reentrantlock.java

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/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package edu.emory.mathcs.backport.java.util.concurrent.locks;

import java.util.*;
import edu.emory.mathcs.backport.java.util.concurrent.*;
import edu.emory.mathcs.backport.java.util.concurrent.helpers.*;

/**
 * A reentrant mutual exclusion {@link Lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * <tt>synchronized</tt> methods and statements, but with extended
 * capabilities.
 *
 * <p> A <tt>ReentrantLock</tt> is <em>owned</em> by the thread last
 * successfully locking, but not yet unlocking it. A thread invoking
 * <tt>lock</tt> will return, successfully acquiring the lock, when
 * the lock is not owned by another thread. The method will return
 * immediately if the current thread already owns the lock. This can
 * be checked using methods {@link #isHeldByCurrentThread}, and {@link
 * #getHoldCount}.
 *
 * <p> The constructor for this class accepts an optional
 * <em>fairness</em> parameter.  When set <tt>true</tt>, under
 * contention, locks favor granting access to the longest-waiting
 * thread.  Otherwise this lock does not guarantee any particular
 * access order.  Programs using fair locks accessed by many threads
 * may display lower overall throughput (i.e., are slower; often much
 * slower) than those using the default setting, but have smaller
 * variances in times to obtain locks and guarantee lack of
 * starvation. Note however, that fairness of locks does not guarantee
 * fairness of thread scheduling. Thus, one of many threads using a
 * fair lock may obtain it multiple times in succession while other
 * active threads are not progressing and not currently holding the
 * lock.
 * Also note that the untimed {@link #tryLock() tryLock} method does not
 * honor the fairness setting. It will succeed if the lock
 * is available even if other threads are waiting.
 *
 * <p> It is recommended practice to <em>always</em> immediately
 * follow a call to <tt>lock</tt> with a <tt>try</tt> block, most
 * typically in a before/after construction such as:
 *
 * <pre>
 * class X {
 *   private final ReentrantLock lock = new ReentrantLock();
 *   // ...
 *
 *   public void m() {
 *     lock.lock();  // block until condition holds
 *     try {
 *       // ... method body
 *     } finally {
 *       lock.unlock()
 *     }
 *   }
 * }
 * </pre>
 *
 * <p>In addition to implementing the {@link Lock} interface, this
 * class defines methods <tt>isLocked</tt> and
 * <tt>getLockQueueLength</tt>, as well as some associated
 * <tt>protected</tt> access methods that may be useful for
 * instrumentation and monitoring.
 *
 * <p> Serialization of this class behaves in the same way as built-in
 * locks: a deserialized lock is in the unlocked state, regardless of
 * its state when serialized.
 *
 * <p> This lock supports a maximum of 2147483648 recursive locks by
 * the same thread.
 *
 * @since 1.5
 * @author Doug Lea
 * @author Dawid Kurzyniec
 */
public class ReentrantLock implements Lock, java.io.Serializable,
                                      CondVar.LockInfo {
    private static final long serialVersionUID = 7373984872572414699L;

    private final Impl impl;

    static abstract class Impl {
        protected Thread owner_ = null;
        protected int holds_ = 0;

        protected Impl() {}

        public abstract void lockInterruptibly() throws InterruptedException;

        public boolean tryLock() {
            Thread caller = Thread.currentThread();
            synchronized (this) {
                if (owner_ == null) {
                    owner_ = caller;
                    holds_ = 1;
                    return true;
                }
                else if (caller == owner_) {
                    ++holds_;
                    return true;
                }
            }
            return false;
        }

        public abstract boolean tryLock(long nanos) throws InterruptedException;

        public abstract void unlock();

        public synchronized int getHoldCount() {
            return isHeldByCurrentThread() ? holds_ : 0;
        }

        public synchronized boolean isHeldByCurrentThread() {
            return Thread.currentThread() == owner_;
        }

        public synchronized boolean isLocked() {
            return owner_ != null;
        }

        public abstract boolean isFair();

        protected synchronized Thread getOwner() {
            return owner_;
        }

        public boolean hasQueuedThreads() {
            throw new UnsupportedOperationException("Use FAIR version");
        }

        public int getQueueLength() {
            throw new UnsupportedOperationException("Use FAIR version");
        }

        public Collection getQueuedThreads() {
            throw new UnsupportedOperationException("Use FAIR version");
        }

        public boolean isQueued(Thread thread) {
            throw new UnsupportedOperationException("Use FAIR version");
        }
    }

    final static class NonfairImpl extends Impl implements java.io.Serializable {

        NonfairImpl() {}

        public void lockInterruptibly() throws InterruptedException {
            if (Thread.interrupted()) throw new InterruptedException();
            Thread caller = Thread.currentThread();
            synchronized (this) {
                if (owner_ == null) {
                    owner_ = caller;
                    holds_ = 1;
                    return;
                }
                else if (caller == owner_) {
                    ++holds_;
                    return;
                }
                else {
                    try {
                        do { wait(); } while (owner_ != null);
                        owner_ = caller;
                        holds_ = 1;
                        return;
                    }
                    catch (InterruptedException ex) {
                        notify();
                        throw ex;
                    }
                }
            }
        }

        public boolean tryLock(long nanos) throws InterruptedException {
            if (Thread.interrupted()) throw new InterruptedException();
            Thread caller = Thread.currentThread();

            synchronized (this) {
                if (owner_ == null) {
                    owner_ = caller;
                    holds_ = 1;
                    return true;
                }
                else if (caller == owner_) {
                    ++holds_;
                    return true;
                }
                else if (nanos <= 0)
                    return false;
                else {
                    long deadline = Utils.nanoTime() + nanos;
                    try {
                        for (; ; ) {
                            TimeUnit.NANOSECONDS.timedWait(this, nanos);
                            if (caller == owner_) {
                                ++holds_;
                                return true;
                            }
                            else if (owner_ == null) {
                                owner_ = caller;
                                holds_ = 1;
                                return true;
                            }
                            else {
                                nanos = deadline - Utils.nanoTime();
                                if (nanos <= 0)
                                    return false;
                            }
                        }
                    }
                    catch (InterruptedException ex) {
                        notify();
                        throw ex;
                    }
                }
            }
        }

        public synchronized void unlock() {
            if (Thread.currentThread() != owner_)
                throw new IllegalMonitorStateException("Not owner");

            if (--holds_ == 0) {
                owner_ = null;
                notify();
            }
        }

        public final boolean isFair() {
            return false;
        }
    }

    final static class FairImpl extends Impl implements WaitQueue.QueuedSync,
                                                        java.io.Serializable  {

        private final WaitQueue wq_ = new FIFOWaitQueue();

        FairImpl() {}

        public synchronized boolean recheck(WaitQueue.WaitNode node) {
            Thread caller = Thread.currentThread();
            if (owner_ == null) {
                owner_ = caller;
                holds_ = 1;
                return true;
            }
            else if (caller == owner_) {
                ++holds_;
                return true;
            }
            wq_.insert(node);
            return false;
        }

        public synchronized void takeOver(WaitQueue.WaitNode node) {
            // assert (holds_ == 1 && owner_ == Thread.currentThread()
            owner_ = node.getOwner();
        }

        public void lockInterruptibly() throws InterruptedException {
            if (Thread.interrupted()) throw new InterruptedException();
            Thread caller = Thread.currentThread();
            synchronized (this) {
                if (owner_ == null) {
                    owner_ = caller;
                    holds_ = 1;
                    return;
                }
                else if (caller == owner_) {
                    ++holds_;
                    return;
                }
            }
            WaitQueue.WaitNode n = new WaitQueue.WaitNode();
            n.doWait(this);
        }

        public boolean tryLock(long nanos) throws InterruptedException {
            if (Thread.interrupted()) throw new InterruptedException();
            Thread caller = Thread.currentThread();
            synchronized (this) {
                if (owner_ == null) {
                    owner_ = caller;