log/src/main/java/org/apache/felix/log/LogListenerThread.java

/* 
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.felix.log;

import java.util.Iterator;
import java.util.List;
import java.util.Stack;
import java.util.Vector;

import org.osgi.service.log.LogEntry;
import org.osgi.service.log.LogListener;

/**
 * This class is responsible for asynchronously delivering log messages to
 * any {@link LogListener} subscribers.  A subscriber can be added using the
 * {@link org.osgi.service.log.LogReaderService#addLogListener(LogListener)}
 * method.
 */
final class LogListenerThread extends Thread {

    /** Whether the thread is stopping or not. */
    private boolean m_stopping = false;

    /** The stack of entries waiting to be delivered to the log listeners. **/
    private final Stack m_entriesToDeliver = new Stack();

    /** The list of listeners. */
    private final List m_listeners = new Vector();

    /**
     * Add an entry to the list of messages to deliver.
     * @param entry the log entry to deliver
     */
    void addEntry(final LogEntry entry) {
        synchronized (m_entriesToDeliver) {
            m_entriesToDeliver.add(entry);
            m_entriesToDeliver.notifyAll();
        }
    }

    /**
     * Add a listener to the list of listeners that are subscribed.
     * @param listener the listener to add to the list of subscribed listeners
     */
    void addListener(final LogListener listener) {
        synchronized (m_listeners) {
            m_listeners.add(listener);
        }
    }

    /**
     * Remove a listener from the list of listeners that are subscribed.
     * @param listener the listener to remove from the list of subscribed listeners
     */
    void removeListener(final LogListener listener) {
        synchronized (m_listeners) {
            m_listeners.remove(listener);
        }
    }

    /**
     * Returns the number of listeners that are currently registered.
     * @return the number of listeners that are currently registered
     */
    int getListenerCount() {
        return m_listeners.size();
    }

    /**
     * Stop the thread.  This will happen asynchronously.
     */
    void shutdown() {
        m_stopping = true;

        synchronized (m_entriesToDeliver) {
            m_entriesToDeliver.notifyAll();
        }
    }

    /**
     * The main method of the thread: waits for new messages to be receieved
     * and then delivers them to any registered log listeners.
     */
    public void run() {
        boolean stop = false;

        for (; !stop;) {
            synchronized (m_entriesToDeliver) {
                if (!m_entriesToDeliver.isEmpty()) {
                    LogEntry entry = (LogEntry) m_entriesToDeliver.pop();

                    synchronized (m_listeners) {
                        Iterator listenerIt = m_listeners.iterator();
                        while (listenerIt.hasNext()) {
                            try {
                                LogListener listener = (LogListener) listenerIt.next();
                                listener.logged(entry);
                            } catch (Throwable t) {
                                // catch and discard any exceptions thrown by the listener
                            }
                        }
                    }
                }

                if (m_entriesToDeliver.isEmpty()) {
                    try {
                        m_entriesToDeliver.wait();
                    } catch (InterruptedException e) {
                        // do nothing
                    }
                }
            }

            if (m_stopping) {
                stop = true;
            }
        }
    }
}