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 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. 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 a number of public and protected methods for inspecting the state of the lock. Some of these methods are only 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.

Constructors

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 should use a fair ordering policy

Methods

lock

public void lock()

Specified by:

lock in interface Lock

lockInterruptibly

public void lockInterruptibly()
                       throws InterruptedException

Specified by:

lockInterruptibly in interface Lock

Throws:

InterruptedException - if the current thread is interrupted

tryLock

public boolean tryLock()

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

if (lock.tryLock() ||
     lock.tryLock(timeout, unit)) {
   ...
 }

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 the time unit is null

unlock

public void unlock()

Specified by:

unlock in interface Lock

Throws:

IllegalMonitorStateException - if the current thread does not hold this lock

newCondition

public Condition newCondition()

Specified by:

newCondition in interface Lock

Returns:

the Condition object

getHoldCount

public int getHoldCount()

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()

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

   public void m() {
       assert lock.isHeldByCurrentThread();
       // ... method body
   }
 }

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 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 null if not owned

hasQueuedThreads

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.

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 lock. 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 the thread is null

getQueueLength

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.

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 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 the condition is 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 the condition is 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 the condition is 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 name of the owning thread.

Overrides:

toString in class Object

Returns:

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