java.util.concurrent.locks
Class ReentrantLock

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

All Implemented Interfaces:

Lock, Serializable

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(). A ReentrantLock may be used in a non-reentrant way by checking that the lock is not already held by the current thread prior to locking.

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.

Implementation Notes

This implementation supports the interruption of lock acquisition and provides a Condition implementation that supports the interruption of thread suspension. It also favors interruption over normal method return.

Since:

1.5

See Also:

Serialized Form

Nested Class Summary
static class	ReentrantLock.ConditionObject           Condition implementation for use with ReentrantLock.

 

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 lock, or null if not owned.
protected  Collection<Thread>	getQueuedThreads()           Returns a collection containing threads that may be waiting to acquire this lock.
int	getQueueLength()           Returns an estimate of the number of threads waiting to acquire this lock.
boolean	isFair()           Return 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()           Acquire the lock.
void	lockInterruptibly()           Acquires the lock unless the current thread is interrupted.
ReentrantLock.ConditionObject	newCondition()           Returns a Condition instance for use with this Lock instance.
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, toString, 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.

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.

lock

public void lock()

Acquire 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:

• The lock is acquired by the current thread; or
• Some other thread 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 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.

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:

• The lock is acquired by the current thread; or
• Some other thread 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 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.

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()

newCondition

public ReentrantLock.ConditionObject newCondition()

Returns a Condition instance for use with this Lock instance.

Specified by:

newCondition in interface Lock

Returns:

the Condition object

isFair

public boolean isFair()

Return true if this lock has fairness set true.

Returns:

true if this lock has fairness set true.

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.

getQueueLength

public 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.

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 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 lock monitoring facilities.

Returns:

the collection of threads

getOwner

protected Thread getOwner()

Returns the thread that currently owns the 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.