ReentrantLock.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.shell.remote;

/**
 * Implements a reentrant lock.
 * <p/>
 * Public domain code.
 */
class ReentrantLock
{
    protected Thread m_owner = null;
    protected long m_holds = 0;

    public void acquire() throws InterruptedException
    {
        //log.debug("acquire()::" + Thread.currentThread().toString());
        if (Thread.interrupted())
        {
            throw new InterruptedException();
        }
        Thread caller = Thread.currentThread();
        synchronized (this)
        {
            if (caller == m_owner)
            {
                ++m_holds;
            }
            else
            {
                try
                {
                    while (m_owner != null)
                    {
                        wait();
                    }
                    m_owner = caller;
                    m_holds = 1;
                }
                catch (InterruptedException ex)
                {
                    notify();
                    throw ex;
                }
            }
        }
    }//acquire

    public boolean attempt(long msecs) throws InterruptedException
    {
        //log.debug("attempt()::" + Thread.currentThread().toString());
        if (Thread.interrupted())
        {
            throw new InterruptedException();
        }
        Thread caller = Thread.currentThread();
        synchronized (this)
        {
            if (caller == m_owner)
            {
                ++m_holds;
                return true;
            }
            else if (m_owner == null)
            {
                m_owner = caller;
                m_holds = 1;
                return true;
            }
            else if (msecs <= 0)
            {
                return false;
            }
            else
            {
                long waitTime = msecs;
                long start = System.currentTimeMillis();
                try
                {
                    for (;;)
                    {
                        wait(waitTime);
                        if (caller == m_owner)
                        {
                            ++m_holds;
                            return true;
                        }
                        else if (m_owner == null)
                        {
                            m_owner = caller;
                            m_holds = 1;
                            return true;
                        }
                        else
                        {
                            waitTime = msecs - (System.currentTimeMillis() - start);
                            if (waitTime <= 0)
                            {
                                return false;
                            }
                        }
                    }
                }
                catch (InterruptedException ex)
                {
                    notify();
                    throw ex;
                }
            }
        }
    }//attempt

    /**
     * Release the lock.
     *
     * @throws Error thrown if not current owner of lock
     */
    public synchronized void release()
    {
        //log.debug("release()::" + Thread.currentThread().toString());
        if (Thread.currentThread() != m_owner)
        {
            throw new Error("Illegal Lock usage");
        }

        if (--m_holds == 0)
        {
            m_owner = null;
            notify();
        }
    }//release

    /**
     * Release the lock N times. <code>release(n)</code> is
     * equivalent in effect to:
     * <pre>
     *   for (int i = 0; i < n; ++i) release();
     * </pre>
     * <p/>
     *
     * @param n times the lock should be released.
     * @throws Error thrown if not current owner of lock
     *               or has fewer than N holds on the lock
     */
    public synchronized void release(long n)
    {
        if (Thread.currentThread() != m_owner || n > m_holds)
        {
            throw new Error("Illegal Lock usage");
        }

        m_holds -= n;
        if (m_holds == 0)
        {
            m_owner = null;
            notify();
        }
    }//release

    /**
     * Return the number of unreleased acquires performed
     * by the current thread.
     * Returns zero if current thread does not hold lock.
     *
     * @return the number of unreleased acquires performed by the owner thread.
     */
    public synchronized long holds()
    {
        if (Thread.currentThread() != m_owner)
        {
            return 0;
        }
        return m_holds;
    }//holds
}//class ReentrantLock