reentrantreadwritelock.java

JAVA的一些源码 JAVA2 STANDARD EDITION DEVELOPMENT KIT 5.0

         * <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>
         *
         * <li>The write 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 write lock is acquired then the value <tt>true</tt> is
         * returned and the write 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 write 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 write 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 write 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 write lock is not held when any {@link
         * Condition} method is called then an {@link
         * IllegalMonitorStateException} is thrown.  (Read locks are
         * held independently of write locks, so are not checked or
         * affected. However it is essentially always an error to
         * invoke a condition waiting method when the current thread
         * has also acquired read locks, since other threads that
         * could unblock it will not be able to acquire the write
         * lock.)
         *
         * <li>When the condition {@link Condition#await() waiting}
         * methods are called the write lock is released and, before
         * they return, the write 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();
        }

        /**
         * 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() + "]");
        }

    }


    // Instrumentation and status

    /**
     * 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 the write 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 the number of read locks held for this lock. This
     * method is designed for use in monitoring system state, not for
     * synchronization control.
     * @return the number of read locks held.
     */
    public int getReadLockCount() {
        return sync.getReadLockCount();
    }

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

    /**
     * Queries if the write lock is held by the current thread. 
     * @return <tt>true</tt> if the current thread holds the write lock and 
     * <tt>false</tt> otherwise.
     */
    public boolean isWriteLockedByCurrentThread() {
        return sync.isHeldExclusively();
    }

    /**
     * Queries the number of reentrant write holds on this lock by the
     * current thread.  A writer thread has a hold on a lock for
     * each lock action that is not matched by an unlock action.
     *
     * @return the number of holds on the write lock by the current thread,
     * or zero if the write lock is not held by the current thread.
     */
    public int getWriteHoldCount() {
        return sync.getWriteHoldCount();
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire the write 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 lock monitoring facilities.
     * @return the collection of threads
     */
    protected Collection<Thread> getQueuedWriterThreads() {
        return sync.getExclusiveQueuedThreads();
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire the read 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 lock monitoring facilities.
     * @return the collection of threads
     */
    protected Collection<Thread> getQueuedReaderThreads() {
        return sync.getSharedQueuedThreads();
    }

    /**
     * Queries whether any threads are waiting to acquire the read or
     * write 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 a 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 either
     * the read or write lock. Note that because cancellations may
     * occur at any time, a <tt>true</tt> return does not guarantee
     * that this thread will ever acquire a 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
     * either the read or write 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 either the read or write 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 the write 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 the write 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 the write 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 the String &quot;Write locks =&quot;
     * followed by the number of reentrantly held write locks, and the
     * String &quot;Read locks =&quot; followed by the number of held
     * read locks.
     * @return a string identifying this lock, as well as its lock state.
     */
    public String toString() {
        int c = sync.getCount();
        int w = exclusiveCount(c);
        int r = sharedCount(c);
        
        return super.toString() + 
            "[Write locks = " + w + ", Read locks = " + r + "]";
    }

}