reentrantlock.java

JAVA的一些源码 JAVA2 STANDARD EDITION DEVELOPMENT KIT 5.0

     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     *
     * <li>The specified waiting time elapses
     *
     * </ul>
     *
     * <p>If the lock is acquired then the value <tt>true</tt> is returned and
     * 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>If the specified waiting time elapses then the value <tt>false</tt>
     * is returned.
     * If the time is 
     * less than or equal to zero, the method will not wait at all.
     *
     * <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, and over reporting the elapse of the waiting
     * time.
     *
     * @param timeout the time to wait for the lock
     * @param unit the time unit of the timeout argument
     *
     * @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> if the waiting time elapsed before the lock could be 
     * acquired.
     *
     * @throws InterruptedException if the current thread is interrupted
     * @throws NullPointerException if unit is null
     *
     */
    public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

    /**
     * Attempts to release this lock.  
     *
     * <p>If the current thread is the
     * holder of this lock then the hold count is decremented. If the
     * hold count is now zero then the lock is released.  If the
     * current thread is not the holder of this lock then {@link
     * IllegalMonitorStateException} is thrown.
     * @throws IllegalMonitorStateException if the current thread does not
     * hold this lock.
     */
    public void unlock() {
        sync.release(1);
    }

    /**
     * Returns a {@link Condition} instance for use with this 
     * {@link Lock} instance.
     *
     * <p>The returned {@link Condition} instance supports the same
     * usages as do the {@link Object} monitor methods ({@link
     * Object#wait() wait}, {@link Object#notify notify}, and {@link
     * Object#notifyAll notifyAll}) when used with the built-in
     * monitor lock.
     *
     * <ul>
     *
     * <li>If this lock is not held when any of the {@link Condition}
     * {@link Condition#await() waiting} or {@link Condition#signal
     * signalling} methods are called, then an {@link
     * IllegalMonitorStateException} is thrown.
     *
     * <li>When the condition {@link Condition#await() waiting}
     * methods are called the lock is released and, before they
     * return, the lock is reacquired and the lock hold count restored
     * to what it was when the method was called.
     *
     * <li>If a thread is {@link Thread#interrupt interrupted} while
     * waiting then the wait will terminate, an {@link
     * InterruptedException} will be thrown, and the thread's
     * interrupted status will be cleared.
     *
     * <li> Waiting threads are signalled in FIFO order
     *
     * <li>The ordering of lock reacquisition for threads returning
     * from waiting methods is the same as for threads initially
     * acquiring the lock, which is in the default case not specified,
     * but for <em>fair</em> locks favors those threads that have been
     * waiting the longest.
     * 
     * </ul>
     *
     * @return the Condition object
     */
    public Condition newCondition() {
        return sync.newCondition();
    }

    /**
     * Queries the number of holds on this lock by the current thread.
     *
     * <p>A thread has a hold on a lock for each lock action that is not 
     * matched by an unlock action.
     *
     * <p>The hold count information is typically only used for testing and
     * debugging purposes. For example, if a certain section of code should
     * not be entered with the lock already held then we can assert that
     * fact:
     *
     * <pre>
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...     
     *   public void m() { 
     *     assert lock.getHoldCount() == 0;
     *     lock.lock();
     *     try {
     *       // ... method body
     *     } finally {
     *       lock.unlock();
     *     }
     *   }
     * }
     * </pre>
     *
     * @return the number of holds on this lock by the current thread,
     * or zero if this lock is not held by the current thread.
     */
    public int getHoldCount() {
        return sync.getHoldCount();
    }

    /**
     * Queries if this lock is held by the current thread.
     *
     * <p>Analogous to the {@link Thread#holdsLock} method for built-in
     * monitor locks, this method is typically used for debugging and
     * testing. For example, a method that should only be called while
     * a lock is held can assert that this is the case:
     *
     * <pre>
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() { 
     *       assert lock.isHeldByCurrentThread();
     *       // ... method body
     *   }
     * }
     * </pre>
     *
     * <p>It can also be used to ensure that a reentrant lock is used
     * in a non-reentrant manner, for example:
     *
     * <pre>
     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() { 
     *       assert !lock.isHeldByCurrentThread();
     *       lock.lock();
     *       try {
     *           // ... method body
     *       } finally {
     *           lock.unlock();
     *       }
     *   }
     * }
     * </pre>
     * @return <tt>true</tt> if current thread holds this lock and 
     * <tt>false</tt> otherwise.
     */
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    /**
     * Queries if this lock is held by any thread. This method is
     * designed for use in monitoring of the system state, 
     * not for synchronization control.
     * @return <tt>true</tt> if any thread holds this lock and 
     * <tt>false</tt> otherwise.
     */
    public boolean isLocked() {
        return sync.isLocked();
    }

    /**
     * Returns true if this lock has fairness set true.
     * @return true if this lock has fairness set true.
     */
    public final boolean isFair() {
        return sync instanceof FairSync;
    }

    /**
     * Returns the thread that currently owns this lock, or
     * <tt>null</tt> if not owned. Note that the owner may be
     * momentarily <tt>null</tt> even if there are threads trying to
     * acquire the lock but have not yet done so.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive lock monitoring facilities.
     * @return the owner, or <tt>null</tt> if not owned.
     */
    protected Thread getOwner() {
        return sync.getOwner();
    }

    /**
     * Queries whether any threads are waiting to acquire this lock. Note that
     * because cancellations may occur at any time, a <tt>true</tt>
     * return does not guarantee that any other thread will ever
     * acquire this lock.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @return true if there may be other threads waiting to acquire
     * the lock.
     */
    public final boolean hasQueuedThreads() { 
        return sync.hasQueuedThreads();
    }


    /**
     * Queries whether the given thread is waiting to acquire this
     * lock. Note that because cancellations may occur at any time, a
     * <tt>true</tt> return does not guarantee that this thread
     * will ever acquire this lock.  This method is designed primarily for use
     * in monitoring of the system state.
     *
     * @param thread the thread
     * @return true if the given thread is queued waiting for this lock.
     * @throws NullPointerException if thread is null
     */
    public final boolean hasQueuedThread(Thread thread) { 
        return sync.isQueued(thread);
    }


    /**
     * Returns an estimate of the number of threads waiting to
     * acquire this lock.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring of the system state, not for synchronization
     * control.
     * @return the estimated number of threads waiting for this lock
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire this lock.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     * @return the collection of threads
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this lock. Note that because timeouts and
     * interrupts may occur at any time, a <tt>true</tt> return does
     * not guarantee that a future <tt>signal</tt> will awaken any
     * threads.  This method is designed primarily for use in
     * monitoring of the system state.
     * @param condition the condition
     * @return <tt>true</tt> if there are any waiting threads.
     * @throws IllegalMonitorStateException if this lock 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this lock
     * @throws NullPointerException if condition null
     */ 
    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this lock. Note that because
     * timeouts and interrupts may occur at any time, the estimate
     * serves only as an upper bound on the actual number of waiters.
     * This method is designed for use in monitoring of the system
     * state, not for synchronization control.
     * @param condition the condition
     * @return the estimated number of waiting threads.
     * @throws IllegalMonitorStateException if this lock 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this lock
     * @throws NullPointerException if condition null
     */ 
    public int getWaitQueueLength(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this lock.
     * Because the actual set of threads may change dynamically while
     * constructing this result, the returned collection is only a
     * best-effort estimate. The elements of the returned collection
     * are in no particular order.  This method is designed to
     * facilitate construction of subclasses that provide more
     * extensive condition monitoring facilities.
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if this lock 
     * is not held
     * @throws IllegalArgumentException if the given condition is
     * not associated with this lock
     * @throws NullPointerException if condition null
     */
    protected Collection<Thread> getWaitingThreads(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns a string identifying this lock, as well as its lock
     * state.  The state, in brackets, includes either the String
     * &quot;Unlocked&quot; or the String &quot;Locked by&quot;
     * followed by the {@link Thread#getName} of the owning thread.
     * @return a string identifying this lock, as well as its lock state.
     */
    public String toString() {
        Thread owner = sync.getOwner();
        return super.toString() + ((owner == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + owner.getName() + "]");
    }
}