Package com.oracle.coherence.concurrent.locks

Class RemoteLock

java.lang.Object

com.oracle.coherence.concurrent.locks.RemoteLock

All Implemented Interfaces:

Lock

public class RemoteLock
extends Object
implements Lock

A reentrant mutual exclusion distributed Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, and the ReentrantLock class, but with support for access synchronization across multiple cluster members.

A DistributedLock is owned by the member and 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().

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 DistributedLock lock = Locks.remoteLock("myLock");
   // ...

   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.

Unlike ReentrantLock, this class does not support serialization, or Conditions.

This lock supports a maximum of Long#MAX_VALUE recursive locks by the same thread. Attempts to exceed this limit result in Error throws from locking methods.

Since:

21.12

Author:

Aleks Seovic 2021.10.19

Method Summary

Modifier and Type	Method	Description
long	getHoldCount()	Queries the number of holds on this lock by the current thread.
LockOwner	getOwner()	Returns the identity of the process and thread that currently owns this lock, or null if not owned.
int	getQueueLength()	Returns an estimate of the number of threads waiting to acquire this lock.
boolean	hasQueuedThread(Thread thread)	Queries whether the given thread is waiting to acquire this lock.
boolean	hasQueuedThreads()	Queries whether any threads are waiting to acquire this lock.
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()	Acquires the lock.
void	lockInterruptibly()	Acquires the lock unless the current thread is interrupted.
Condition	newCondition()
String	toString()	Returns a string identifying this lock, as well as its lock state.
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, wait, wait, wait

Method Detail

lock

public void lock()

Acquires 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

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.

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 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 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()

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

newCondition

public final Condition newCondition()

Specified by:

newCondition in interface Lock

getHoldCount

public long 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 {
   DistributedLock lock = Locks.remoteLock("myLock");
   // ...
   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(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 {
   DistributedLock lock = Locks.remoteLock("myLock");
   // ...

   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 {
   DistributedLock lock = Locks.remoteLock("myLock");
   // ...

   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

getOwner

public LockOwner getOwner()

Returns the identity of the process and thread that currently owns this lock, or null if not owned.

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 system state, not for synchronization control.

Returns:

the estimated number of threads waiting for this lock

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