edu.emory.mathcs.backport.java.util.concurrent.locks

Class ReentrantReadWriteLock

Implemented Interfaces:
ReadWriteLock, java.io.Serializable

public class ReentrantReadWriteLock
extends java.lang.Object
implements ReadWriteLock, java.io.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.

Author:
Doug Lea
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.

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.
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
getWriteHoldCount()
Queries the number of reentrant write holds on this lock by the current thread.
boolean
hasQueuedThreads()
Queries whether any threads are waiting to acquire the read or 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.
Lock
readLock()
String
toString()
Returns a string identifying this lock, as well as its lock state.
Lock
writeLock()

Constructor Details

ReentrantReadWriteLock

public ReentrantReadWriteLock()
Creates a new ReentrantReadWriteLock with default (nonfair) ordering properties.

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

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.

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

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

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 Lock 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.
Returns:
a string identifying this lock, as well as its lock state

writeLock

public Lock writeLock()
Specified by:
writeLock in interface ReadWriteLock