ReentrantReadWriteLock

public class ReentrantReadWriteLock
extends Object implements ReadWriteLock, 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.

  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 immediately 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();
     }
   }
 }

 class RWDictionary {
   private final Map<String, Data> 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 List<String> allKeys() {
     r.lock();
     try { return new ArrayList<>(m.keySet()); }
     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

Inherited methods
From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long millis, int nanos) Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed. final void wait(long millis) Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed. final void wait() Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.
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.

Public constructors

ReentrantReadWriteLock ()

ReentrantReadWriteLock (boolean fair)

Public methods

int getQueueLength ()

int getReadHoldCount ()

int getReadLockCount ()

int getWaitQueueLength (Condition condition)

int getWriteHoldCount ()

boolean hasQueuedThread (Thread thread)

boolean hasQueuedThreads ()

boolean hasWaiters (Condition condition)

boolean isFair ()

boolean isWriteLocked ()

boolean isWriteLockedByCurrentThread ()

ReentrantReadWriteLock.ReadLock readLock ()

String toString ()

ReentrantReadWriteLock.WriteLock writeLock ()

Protected methods

Thread getOwner ()

Collection<Thread> getQueuedReaderThreads ()

Collection<Thread> getQueuedThreads ()

Collection<Thread> getQueuedWriterThreads ()

Collection<Thread> getWaitingThreads (Condition condition)