java.util.concurrent.locks
Class ReentrantReadWriteLock

java.lang.Object
  java.util.concurrent.locks.ReentrantReadWriteLock

All Implemented Interfaces:

ReadWriteLock, java.io.Serializable

public class ReentrantReadWriteLock

extends java.lang.Object

implements ReadWriteLock, java.io.Serializable

An implementation of ReadWriteLock supporting similar semantics to ReentrantLock.

This class has the following properties:

• Acquisition order

  This class does not impose a reader or writer preference ordering for lock access. However, it does support an optional fairness policy. When constructed as fair, threads contend for entry using an approximately arrival-order policy. When the write lock is released either the longest-waiting single writer will be assigned the write lock, or if there is a reader waiting longer than any writer, the set of readers will be assigned the read lock. When constructed as non-fair, the order of entry to the lock need not be in arrival order. In either case, if readers are active and a writer enters the lock then no subsequent readers will be granted the read lock until after that writer has acquired and released the write lock.

• Reentrancy

  This lock allows both readers and writers to reacquire read or write locks in the style of a ReentrantLock. 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. And both view the interruptible lock methods as explicit interruption points and give preference to responding to the interrupt over normal or reentrant acquisition of the lock, or over reporting the elapse of the waiting time, as applicable.

• Condition support

  The write lock provides a Condition implementation that behaves in the same way, with respect to the write lock, as the Condition implementation provided by ReentrantLock.newCondition() does for ReentrantLock. This Condition can, of course, only be used with the write lock.

  The read lock does not support a Condition and readLock().newCondition() throws UnsupportedOperationException.

Sample usage. Here is a code sketch showing how to exploit reentrancy to perform lock downgrading after updating a cache (exception handling is elided for simplicity):

 class CachedData {
   Object data;
   volatile boolean cacheValid;
   ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   void processCachedData() {
     rwl.readLock().lock();
     if (!cacheValid) {
        // upgrade lock manually
        rwl.readLock().unlock();   // must unlock first to obtain writelock
        rwl.writeLock().lock();
        if (!cacheValid) { // recheck
          data = ...
          cacheValid = true;
        }
        // downgrade lock
        rwl.readLock().lock();  // reacquire read without giving up write lock
        rwl.writeLock().unlock(); // unlock write, still hold read
     }

     use(data);
     rwl.readLock().unlock();
   }
 }
 

Implementation Considerations

In addition to the above, this reference implementation has the following property:

• Ownership

  While not exposing a means to query the owner, the write lock does internally define an owner and so the write lock can only be released by the thread that acquired it.

  In contrast, the read lock has no concept of ownership. Consequently, while not a particularly good practice, it is possible to acquire a read lock in one thread, and release it in another. This can occasionally be useful.

Since:

1.5

Author:

Doug Lea

See Also:

Serialized Form

Constructor Summary

ReentrantReadWriteLock()
Creates a new ReentrantReadWriteLock with default ordering properties.

ReentrantReadWriteLock(boolean fair)
Creates a new ReentrantReadWriteLock with the given fairness policy.

Method Summary

Lock	readLock() Return the lock used for reading.
Lock	writeLock() Return the lock used for writing.

Methods inherited from class java.lang.Object

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

Constructor Detail

ReentrantReadWriteLock

public ReentrantReadWriteLock()

Creates a new ReentrantReadWriteLock with default ordering properties.

ReentrantReadWriteLock

public ReentrantReadWriteLock(boolean fair)

Creates a new ReentrantReadWriteLock with the given fairness policy.

Parameters:

fair - true if this lock should use a fair ordering policy

Method Detail

writeLock

public Lock writeLock()

Description copied from interface: ReadWriteLock

Return the lock used for writing.

Specified by:

writeLock in interface ReadWriteLock

readLock

public Lock readLock()

Description copied from interface: ReadWriteLock

Return the lock used for reading.

Specified by:

readLock in interface ReadWriteLock