ReentrantReadWriteLock

Class Overview

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.

  Non-fair mode (default)

  When constructed as non-fair (the default), the order of entry to the read and write lock is unspecified, subject to reentrancy constraints. A nonfair lock that is continuously contended may indefinitely postpone one or more reader or writer threads, but will normally have higher throughput than a fair lock.

  Fair mode

  When constructed as fair, threads contend for entry using an approximately arrival-order policy. When the currently held lock is released either the longest-waiting single writer thread will be assigned the write lock, or if there is a group of reader threads waiting longer than all waiting writer threads, that group will be assigned the read lock.

  A thread that tries to acquire a fair read lock (non-reentrantly) will block if either the write lock is held, or there is a waiting writer thread. The thread will not acquire the read lock until after the oldest currently waiting writer thread has acquired and released the write lock. Of course, if a waiting writer abandons its wait, leaving one or more reader threads as the longest waiters in the queue with the write lock free, then those readers will be assigned the read lock.

  A thread that tries to acquire a fair write lock (non-reentrantly) will block unless both the read lock and write lock are free (which implies there are no waiting threads). (Note that the non-blocking tryLock() and tryLock() methods do not honor this fair setting and will acquire the lock if it is possible, regardless of waiting threads.)

• Reentrancy

  This lock allows both readers and writers to reacquire read or write locks in the style of a ReentrantLock. 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

  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 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.

• 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.

Serialization of this class behaves in the same way as built-in locks: a deserialized lock is in the unlocked state, regardless of its state when serialized.

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;
   volatile boolean cacheValid;
   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   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();
     }
   }
 }}

ReentrantReadWriteLocks can be used to improve concurrency in some uses of some kinds of Collections. This is typically worthwhile only when the collections are expected to be large, accessed by more reader threads than writer threads, and entail operations with overhead that outweighs synchronization overhead. For example, here is a class using a TreeMap that is expected to be large and concurrently accessed.

class RWDictionary {
    private final Map m = new TreeMap();
    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private final Lock r = rwl.readLock();
    private final Lock w = rwl.writeLock();

    public Data get(String key) {
        r.lock();
        try { return m.get(key); 
        finally { r.unlock(); }
    }
    public String[] allKeys() {
        r.lock();
        try { return m.keySet().toArray(); }
        finally { r.unlock(); }
    }
    public Data put(String key, Data value) {
        w.lock();
        try { return m.put(key, value); }
        finally { w.unlock(); }
    }
    public void clear() {
        w.lock();
        try { m.clear(); }
        finally { w.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.

Summary

Nested Classes
class	ReentrantReadWriteLock.ReadLock	The lock returned by method readLock().
class	ReentrantReadWriteLock.WriteLock	The lock returned by method writeLock().

Public Constructors
	ReentrantReadWriteLock() Creates a new ReentrantReadWriteLock with default (nonfair) ordering properties.
	ReentrantReadWriteLock(boolean fair) Creates a new ReentrantReadWriteLock with the given fairness policy.

Public Methods
final int	getQueueLength() Returns an estimate of the number of threads waiting to acquire either the read or write lock.
int	getReadHoldCount() Queries the number of reentrant read holds on this lock by the current thread.
int	getReadLockCount() Queries the number of read locks held for this lock.
int	getWaitQueueLength(Condition condition) Returns an estimate of the number of threads waiting on the given condition associated with the write lock.
int	getWriteHoldCount() Queries the number of reentrant write holds on this lock by the current thread.
final boolean	hasQueuedThread(Thread thread) Queries whether the given thread is waiting to acquire either the read or write lock.
final boolean	hasQueuedThreads() Queries whether any threads are waiting to acquire the read or write lock.
boolean	hasWaiters(Condition condition) Queries whether any threads are waiting on the given condition associated with the write lock.
final boolean	isFair() Returns true if this lock has fairness set true.
boolean	isWriteLocked() Queries if the write lock is held by any thread.
boolean	isWriteLockedByCurrentThread() Queries if the write lock is held by the current thread.
ReentrantReadWriteLock.ReadLock	readLock() Returns the lock used for reading.
String	toString() Returns a string identifying this lock, as well as its lock state.
ReentrantReadWriteLock.WriteLock	writeLock() Returns the lock used for writing.

Protected Methods
Thread	getOwner() Returns the thread that currently owns the write lock, or null if not owned.
Collection<Thread>	getQueuedReaderThreads() Returns a collection containing threads that may be waiting to acquire the read lock.
Collection<Thread>	getQueuedThreads() Returns a collection containing threads that may be waiting to acquire either the read or write lock.
Collection<Thread>	getQueuedWriterThreads() Returns a collection containing threads that may be waiting to acquire the write lock.
Collection<Thread>	getWaitingThreads(Condition condition) Returns a collection containing those threads that may be waiting on the given condition associated with the write lock.

[Expand] Inherited Methods
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals(Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Called before the object's memory is reclaimed by the VM. 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() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, 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 millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
From interface java.util.concurrent.locks.ReadWriteLock abstract Lock readLock() Returns the lock used for reading. abstract Lock writeLock() Returns the lock used for writing.