reentrantlock.java

JAVA的一些源码 JAVA2 STANDARD EDITION DEVELOPMENT KIT 5.0

/*
 * @(#)ReentrantLock.java	1.7 04/07/13
 *
 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util.concurrent.locks;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

/**
 * 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
 * 
 */
public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    static abstract class Sync  extends AbstractQueuedSynchronizer {
        /** Current owner thread */
        transient Thread owner;

        /**
         * Perform {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();

        /** 
         * Perform non-fair tryLock.  tryAcquire is
         * implemented in subclasses, but both need nonfair
         * try for trylock method
         */
        final boolean nonfairTryAcquire(int acquires) { 
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    owner = current;
                    return true;
                }
            }
            else if (current == owner) {
                setState(c+acquires);
                return true;
            }
            return false;
        }

        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != owner)
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                owner = null;
            }
            setState(c);
            return free;
        }

        protected final boolean isHeldExclusively() {
            return getState() != 0 && owner == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            int c = getState();
            Thread o = owner;
            return (c == 0)? null : o;
        }
        
        final int getHoldCount() {
            int c = getState();
            Thread o = owner;
            return (o == Thread.currentThread())? c : 0;
        }
        
        final boolean isLocked() {
            return getState() != 0;
        }

        /**
         * Reconstitute this lock instance from a stream
         * @param s the stream
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    /**
     * Sync object for non-fair locks
     */
    final static class NonfairSync extends Sync {
        /**
         * Perform lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                owner = Thread.currentThread();
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) { 
            return nonfairTryAcquire(acquires);
        }
    }

    /**
     * Sync object for fair locks
     */
    final static class FairSync  extends Sync {
        final void lock() { 
            acquire(1); 
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) { 
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                Thread first = getFirstQueuedThread();
                if ((first == null || first == current) && 
                    compareAndSetState(0, acquires)) {
                    owner = current;
                    return true;
                }
            }
            else if (current == owner) {
                setState(c+acquires);
                return true;
            }
            return false;
        }
    }

    /**
     * Creates an instance of <tt>ReentrantLock</tt>.
     * This is equivalent to using <tt>ReentrantLock(false)</tt>.
     */
    public ReentrantLock() { 
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of <tt>ReentrantLock</tt> with the
     * given fairness policy.
     * @param fair true if this lock will be fair; else false
     */
    public ReentrantLock(boolean fair) { 
        sync = (fair)? new FairSync() : new NonfairSync();
    }

    /**
     * Acquires the lock. 
     *
     * <p>Acquires the lock if it is not held by another thread and returns 
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread
     * already holds the lock then the hold count is incremented by one and
     * the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one. 
     */
    public void lock() {
        sync.lock();
    }

    /**
     * Acquires the lock unless the current thread is 
     * {@link Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns 
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds this lock then the hold count 
     * is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until one of two things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread.
     *
     * </ul>
     *
     * <p>If the lock is acquired by the current thread then the lock hold 
     * count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or 
     *
     * <li>is {@link Thread#interrupt interrupted} while acquiring 
     * the lock,
     *
     * </ul>
     *
     * then {@link InterruptedException} is thrown and the current thread's 
     * interrupted status is cleared. 
     *
     * <p>In this implementation, as this method is an explicit interruption 
     * point, preference is 
     * given to responding to the interrupt over normal or reentrant 
     * acquisition of the lock.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void lockInterruptibly() throws InterruptedException { 
        sync.acquireInterruptibly(1);
    }

    /**
     * Acquires the lock only if it is not held by another thread at the time
     * of invocation.
     *
     * <p>Acquires the lock if it is not held by another thread and
     * returns immediately with the value <tt>true</tt>, setting the
     * lock hold count to one. Even when this lock has been set to use a
     * fair ordering policy, a call to <tt>tryLock()</tt> <em>will</em>
     * immediately acquire the lock if it is available, whether or not
     * other threads are currently waiting for the lock. 
     * This &quot;barging&quot; behavior can be useful in certain 
     * circumstances, even though it breaks fairness. If you want to honor
     * the fairness setting for this lock, then use 
     * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
     * which is almost equivalent (it also detects interruption).
     *
     * <p> If the current thread
     * already holds this lock then the hold count is incremented by one and
     * the method returns <tt>true</tt>.
     *
     * <p>If the lock is held by another thread then this method will return 
     * immediately with the value <tt>false</tt>.  
     *
     * @return <tt>true</tt> if the lock was free and was acquired by the
     * current thread, or the lock was already held by the current thread; and
     * <tt>false</tt> otherwise.
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

    /**
     * Acquires the lock if it is not held by another thread within the given 
     * waiting time and the current thread has not been 
     * {@link Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns 
     * immediately with the value <tt>true</tt>, setting the lock hold count 
     * to one. If this lock has been set to use a fair ordering policy then
     * an available lock <em>will not</em> be acquired if any other threads
     * are waiting for the lock. This is in contrast to the {@link #tryLock()}
     * method. If you want a timed <tt>tryLock</tt> that does permit barging on
     * a fair lock then combine the timed and un-timed forms together:
     *
     * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
     * </pre>
     *
     * <p>If the current thread
     * already holds this lock then the hold count is incremented by one and
     * the method returns <tt>true</tt>.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling 
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     *