Package co.paralleluniverse.strands.concurrent

Class ReentrantReadWriteLock

  • All Implemented Interfaces:
    java.io.Serializable, java.util.concurrent.locks.ReadWriteLock
    public class ReentrantReadWriteLock
extends java.lang.Object
implements java.util.concurrent.locks.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.

      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 strands, but will normally have higher throughput than a fair lock.
      Fair mode
      When constructed as fair, strands contend for entry using an approximately arrival-order policy. When the currently held lock is released either the longest-waiting single writer strand will be assigned the write lock, or if there is a group of reader strands waiting longer than all waiting writer strands, that group will be assigned the read lock.

      A strand 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 strand. The strand will not acquire the read lock until after the oldest currently waiting writer strand has acquired and released the write lock. Of course, if a waiting writer abandons its wait, leaving one or more reader strands as the longest waiters in the queue with the write lock free, then those readers will be assigned the read lock.

      A strand 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 strands). (Note that the non-blocking ReentrantReadWriteLock.ReadLock.tryLock() and ReentrantReadWriteLock.WriteLock.tryLock() methods do not honor this fair setting and will acquire the lock if it is possible, regardless of waiting strands.)

    • 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 strand 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 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.

    • 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 strand 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 strands than writer strands, 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<String, Data> m = new TreeMap<String, Data>();
    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.

    Since:
    1.5
    See Also:
    Serialized Form

    • Constructor Summary

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

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      protected Strand getOwner()
      Returns the strand that currently owns the write lock, or null if not owned.
      protected java.util.Collection<Strand> getQueuedReaderStrands()
      Returns a collection containing strands that may be waiting to acquire the read lock.
      protected java.util.Collection<Strand> getQueuedStrands()
      Returns a collection containing strands that may be waiting to acquire either the read or write lock.
      protected java.util.Collection<Strand> getQueuedWriterStrands()
      Returns a collection containing strands that may be waiting to acquire the write lock.
      int getQueueLength()
      Returns an estimate of the number of strands waiting to acquire either the read or write lock.
      int getReadHoldCount()
      Queries the number of reentrant read holds on this lock by the current strand.
      int getReadLockCount()
      Queries the number of read locks held for this lock.
      protected java.util.Collection<Strand> getWaitingStrands​(java.util.concurrent.locks.Condition condition)
      Returns a collection containing those strands that may be waiting on the given condition associated with the write lock.
      int getWaitQueueLength​(java.util.concurrent.locks.Condition condition)
      Returns an estimate of the number of strands 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 strand.
      boolean hasQueuedStrand​(Strand strand)
      Queries whether the given strand is waiting to acquire either the read or write lock.
      boolean hasQueuedStrands()
      Queries whether any strands are waiting to acquire the read or write lock.
      boolean hasWaiters​(java.util.concurrent.locks.Condition condition)
      Queries whether any strands are waiting on the given condition associated with the write lock.
      boolean isFair()
      Returns true if this lock has fairness set true.
      boolean isWriteLocked()
      Queries if the write lock is held by any strand.
      boolean isWriteLockedByCurrentStrand()
      Queries if the write lock is held by the current strand.
      ReentrantReadWriteLock.ReadLock readLock()  
      java.lang.String toString()
      Returns a string identifying this lock, as well as its lock state.
      ReentrantReadWriteLock.WriteLock writeLock()  

      • Methods inherited from class java.lang.Object

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

    • Constructor Detail

      • ReentrantReadWriteLock

        public ReentrantReadWriteLock()
        Creates a new ReentrantReadWriteLock with default (nonfair) 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

      • isFair

        public final boolean isFair()
        Returns true if this lock has fairness set true.
        Returns:
        true if this lock has fairness set true

      • getOwner

        protected Strand getOwner()
        Returns the strand that currently owns the write lock, or null if not owned. When this method is called by a strand 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 strands 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.

      • isWriteLocked

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

      • isWriteLockedByCurrentStrand

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

      • getWriteHoldCount

        public int getWriteHoldCount()
        Queries the number of reentrant write holds on this lock by the current strand. A writer strand 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 strand, or zero if the write lock is not held by the current strand

      • getReadHoldCount

        public int getReadHoldCount()
        Queries the number of reentrant read holds on this lock by the current strand. A reader strand 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 strand, or zero if the read lock is not held by the current strand
        Since:
        1.6

      • getQueuedWriterStrands

        protected java.util.Collection<Strand> getQueuedWriterStrands()
        Returns a collection containing strands that may be waiting to acquire the write lock. Because the actual set of strands may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.
        Returns:
        the collection of strands

      • getQueuedReaderStrands

        protected java.util.Collection<Strand> getQueuedReaderStrands()
        Returns a collection containing strands that may be waiting to acquire the read lock. Because the actual set of strands may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.
        Returns:
        the collection of strands

      • hasQueuedStrands

        public final boolean hasQueuedStrands()
        Queries whether any strands are waiting to acquire the read or write lock. Note that because cancellations may occur at any time, a true return does not guarantee that any other strand will ever acquire a lock. This method is designed primarily for use in monitoring of the system state.
        Returns:
        true if there may be other strands waiting to acquire the lock

      • hasQueuedStrand

        public final boolean hasQueuedStrand​(Strand strand)
        Queries whether the given strand is waiting to acquire either the read or write lock. Note that because cancellations may occur at any time, a true return does not guarantee that this strand will ever acquire a lock. This method is designed primarily for use in monitoring of the system state.
        Parameters:
        strand - the strand
        Returns:
        true if the given strand is queued waiting for this lock
        Throws:
        java.lang.NullPointerException - if the strand is null

      • getQueueLength

        public final int getQueueLength()
        Returns an estimate of the number of strands waiting to acquire either the read or write lock. The value is only an estimate because the number of strands may change dynamically while this method traverses internal data structures. This method is designed for use in monitoring of the system state, not for synchronization control.
        Returns:
        the estimated number of strands waiting for this lock

      • getQueuedStrands

        protected java.util.Collection<Strand> getQueuedStrands()
        Returns a collection containing strands that may be waiting to acquire either the read or write lock. Because the actual set of strands may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive monitoring facilities.
        Returns:
        the collection of strands

      • hasWaiters

        public boolean hasWaiters​(java.util.concurrent.locks.Condition condition)
        Queries whether any strands are waiting on the given condition associated with the write lock. Note that because timeouts and interrupts may occur at any time, a true return does not guarantee that a future signal will awaken any strands. This method is designed primarily for use in monitoring of the system state.
        Parameters:
        condition - the condition
        Returns:
        true if there are any waiting strands
        Throws:
        java.lang.IllegalMonitorStateException - if this lock is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this lock
        java.lang.NullPointerException - if the condition is null

      • getWaitQueueLength

        public int getWaitQueueLength​(java.util.concurrent.locks.Condition condition)
        Returns an estimate of the number of strands waiting on the given condition associated with the write lock. Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on the actual number of waiters. This method is designed for use in monitoring of the system state, not for synchronization control.
        Parameters:
        condition - the condition
        Returns:
        the estimated number of waiting strands
        Throws:
        java.lang.IllegalMonitorStateException - if this lock is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this lock
        java.lang.NullPointerException - if the condition is null

      • getWaitingStrands

        protected java.util.Collection<Strand> getWaitingStrands​(java.util.concurrent.locks.Condition condition)
        Returns a collection containing those strands that may be waiting on the given condition associated with the write lock. Because the actual set of strands may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive condition monitoring facilities.
        Parameters:
        condition - the condition
        Returns:
        the collection of strands
        Throws:
        java.lang.IllegalMonitorStateException - if this lock is not held
        java.lang.IllegalArgumentException - if the given condition is not associated with this lock
        java.lang.NullPointerException - if the condition is null

      • toString

        public java.lang.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 java.lang.Object
        Returns:
        a string identifying this lock, as well as its lock state