Package com.oracle.coherence.concurrent.locks

Class RemoteReadWriteLock

java.lang.Object
com.oracle.coherence.concurrent.locks.RemoteReadWriteLock
All Implemented Interfaces:
ReadWriteLock
public class RemoteReadWriteLock extends Object implements ReadWriteLock
An implementation of ReadWriteLock supporting similar semantics to ReentrantReadWriteLock, but with support for access synchronization across multiple cluster members.

This class has the following properties:

  • Acquisition order

    This class does not impose a reader or writer preference ordering for lock access, and unlike ReentrantReadWriteLock, it does NOT support an optional fairness policy. It supports only the non-fair mode, where the order of entry to the read and write lock is unspecified, subject to reentrancy constraints. A non-fair lock that is continuously contended may indefinitely postpone one or more reader or writer threads.

  • Reentrancy

    This lock allows both readers and writers to reacquire read or write locks in the style of a ReentrantReadWriteLock. Non-reentrant readers are not allowed until all write locks held by the writing thread have been released.

    Additionally, a writer can acquire the read lock, but not vice-versa. Among other applications, reentrancy can be useful when write locks are held during calls or callbacks to methods that perform reads under read locks. If a reader tries to acquire the write lock it will never succeed.

  • Lock downgrading

    Reentrancy also allows downgrading from the write lock to a read lock, by acquiring the write lock, then the read lock and then releasing the write lock. However, upgrading from a read lock to the write lock is not possible.

  • Interruption of lock acquisition

    The read lock and write lock both support interruption during lock acquisition.

  • Condition support

    Neither the read lock nor the write lock support a Condition and both readLock().newCondition() and writeLock().newCondition() throw UnsupportedOperationException.

  • Instrumentation

    This class supports methods to determine whether locks are held or contended. These methods are designed for monitoring system state, not for synchronization control.

Unlike ReentrantReadWriteLock, this class does not support serialization.

Sample usages. Here is a code sketch showing how to perform lock downgrading after updating a cache (exception handling is particularly tricky when handling multiple locks in a non-nested fashion): 

 
 class CachedData {
   Object data;
   boolean cacheValid;
   final DistributedReadWriteLock rwl = Locks.remoteReadWriteLock("myRWLock");

   void processCachedData() {
     rwl.readLock().lock();
     if (!cacheValid) {
       // Must release read lock before acquiring write lock
       rwl.readLock().unlock();
       rwl.writeLock().lock();
       try {
         // Recheck state because another thread might have
         // acquired write lock and changed state before we did.
         if (!cacheValid) {
           data = ...
           cacheValid = true;
         }
         // Downgrade by acquiring read lock before releasing write lock
         rwl.readLock().lock();
       } finally {
         rwl.writeLock().unlock(); // Unlock write, still hold read
       }
     }

     try {
       use(data);
     } finally {
       rwl.readLock().unlock();
     }
   }
 }

Implementation Notes

This lock supports a maximum of 65535 recursive write locks and 65535 read locks. Attempts to exceed these limits result in Error throws from locking methods.

Author:
Aleks Seovic 2021.10.27

  • Method Details

    • readLock

      public Lock readLock()
      Returns the lock used for reading.
      Specified by:
      readLock in interface ReadWriteLock
      Returns:
      the lock used for reading

    • writeLock

      public Lock writeLock()
      Returns the lock used for writing.
      Specified by:
      writeLock in interface ReadWriteLock
      Returns:
      the lock used for writing

    • getOwner

      public LockOwner getOwner()
      Returns the thread that currently owns the write 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

    • getReadLockCount

      public int getReadLockCount()
      Queries the number of read locks held for this lock. This method is designed for use in monitoring system state, not for synchronization control.
      Returns:
      the number of read locks held

    • isReadLocked

      public boolean isReadLocked()
      Queries if the read lock is held by any thread. This method is designed for use in monitoring system state, not for synchronization control.
      Returns:
      true if any thread holds the read lock and false otherwise

    • isWriteLocked

      public boolean isWriteLocked()
      Queries if the write lock is held by any thread. This method is designed for use in monitoring system state, not for synchronization control.
      Returns:
      true if any thread holds the write lock and false otherwise

    • isWriteLockedByCurrentThread

      public boolean isWriteLockedByCurrentThread()
      Queries if the write lock is held by the current thread.
      Returns:
      true if the current thread holds the write lock and false otherwise

    • getWriteHoldCount

      public int getWriteHoldCount()
      Queries the number of reentrant write holds on this lock by the current thread. A writer thread has a hold on a lock for each lock action that is not matched by an unlock action.
      Returns:
      the number of holds on the write lock by the current thread, or zero if the write lock is not held by the current thread

    • getReadHoldCount

      public int getReadHoldCount()
      Queries the number of reentrant read holds on this lock by the current thread. A reader thread has a hold on a lock for each lock action that is not matched by an unlock action.
      Returns:
      the number of holds on the read lock by the current thread, or zero if the read lock is not held by the current thread
      Since:
      1.6

    • 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 the String "Write locks =" followed by the number of reentrantly held write locks, and 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