Class ReentrantReadWriteLock.ReadLock

java.lang.Object

java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock

All Implemented Interfaces:

Serializable, Lock

Enclosing class:

ReentrantReadWriteLock

public static class ReentrantReadWriteLock.ReadLock
extends Object
implements Lock, Serializable

The lock returned by method ReentrantReadWriteLock.readLock().

Since:

1.5

See Also:

• Serialized Form

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	ReadLock(ReentrantReadWriteLock lock)	Constructor for use by subclasses.

Method Summary

Modifier and Type	Method	Description
void	lock()	Acquires the read lock.
void	lockInterruptibly()	Acquires the read lock unless the current thread is interrupted.
Condition	newCondition()	Throws UnsupportedOperationException because ReadLocks do not support conditions.
String	toString()	Returns a string identifying this lock, as well as its lock state.
boolean	tryLock()	Acquires the read lock only if the write lock is not held by another thread at the time of invocation.
boolean	tryLock(long timeout, TimeUnit unit)	Acquires the read lock if the write lock 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 declared in class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Modifier and Type	Method	Description
protected Object	clone()	Answers a new instance of the same class as the receiver, whose slots have been filled in with the values in the slots of the receiver.
boolean	equals(Object o)	Compares the argument to the receiver, and answers true if they represent the same object using a class specific comparison.
protected void	finalize()	Deprecated, for removal: This API element is subject to removal in a future version. May cause performance issues, deadlocks and hangs.
final Class<? extends Object>	getClass()	Answers the unique instance of java.lang.Class which represents the class of the receiver.
int	hashCode()	Answers an integer hash code for the receiver.
final void	notify()	Causes one thread which is waiting on the receiver to be made ready to run.
final void	notifyAll()	Causes all threads which are waiting on the receiver to be made ready to run.
final void	wait()	Causes the thread which sent this message to be made not ready to run pending some change in the receiver (as indicated by notify or notifyAll).
final void	wait(long time)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.
final void	wait(long time, int frac)	Causes the thread which sent this message to be made not ready to run either pending some change in the receiver (as indicated by notify or notifyAll) or the expiration of the timeout.

Constructor Details

ReadLock

protected ReadLock(ReentrantReadWriteLock lock)

Constructor for use by subclasses.

Parameters:

lock - the outer lock object

Throws:

NullPointerException - if the lock is null

Method Details

lock

public void lock()

Acquires the read lock.

Acquires the read lock if the write lock is not held by any thread and returns immediately.

If the write lock is held by any thread or the fairness policy prohibits acquisition of the read lock at this time, then the current thread becomes disabled for thread scheduling purposes and lies dormant until the read lock has been acquired.

Specified by:

lock in interface Lock

lockInterruptibly

public void lockInterruptibly()
                       throws InterruptedException

Acquires the read lock unless the current thread is interrupted.

Acquires the read lock if the write lock is not held by any thread and returns immediately.

If the write lock is held by any thread or the fairness policy prohibits acquisition of the read lock at this time, then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:

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

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while acquiring the read 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

tryLock

public boolean tryLock()

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

Acquires the read lock if the write lock is not held by any thread and returns immediately with the value true. Even when this lock has been set to use a fair ordering policy, a call to tryLock() will immediately acquire the read lock if it is available, whether or not other threads are currently waiting for the read 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 write lock is held by any thread then this method will return immediately with the value false.

Specified by:

tryLock in interface Lock

Returns:

true if the read lock was acquired

tryLock

public boolean tryLock(long timeout,
 TimeUnit unit)
                throws InterruptedException

Acquires the read lock if the write lock is not held by another thread within the given waiting time and the current thread has not been interrupted.

Acquires the read lock if the write lock is not held by another thread and returns immediately with the value true. 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 write 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 read lock is acquired by the current thread; or
• Some other thread interrupts the current thread; or
• The specified waiting time elapses.

If the read lock is acquired then the value true is returned.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while acquiring the read 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 read lock

unit - the time unit of the timeout argument

Returns:

true if the read lock was acquired

Throws:

InterruptedException - if the current thread is interrupted

NullPointerException - if the time unit is null

unlock

public void unlock()

Attempts to release this lock.

If the number of readers is now zero then the lock is made available for write lock attempts. If the current thread does not hold 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()

Throws UnsupportedOperationException because ReadLocks do not support conditions.

Specified by:

newCondition in interface Lock

Returns:

A new Condition instance for this Lock instance

Throws:

UnsupportedOperationException - always

toString

public String toString()

Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes the String "Read locks =" followed by the number of held read locks.

Overrides:

toString in class Object

Returns:

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