * 

 *  For the first minute it will attempt to connect once every ten seconds.
 *  
For the next five minutes it will attempt to connect once a minute.
 *  
If that fails it will indefinitely try to connect once every five minutes.
 * 
 *
 * @author Francisco Vives
 */
public class ReconnectionManager implements ConnectionListener {

    // Holds the connection to the server
    private Connection connection;
    private Thread reconnectionThread;
    private int randomBase = new Random().nextInt(11) + 5; // between 5 and 15 seconds
    
    // Holds the state of the reconnection
    boolean done = false;

    static {
        // Create a new PrivacyListManager on every established connection. In the init()
        // method of PrivacyListManager, we'll add a listener that will delete the
        // instance when the connection is closed.
        Connection.addConnectionCreationListener(new ConnectionCreationListener() {
            public void connectionCreated(Connection connection) {
                connection.addConnectionListener(new ReconnectionManager(connection));
            }
        });
    }

    private ReconnectionManager(Connection connection) {
        this.connection = connection;
    }


    /**
     * Returns true if the reconnection mechanism is enabled.
     *
     * @return true if automatic reconnections are allowed.
     */
    private boolean isReconnectionAllowed() {
        return !done && !connection.isConnected()
                && connection.isReconnectionAllowed();
    }

    /**
     * Starts a reconnection mechanism if it was configured to do that.
     * The algorithm is been executed when the first connection error is detected.
     * 
     * The reconnection mechanism will try to reconnect periodically in this way:
     * 

     * First it will try 6 times every 10 seconds.
     * 
Then it will try 10 times every 1 minute.
     * 
Finally it will try indefinitely every 5 minutes.
     * 
     */
    synchronized protected void reconnect() {
        if (this.isReconnectionAllowed()) {
            // Since there is no thread running, creates a new one to attempt
            // the reconnection.
            // avoid to run duplicated reconnectionThread -- fd: 16/09/2010
            if (reconnectionThread!=null && reconnectionThread.isAlive()) return;
            
            reconnectionThread = new Thread() {
             			
                /**
                 * Holds the current number of reconnection attempts
                 */
                private int attempts = 0;

                /**
                 * Returns the number of seconds until the next reconnection attempt.
                 *
                 * @return the number of seconds until the next reconnection attempt.
                 */
                private int timeDelay() {
                    attempts++;
                    if (attempts > 13) {
                	return randomBase*6*5;      // between 2.5 and 7.5 minutes (~5 minutes)
                    }
                    if (attempts > 7) {
                	return randomBase*6;       // between 30 and 90 seconds (~1 minutes)
                    }
                    return randomBase;       // 10 seconds
                }

                /**
                 * The process will try the reconnection until the connection succeed or the user
                 * cancell it
                 */
                public void run() {
                    // The process will try to reconnect until the connection is established or
                    // the user cancel the reconnection process {@link Connection#disconnect()}
                    while (ReconnectionManager.this.isReconnectionAllowed()) {
                        // Find how much time we should wait until the next reconnection
                        int remainingSeconds = timeDelay();
                        // Sleep until we're ready for the next reconnection attempt. Notify
                        // listeners once per second about how much time remains before the next
                        // reconnection attempt.
                        while (ReconnectionManager.this.isReconnectionAllowed() &&
                                remainingSeconds > 0)
                        {
                            try {
                                Thread.sleep(1000);
                                remainingSeconds--;
                                ReconnectionManager.this
                                        .notifyAttemptToReconnectIn(remainingSeconds);
                            }
                            catch (InterruptedException e1) {
                                e1.printStackTrace();
                                // Notify the reconnection has failed
                                ReconnectionManager.this.notifyReconnectionFailed(e1);
                            }
                        }

                        // Makes a reconnection attempt
                        try {
                            if (ReconnectionManager.this.isReconnectionAllowed()) {
                                connection.connect();
                            }
                        }
                        catch (XMPPException e) {
                            // Fires the failed reconnection notification
                            ReconnectionManager.this.notifyReconnectionFailed(e);
                        }
                    }
                }
            };
            reconnectionThread.setName("Smack Reconnection Manager");
            reconnectionThread.setDaemon(true);
            reconnectionThread.start();
        }
    }

    /**
     * Fires listeners when a reconnection attempt has failed.
     *
     * @param exception the exception that occured.
     */
    protected void notifyReconnectionFailed(Exception exception) {
        if (isReconnectionAllowed()) {
            for (ConnectionListener listener : connection.connectionListeners) {
                listener.reconnectionFailed(exception);
            }
        }
    }

    /**
     * Fires listeners when The Connection will retry a reconnection. Expressed in seconds.
     *
     * @param seconds the number of seconds that a reconnection will be attempted in.
     */
    protected void notifyAttemptToReconnectIn(int seconds) {
        if (isReconnectionAllowed()) {
            for (ConnectionListener listener : connection.connectionListeners) {
                listener.reconnectingIn(seconds);
            }
        }
    }

    public void connectionClosed() {
        done = true;
    }

    public void connectionClosedOnError(Exception e) {
        done = false;
        if (e instanceof XMPPException) {
            XMPPException xmppEx = (XMPPException) e;
            StreamError error = xmppEx.getStreamError();

            // Make sure the error is not null
            if (error != null) {
                String reason = error.getCode();

                if ("conflict".equals(reason)) {
                    return;
                }
            }
        }

        if (this.isReconnectionAllowed()) {
            this.reconnect();
        }
    }

    public void reconnectingIn(int seconds) {
        // ignore
    }

    public void reconnectionFailed(Exception e) {
        // ignore
    }

    /**
     * The connection has successfull gotten connected.
     */
    public void reconnectionSuccessful() {
        // ignore
    }

}