co.paralleluniverse.strands.concurrent

Class ReentrantReadWriteLock

java.lang.Object

co.paralleluniverse.strands.concurrent.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

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	ReentrantReadWriteLock.ReadLock The lock returned by method readLock().
static class	ReentrantReadWriteLock.WriteLock The lock returned by method writeLock().

Constructor Summary

Constructors

Constructor and 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

Modifier and Type	Method and 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

writeLock

public ReentrantReadWriteLock.WriteLock writeLock()

Specified by:

writeLock in interface java.util.concurrent.locks.ReadWriteLock

readLock

public ReentrantReadWriteLock.ReadLock readLock()

Specified by:

readLock in interface java.util.concurrent.locks.ReadWriteLock

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