Build 1.0_r1(from source)

java.util.concurrent.locks
Class ReentrantLock

java.lang.Object
  extended by java.util.concurrent.locks.ReentrantLock
All Implemented Interfaces:
Serializable, Lock
Direct Known Subclasses:
ConcurrentHashMap.Segment

public class ReentrantLock
extends Object
implements Lock, Serializable

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 isHeldByCurrentThread(), and 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
See Also:
Serialized Form

Nested Class Summary
(package private) static class ReentrantLock.FairSync
          Sync object for fair locks
(package private) static class ReentrantLock.NonfairSync
          Sync object for non-fair locks
(package private) static class ReentrantLock.Sync
          Base of synchronization control for this lock.
 
Constructor Summary
ReentrantLock()
          Creates an instance of ReentrantLock.
ReentrantLock(boolean fair)
          Creates an instance of ReentrantLock with the given fairness policy.
 
Method Summary
 int getHoldCount()
          Queries the number of holds on this lock by the current thread.
protected  Thread getOwner()
          Returns the thread that currently owns the exclusive lock, or null if not owned.
protected  Collection<Thread> getQueuedThreads()
          Returns a collection containing threads that may be waiting to acquire.
 int getQueueLength()
          Returns an estimate of the number of threads waiting to acquire.
protected  Collection<Thread> getWaitingThreads(Condition condition)
          Returns a collection containing those threads that may be waiting on the given condition associated with this lock.
 int getWaitQueueLength(Condition condition)
          Returns an estimate of the number of threads waiting on the given condition associated with this lock.
 boolean hasQueuedThread(Thread thread)
          Queries whether the given thread is waiting to acquire this lock.
 boolean hasQueuedThreads()
          Queries whether any threads are waiting to acquire.
 boolean hasWaiters(Condition condition)
          Queries whether any threads are waiting on the given condition associated with this lock.
 boolean isFair()
          Returns true if this lock has fairness set true.
 boolean isHeldByCurrentThread()
          Queries if this lock is held by the current thread.
 boolean isLocked()
          Queries if this lock is held by any thread.
 void lock()
          Acquires the lock.
 void lockInterruptibly()
          Acquires the lock unless the current thread is interrupted.
 Condition newCondition()
          Returns a Condition instance for use with this Lock instance.
 String toString()
          Returns a string identifying this lock, as well as its lock state.
 boolean tryLock()
          Acquires the lock only if it is not held by another thread at the time of invocation.
 boolean tryLock(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 interrupted.
 void unlock()
          Attempts to release this lock.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

ReentrantLock

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


ReentrantLock

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

Parameters:
fair - true if this lock will be fair; else false
Method Detail

lock

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.

Specified by:
lock in interface Lock

lockInterruptibly

public void lockInterruptibly()
                       throws InterruptedException
Acquires the lock unless the current thread is 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:

If the lock is acquired by the current thread then the lock hold count is set to one.

If the current thread:

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.

Specified by:
lockInterruptibly in interface Lock
Throws:
InterruptedException - if the current thread is interrupted
See Also:
Thread.interrupt()

tryLock

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 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.

Specified by:
tryLock in interface Lock
Returns:
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.

tryLock

public boolean tryLock(long timeout,
                       TimeUnit unit)
                throws InterruptedException
Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been 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 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:

If the lock is acquired then the value true is returned and the lock hold count is set to one.

If the current thread:

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.

Specified by:
tryLock in interface Lock
Parameters:
timeout - the time to wait for the lock
unit - the time unit of the timeout argument
Returns:
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
NullPointerException - if unit is null
See Also:
Thread.interrupt()

unlock

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 IllegalMonitorStateException is thrown.

Specified by:
unlock in interface Lock
Throws:
IllegalMonitorStateException - if the current thread does not hold this lock.

newCondition

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

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

Specified by:
newCondition in interface Lock
Returns:
the Condition object

getHoldCount

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();
     }
   }
 }
 

Returns:
the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread.

isHeldByCurrentThread

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

Analogous to the Thread.holdsLock(java.lang.Object) 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();
       }
   }
 }
 

Returns:
true if current thread holds this lock and false otherwise.

isLocked

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.

Returns:
true if any thread holds this lock and false otherwise.

isFair

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

Returns:
true if this lock has fairness set true.

getOwner

protected Thread getOwner()
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.

Returns:
the owner, or null if not owned.

hasQueuedThreads

public final boolean hasQueuedThreads()
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.

Returns:
true if there may be other threads waiting to acquire the lock.

hasQueuedThread

public final boolean hasQueuedThread(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 method is designed primarily for use in monitoring of the system state.

Parameters:
thread - the thread
Returns:
true if the given thread is queued waiting for this lock.
Throws:
NullPointerException - if thread is null

getQueueLength

public final int getQueueLength()
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.

Returns:
the estimated number of threads waiting for this lock

getQueuedThreads

protected Collection<Thread> getQueuedThreads()
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.

Returns:
the collection of threads

hasWaiters

public boolean hasWaiters(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.

Parameters:
condition - the condition
Returns:
true if there are any waiting threads.
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if condition null

getWaitQueueLength

public int getWaitQueueLength(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.

Parameters:
condition - the condition
Returns:
the estimated number of waiting threads.
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if condition null

getWaitingThreads

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. 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
Returns:
the collection of threads
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if condition null

toString

public 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 Thread.getName() of the owning thread.

Overrides:
toString in class Object
Returns:
a string identifying this lock, as well as its lock state.

Build 1.0_r1(from source)

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