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Java Source Code / Java Documentation » Apache Harmony Java SE » java package » java.util.concurrent.locks 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


java.lang.Object
   java.util.concurrent.locks.ReentrantLock

ReentrantLock
public class ReentrantLock implements Lock,java.io.Serializable(Code)
A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

A ReentrantLock is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking lock 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 ReentrantLock.isHeldByCurrentThread , and ReentrantLock.getHoldCount .

The constructor for this class accepts an optional fairness parameter. When set true, 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.

It is recommended practice to always immediately follow a call to lock with a try block, most typically in a before/after construction such as:

 class X {
 private final ReentrantLock lock = new ReentrantLock();
 // ...
 public void m() { 
 lock.lock();  // block until condition holds
 try {
 // ... method body
 } finally {
 lock.unlock()
 }
 }
 }
 

In addition to implementing the Lock interface, this class defines methods isLocked and getLockQueueLength, as well as some associated protected access methods that may be useful for instrumentation and monitoring.

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.

This lock supports a maximum of 2147483648 recursive locks by the same thread.
since:
   1.5
author:
   Doug Lea


Inner Class :abstract static class Sync extends AbstractQueuedSynchronizer
Inner Class :final static class NonfairSync extends Sync
Inner Class :final static class FairSync extends Sync


Constructor Summary
public  ReentrantLock()
     Creates an instance of ReentrantLock.
public  ReentrantLock(boolean fair)
     Creates an instance of ReentrantLock with the given fairness policy.

Method Summary
public  intgetHoldCount()
     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.

protected  ThreadgetOwner()
     Returns the thread that currently owns the exclusive lock, or null if not owned.
final public  intgetQueueLength()
     Returns an estimate of the number of threads waiting to acquire.
protected  Collection<Thread>getQueuedThreads()
     Returns a collection containing threads that may be waiting to acquire.
public  intgetWaitQueueLength(Condition condition)
     Returns an estimate of the number of threads waiting on the given condition associated with this lock.
protected  Collection<Thread>getWaitingThreads(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.
final public  booleanhasQueuedThread(Thread thread)
     Queries whether the given thread is waiting to acquire this lock.
final public  booleanhasQueuedThreads()
     Queries whether any threads are waiting to acquire.
public  booleanhasWaiters(Condition condition)
     Queries whether any threads are waiting on the given condition associated with this lock.
final public  booleanisFair()
     Returns true if this lock has fairness set true.
public  booleanisHeldByCurrentThread()
     Queries if this lock is held by the current thread.

Analogous to the Thread.holdsLock method for built-in monitor locks, this method is typically used for debugging and testing.

public  booleanisLocked()
     Queries if this lock is held by any thread.
public  voidlock()
     Acquires the lock.
public  voidlockInterruptibly()
     Acquires the lock unless the current thread is 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 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 Thread.interrupt interrupted while acquiring the lock,
then InterruptedException is thrown and the current thread's interrupted status is cleared.
public  ConditionnewCondition()
     Returns a Condition instance for use with this Lock instance.
public  StringtoString()
     Returns a string identifying this lock, as well as its lock state.
public  booleantryLock()
     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.

public  booleantryLock(long timeout, 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 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.

public  voidunlock()
     Attempts to release this lock.


Constructor Detail
ReentrantLock
public ReentrantLock()(Code)
Creates an instance of ReentrantLock. This is equivalent to using ReentrantLock(false).



ReentrantLock
public ReentrantLock(boolean fair)(Code)
Creates an instance of ReentrantLock with the given fairness policy.
Parameters:
  fair - true if this lock will be fair; else false




Method Detail
getHoldCount
public int getHoldCount()(Code)
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();
 }
 }
 }
 
the number of holds on this lock by the current thread,or zero if this lock is not held by the current thread.



getOwner
protected Thread getOwner()(Code)
Returns the thread that currently owns the exclusive 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. the owner, or null if not owned.



getQueueLength
final public int getQueueLength()(Code)
Returns an estimate of the number of threads waiting to acquire. 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. the estimated number of threads waiting for this lock



getQueuedThreads
protected Collection<Thread> getQueuedThreads()(Code)
Returns a collection containing threads that may be waiting to acquire. 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. the collection of threads



getWaitQueueLength
public int getWaitQueueLength(Condition condition)(Code)
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.
Parameters:
  condition - the condition the estimated number of waiting threads.
throws:
  IllegalMonitorStateException - if this lock is not held
throws:
  IllegalArgumentException - if the given condition isnot associated with this lock
throws:
  NullPointerException - if condition null



getWaitingThreads
protected Collection<Thread> getWaitingThreads(Condition condition)(Code)
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.
Parameters:
  condition - the condition the collection of threads
throws:
  IllegalMonitorStateException - if this lock is not held
throws:
  IllegalArgumentException - if the given condition isnot associated with this lock
throws:
  NullPointerException - if condition null



hasQueuedThread
final public boolean hasQueuedThread(Thread thread)(Code)
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 method is designed primarily for use in monitoring of the system state.
Parameters:
  thread - the thread true if the given thread is queued waiting for this lock.
throws:
  NullPointerException - if thread is null



hasQueuedThreads
final public boolean hasQueuedThreads()(Code)
Queries whether any threads are waiting to acquire. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire. This method is designed primarily for use in monitoring of the system state. true if there may be other threads waiting to acquirethe lock.



hasWaiters
public boolean hasWaiters(Condition condition)(Code)
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.
Parameters:
  condition - the condition true if there are any waiting threads.
throws:
  IllegalMonitorStateException - if this lock is not held
throws:
  IllegalArgumentException - if the given condition isnot associated with this lock
throws:
  NullPointerException - if condition null



isFair
final public boolean isFair()(Code)
Returns true if this lock has fairness set true. true if this lock has fairness set true.



isHeldByCurrentThread
public boolean isHeldByCurrentThread()(Code)
Queries if this lock is held by the current thread.

Analogous to the 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();
 }
 }
 }
 
true if current thread holds this lock and false otherwise.



isLocked
public boolean isLocked()(Code)
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. true if any thread holds this lock and false otherwise.



lock
public void lock()(Code)
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.




lockInterruptibly
public void lockInterruptibly() throws InterruptedException(Code)
Acquires the lock unless the current thread is 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 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 Thread.interrupt interrupted while acquiring the lock,
then 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




newCondition
public Condition newCondition()(Code)
Returns a Condition instance for use with this Lock instance.

The returned Condition instance supports the same usages as do the Object monitor methods ( Object.wait wait , Object.notify notify , and Object.notifyAll notifyAll ) when used with the built-in monitor lock.

  • If this lock is not held when any of the ConditionCondition.await waiting or Condition.signalsignalling methods are called, then an IllegalMonitorStateException is thrown.
  • When the condition 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 Thread.interrupt interrupted while waiting then the wait will terminate, an 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.
the Condition object



toString
public String toString()(Code)
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 Thread.getName of the owning thread. a string identifying this lock, as well as its lock state.



tryLock
public boolean tryLock()(Code)
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 ReentrantLock.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. true if the lock was free and was acquired by thecurrent thread, or the lock was already held by the current thread; andfalse otherwise.




tryLock
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException(Code)
Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been 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 ReentrantLock.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 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 Thread.interrupt interrupted while acquiring the lock,
then 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.
Parameters:
  timeout - the time to wait for the lock
Parameters:
  unit - the time unit of the timeout argument true if the lock was free and was acquired by thecurrent thread, or the lock was already held by the current thread; andfalse 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




unlock
public void unlock()(Code)
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 IllegalMonitorStateException is thrown.
throws:
  IllegalMonitorStateException - if the current thread does nothold this lock.




Methods inherited from java.lang.Object
protected Object clone() throws CloneNotSupportedException(Code)(Java Doc)
public boolean equals(Object object)(Code)(Java Doc)
protected void finalize() throws Throwable(Code)(Java Doc)
final public Class<? extends Object> getClass()(Code)(Java Doc)
public int hashCode()(Code)(Java Doc)
final public void notify()(Code)(Java Doc)
final public void notifyAll()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
final public void wait(long millis, int nanos) throws InterruptedException(Code)(Java Doc)
final public void wait(long millis) throws InterruptedException(Code)(Java Doc)
final public void wait() throws InterruptedException(Code)(Java Doc)

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