ReentrantReadWriteLock.ReadLock

AI-generated Key Takeaways

• ReentrantReadWriteLock.ReadLock provides a way to acquire a read lock within a ReentrantReadWriteLock, allowing multiple threads to concurrently read shared resources.

• It offers methods like lock(), lockInterruptibly(), and tryLock() to acquire the read lock, ensuring thread safety and handling potential interruptions.

• ReadLock does not support conditions, so newCondition() throws an UnsupportedOperationException.

• unlock() releases the read lock, potentially making the write lock available if no other readers exist.

public static class ReentrantReadWriteLock.ReadLock extends Object
implements Lock Serializable

The lock returned by method ReentrantReadWriteLock.readLock().

Protected Constructor Summary

ReadLock(ReentrantReadWriteLock lock) Constructor for use by subclasses.

Public Method Summary

void	lock() Acquires the read lock.
void	lockInterruptibly() Acquires the read lock unless the current thread is {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted}.
void	unlock() Attempts to release this lock.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.concurrent.locks.Lock

abstract void	lock() Acquires the lock.
abstract void	lockInterruptibly() Acquires the lock unless the current thread is {@linkplain Thread#interrupt interrupted}.
abstract Condition	newCondition() Returns a new Condition instance that is bound to this Lock instance.
abstract boolean	tryLock() Acquires the lock only if it is free at the time of invocation.
abstract boolean	tryLock(long time, TimeUnit unit) Acquires the lock if it is free within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}.
abstract void	unlock() Releases the lock.

Protected Constructors

protected ReadLock (ReentrantReadWriteLock lock)

Constructor for use by subclasses.

Parameters

lock	the outer lock object

Throws

NullPointerException	if the lock is null

Public Methods

public void lock ()

Acquires the read lock.

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

If the write lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until the read lock has been acquired.

public void lockInterruptibly ()

Acquires the read lock unless the current thread is {@linkplain Thread#interrupt interrupted}.

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

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 two things happens:

• The read lock is acquired by the current thread; or
• Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

• has its interrupted status set on entry to this method; or
• is {@linkplain Thread#interrupt 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.

Throws

InterruptedException	if the current thread is interrupted

public Condition newCondition ()

Throws UnsupportedOperationException because ReadLocks do not support conditions.

Returns

• A new Condition instance for this Lock instance

Throws

UnsupportedOperationException	always

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.

Returns

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

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 another 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 another thread then this method will return immediately with the value false.

Returns

• true if the read lock was acquired

public 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 {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt 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.

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

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.

Throws

IllegalMonitorStateException	if the current thread does not hold this lock