File	Doc	Category	Size	Date	Package
ReentrantLock.java	API Doc	Java SE 5 API	26536	Fri Aug 26 14:57:26 BST 2005	java.util.concurrent.locks

ReentrantLock

• java.lang.Object

class X {
private final ReentrantLock lock = new ReentrantLock();
// ...

public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock()
}
}
}

Fields Summary
private static final long	serialVersionUID
private final Sync	sync Synchronizer providing all implementation mechanics

Constructors Summary
public ReentrantLock() Creates an instance of ReentrantLock. This is equivalent to using ReentrantLock(false). sync = new NonfairSync();
public ReentrantLock(boolean fair) Creates an instance of ReentrantLock with the given fairness policy. param fair true if this lock will be fair; else false sync = (fair)? new FairSync() : new NonfairSync();

Methods Summary
public int	getHoldCount() Queries the number of holds on this lock by the current thread. A thread has a hold on a lock for each lock action that is not matched by an unlock action. 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: class X { ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert lock.getHoldCount() == 0; lock.lock(); try { // ... method body } finally { lock.unlock(); } } } return the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread. return sync.getHoldCount();
protected java.lang.Thread	getOwner() Returns the thread that currently owns this lock, or null if not owned. Note that the owner may be momentarily null 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 null if not owned. return sync.getOwner();
public final int	getQueueLength() 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 return sync.getQueueLength();
protected java.util.Collection	getQueuedThreads() 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 return sync.getQueuedThreads();
public int	getWaitQueueLength(java.util.concurrent.locks.Condition 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 if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
protected java.util.Collection	getWaitingThreads(java.util.concurrent.locks.Condition 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 if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
public final boolean	hasQueuedThread(java.lang.Thread thread) Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, a true 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 return sync.isQueued(thread);
public final boolean	hasQueuedThreads() Queries whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, a true 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. return sync.hasQueuedThreads();
public boolean	hasWaiters(java.util.concurrent.locks.Condition condition) 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 true return does not guarantee that a future signal will awaken any threads. This method is designed primarily for use in monitoring of the system state. param condition the condition return true 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 if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
public final boolean	isFair() Returns true if this lock has fairness set true. return true if this lock has fairness set true. return sync instanceof FairSync;
public boolean	isHeldByCurrentThread() Queries if this lock is held by the current thread. 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: class X { ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert lock.isHeldByCurrentThread(); // ... method body } } It can also be used to ensure that a reentrant lock is used in a non-reentrant manner, for example: class X { ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert !lock.isHeldByCurrentThread(); lock.lock(); try { // ... method body } finally { lock.unlock(); } } } return true if current thread holds this lock and false otherwise. return sync.isHeldExclusively();
public boolean	isLocked() 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 true if any thread holds this lock and false otherwise. return sync.isLocked();
public void	lock() Acquires the lock. Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one. If the current thread already holds the lock then the hold count is incremented by one and the method returns immediately. 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. sync.lock();
public void	lockInterruptibly() Acquires the lock unless the current thread is {@link Thread#interrupt interrupted}. Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one. If the current thread already holds this lock then the hold count is incremented by one and the method returns immediately. 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: The lock is acquired by the current thread; or Some other thread {@link Thread#interrupt interrupts} the current thread. If the lock is acquired by the current thread then the lock hold count is set to one. If the current thread: has its interrupted status set on entry to this method; or is {@link Thread#interrupt interrupted} while acquiring the lock, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. 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 sync.acquireInterruptibly(1);
public java.util.concurrent.locks.Condition	newCondition() Returns a {@link Condition} instance for use with this {@link Lock} instance. 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. 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. 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. 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. Waiting threads are signalled in FIFO order 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 fair locks favors those threads that have been waiting the longest. return the Condition object return sync.newCondition();
public java.lang.String	toString() Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes either the String "Unlocked" or the String "Locked by" followed by the {@link Thread#getName} of the owning thread. return a string identifying this lock, as well as its lock state. Thread owner = sync.getOwner(); return super.toString() + ((owner == null) ? "[Unlocked]" : "[Locked by thread " + owner.getName() + "]");
public boolean	tryLock() Acquires the lock only if it is not held by another thread at the time of invocation. Acquires the lock if it is not held by another thread and returns immediately with the value true, setting the lock hold count to one. Even when this lock has been set to use a fair ordering policy, a call to tryLock() will immediately acquire the lock if it is available, whether or not other threads are currently waiting for the lock. This "barging" 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). If the current thread already holds this lock then the hold count is incremented by one and the method returns true. If the lock is held by another thread then this method will return immediately with the value false. return true if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and false otherwise. return sync.nonfairTryAcquire(1);
public boolean	tryLock(long timeout, java.util.concurrent.TimeUnit unit) 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}. Acquires the lock if it is not held by another thread and returns immediately with the value true, setting the lock hold count to one. If this lock has been set to use a fair ordering policy then an available lock will not 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 tryLock that does permit barging on a fair lock then combine the timed and un-timed forms together: if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } If the current thread already holds this lock then the hold count is incremented by one and the method returns true. 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: The lock is acquired by the current thread; or Some other thread {@link Thread#interrupt interrupts} the current thread; or The specified waiting time elapses If the lock is acquired then the value true is returned and the lock hold count is set to one. If the current thread: has its interrupted status set on entry to this method; or is {@link Thread#interrupt interrupted} while acquiring the lock, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all. 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 true if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and false 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 return sync.tryAcquireNanos(1, unit.toNanos(timeout));
public void	unlock() Attempts to release this lock. 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. sync.release(1);