java.util.concurrent.locks

Class ReentrantReadWriteLock

Implemented Interfaces:
ReadWriteLock, Serializable

public class ReentrantReadWriteLock
extends Object
implements ReadWriteLock, Serializable

An implementation of ReadWriteLock supporting similar semantics to ReentrantLock.

This class has the following properties:

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 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) {
        // Must release read lock before acquiring write lock
        rwl.readLock().unlock();
        rwl.writeLock().lock();
        // 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();
        rwl.writeLock().unlock(); // Unlock write, still hold read
     }

     use(data);
     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.

Since:
1.5
See Also:
Serialized Form

Nested Class Summary

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

Constructor Summary

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

Method Summary

protected Thread
getOwner()
Returns the thread that currently owns the write lock, or null if not owned.
int
getQueueLength()
Returns an estimate of the number of threads waiting to acquire either the read or write lock.
protected Collection
getQueuedReaderThreads()
Returns a collection containing threads that may be waiting to acquire the read lock.
protected Collection
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire either the read or write lock.
protected Collection
getQueuedWriterThreads()
Returns a collection containing threads that may be waiting to acquire the 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.
protected Collection
getWaitingThreads(Condition condition)
Returns a collection containing those threads that may be 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.
boolean
hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to acquire either the read or write lock.
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.
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()
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, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

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 Details

getOwner

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

getQueueLength

public final int getQueueLength()
Returns an estimate of the number of threads waiting to acquire either the read or write lock. The value is only an estimate because the number of threads 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 threads waiting for this lock

getQueuedReaderThreads

protected Collection getQueuedReaderThreads()
Returns a collection containing threads that may be waiting to acquire the read lock. Because the actual set of threads 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 threads

getQueuedThreads

protected Collection getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire either the read or write lock. Because the actual set of threads 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 threads

getQueuedWriterThreads

protected Collection getQueuedWriterThreads()
Returns a collection containing threads that may be waiting to acquire the write lock. Because the actual set of threads 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 threads

getReadHoldCount

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

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.

getWaitQueueLength

public int getWaitQueueLength(Condition condition)
Returns an estimate of the number of threads 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 threads
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if the condition is null

getWaitingThreads

protected Collection getWaitingThreads(Condition condition)
Returns a collection containing those threads that may be waiting on the given condition associated with the write lock. Because the actual set of threads 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 threads
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if the condition is null

getWriteHoldCount

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

hasQueuedThread

public final boolean hasQueuedThread(Thread thread)
Queries whether the given thread 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 thread will ever acquire a lock. This method is designed primarily for use in monitoring of the system state.
Parameters:
thread - the thread
Returns:
true if the given thread is queued waiting for this lock
Throws:
NullPointerException - if the thread is null

hasQueuedThreads

public final boolean hasQueuedThreads()
Queries whether any threads 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 thread 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 threads waiting to acquire the lock

hasWaiters

public boolean hasWaiters(Condition condition)
Queries whether any threads 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 threads. This method is designed primarily for use in monitoring of the system state.
Parameters:
condition - the condition
Returns:
true if there are any waiting threads
Throws:
IllegalMonitorStateException - if this lock is not held
IllegalArgumentException - if the given condition is not associated with this lock
NullPointerException - if the condition is null

isFair

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

isWriteLocked

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

isWriteLockedByCurrentThread

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

readLock

public ReentrantReadWriteLock.ReadLock readLock()
Specified by:
readLock in interface ReadWriteLock

toString

public 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 interface Object
Returns:
a string identifying this lock, as well as its lock state

writeLock

public ReentrantReadWriteLock.WriteLock writeLock()
Specified by:
writeLock in interface ReadWriteLock