/*
 * 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.dm.runtime;

import java.util.LinkedList;

/**
 * Allows you to enqueue tasks from multiple threads and then execute
 * them on one thread sequentially. It assumes more than one thread will
 * try to execute the tasks and it will make an effort to pick the first
 * task that comes along whilst making sure subsequent tasks return
 * without waiting.
 * 
 * @author <a href="mailto:dev@felix.apache.org">Felix Project Team</a>
 */
public final class SerialExecutor
{
    private final LinkedList<Runnable> m_workQueue = new LinkedList<Runnable>();
    private Runnable m_active;

    /**
     * Enqueue a new task for later execution. This method is
     * thread-safe, so multiple threads can contribute tasks.
     * 
     * @param runnable the runnable containing the actual task
     */
    public synchronized void enqueue(final Runnable runnable)
    {
        m_workQueue.addLast(new Runnable()
        {
            @SuppressWarnings("synthetic-access")
            public void run()
            {
                try
                {
                    runnable.run();
                }
                finally
                {
                    scheduleNext();
                }
            }
        });
    }

    /**
     * Execute any pending tasks. This method is thread safe,
     * so multiple threads can try to execute the pending
     * tasks, but only the first will be used to actually do
     * so. Other threads will return immediately.
     */
    public void execute()
    {
        Runnable active;
        synchronized (this)
        {
            active = m_active;
        }
        if (active == null)
        {
            scheduleNext();
        }
    }

    private void scheduleNext()
    {
        Runnable active;
        synchronized (this)
        {
            m_active = (m_workQueue.size() == 0) ? null : (Runnable) m_workQueue.removeFirst();
            active = m_active;
        }
        if (active != null)
        {
            active.run();
        }
    }
}