ReentrantLock JDK 1.6)

Description

public class ReentrantLock

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. Also note that the untimed tryLock method does not honor the fairness setting. It will succeed if the lock is available even if other threads are waiting.

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 2147483647 recursive locks by the same thread. Attempts to exceed this limit result in Error throws from locking methods.

See also:

Methods

Hide/Show inherited methods

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

protected Thread getOwner ()

Returns the thread that currently owns this lock, or null if not owned.

Returns the thread that currently owns this lock, or
null if not owned. When this method is called by a
thread that is not the owner, the return value reflects a
best-effort approximation of current lock status. For example,
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 {@code null} if not owned

protected Collection<Thread> getQueuedThreads ()

Returns a collection containing threads that may be waiting to acquire this lock.

publicfinal int getQueueLength ()

Returns an estimate of the number of threads waiting to acquire 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.

public int getWaitQueueLength (Condition condition)

Returns an estimate of the number of threads waiting on the given condition associated with this lock.

publicfinal boolean hasQueuedThread (Thread thread)

Queries whether the given thread is waiting to acquire this lock.

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

publicfinal boolean hasQueuedThreads ()

Queries whether any threads are waiting to acquire this lock.

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.
Returns: {@code true} if there may be other threads waiting to acquire the lock

public boolean hasWaiters (Condition condition)

Queries whether any threads are waiting on the given condition associated with this lock.

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

publicfinal boolean isFair ()

Returns true if this lock has fairness set true.

Returns true if this lock has fairness set true.
Returns: {@code true} if this lock has fairness set true

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: {@code true} if current thread holds this lock and {@code false} otherwise

public boolean isLocked ()

Queries if this lock is held by any thread.

public void lock () [Specified in Lock]

Acquires the lock.

public void lockInterruptibly () throws InterruptedException [Specified in Lock]

Acquires the lock unless the current thread is interrupted.

public Condition newCondition () [Specified in Lock]

Returns a Condition instance for use with this Lock instance.

public String toString ()

Returns a string identifying this lock, as well as its lock state.

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
name of the owning thread.
Returns: a string identifying this lock, as well as its lock state

public boolean tryLock () [Specified in Lock]

Acquires the lock only if it is not held by another thread at the time of invocation.

public boolean tryLock (long timeout, TimeUnit unit) throws InterruptedException [Specified in Lock]

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 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 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.
Returns: {@code true} if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and {@code false} if the waiting time elapsed before the lock could be acquired
Parameters:
- timeout - the time to wait for the lock
- unit - the time unit of the timeout argument
Throws:
- InterruptedException - if the current thread is interrupted
- NullPointerException - if the time unit is null

public void unlock () [Specified in Lock]

Attempts to release this lock.