jetty-util/src/main/java/org/eclipse/jetty/util/thread/Locker.java

//
//  ========================================================================
//  Copyright (c) 1995-2016 Mort Bay Consulting Pty. Ltd.
//  ------------------------------------------------------------------------
//  All rights reserved. This program and the accompanying materials
//  are made available under the terms of the Eclipse Public License v1.0
//  and Apache License v2.0 which accompanies this distribution.
//
//      The Eclipse Public License is available at
//      http://www.eclipse.org/legal/epl-v10.html
//
//      The Apache License v2.0 is available at
//      http://www.opensource.org/licenses/apache2.0.php
//
//  You may elect to redistribute this code under either of these licenses.
//  ========================================================================
//

package org.eclipse.jetty.util.thread;

import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.ReentrantLock;

/**
 * <p>This is a lock designed to protect VERY short sections of
 * critical code.  Threads attempting to take the lock will wait
 * until the lock is available, thus it is important that
 * the code protected by this lock is extremely simple and non
 * blocking.</p>
 * <pre>
 * try(SpinLock.Lock lock = locker.lock())
 * {
 *   // something very quick and non blocking
 * }
 * </pre>
 */
public class Locker
{
    private static final boolean SPIN = Boolean.getBoolean(Locker.class.getName() + ".spin");
    private static final Lock LOCKED = new Lock();

    private final boolean _spin;
    private final ReentrantLock _lock = new ReentrantLock();
    private final AtomicReference<Thread> _spinLockState = new AtomicReference<>(null);
    private final Lock _unlock = new UnLock();

    public Locker()
    {
        this(SPIN);
    }

    public Locker(boolean spin)
    {
        this._spin = spin;
    }

    public Lock lock()
    {
        if (_spin)
            spinLock();
        else
            concLock();
        return _unlock;
    }
    
    
    public Lock lockIfNotHeld ()
    {
        if (_spin)
            throw new UnsupportedOperationException();
        if (_lock.isHeldByCurrentThread())
            return LOCKED;
        _lock.lock();
        return _unlock;
    }

    private void spinLock()
    {
        Thread current = Thread.currentThread();
        while (true)
        {
            // Using test-and-test-and-set for better performance.
            Thread locker = _spinLockState.get();
            if (locker != null || !_spinLockState.compareAndSet(null, current))
            {
                if (locker == current)
                    throw new IllegalStateException("Locker is not reentrant");
                continue;
            }
            return;
        }
    }
    

    private void concLock()
    {
        if (_lock.isHeldByCurrentThread())
            throw new IllegalStateException("Locker is not reentrant");
        _lock.lock();
    }

    public boolean isLocked()
    {
        if (_spin)
            return _spinLockState.get() != null;
        else
            return _lock.isLocked();
    }
    public static class Lock implements AutoCloseable
    {
        @Override
        public void close()
        {
        }
    }
    
    
    public class UnLock extends Lock
    {
        @Override
        public void close()
        {
            if (_spin)
                _spinLockState.set(null);
            else
                _lock.unlock();
        }
    }
}