Skip to content

Most visited

Recently visited

navigation

ReentrantReadWriteLock

public class ReentrantReadWriteLock
extends Object implements ReadWriteLock, Serializable

java.lang.Object
   ↳ java.util.concurrent.locks.ReentrantReadWriteLock


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();
     }
   }
 }
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<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

Nested classes

class ReentrantReadWriteLock.ReadLock

The lock returned by method readLock()

class ReentrantReadWriteLock.WriteLock

The lock returned by method writeLock()

Public constructors

ReentrantReadWriteLock()

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

ReentrantReadWriteLock(boolean fair)

Creates a new ReentrantReadWriteLock with the given fairness policy.

Public methods

final 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 getWaitQueueLength(Condition condition)

Returns an estimate of the number of threads 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.

final boolean hasQueuedThread(Thread thread)

Queries whether the given thread is waiting to acquire either the read or write lock.

final 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.

final 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()

Returns the lock used for reading.

String toString()

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

ReentrantReadWriteLock.WriteLock writeLock()

Returns the lock used for writing.

Protected methods

Thread getOwner()

Returns the thread that currently owns the write lock, or null if not owned.

Collection<Thread> getQueuedReaderThreads()

Returns a collection containing threads that may be waiting to acquire the read lock.

Collection<Thread> getQueuedThreads()

Returns a collection containing threads that may be waiting to acquire either the read or write lock.

Collection<Thread> getQueuedWriterThreads()

Returns a collection containing threads that may be waiting to acquire the write lock.

Collection<Thread> getWaitingThreads(Condition condition)

Returns a collection containing those threads that may be waiting on the given condition associated with the write lock.

Inherited methods

From class java.lang.Object
From interface java.util.concurrent.locks.ReadWriteLock

Public constructors

ReentrantReadWriteLock

Added in API level 1
ReentrantReadWriteLock ()

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

ReentrantReadWriteLock

Added in API level 1
ReentrantReadWriteLock (boolean fair)

Creates a new ReentrantReadWriteLock with the given fairness policy.

Parameters
fair boolean: true if this lock should use a fair ordering policy

Public methods

getQueueLength

Added in API level 1
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 system state, not for synchronization control.

Returns
int the estimated number of threads waiting for this lock

getReadHoldCount

Added in API level 9
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
int 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

getReadLockCount

Added in API level 1
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
int the number of read locks held

getWaitQueueLength

Added in API level 1
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 Condition: the condition
Returns
int 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

getWriteHoldCount

Added in API level 1
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
int 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

Added in API level 1
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 Thread: the thread
Returns
boolean true if the given thread is queued waiting for this lock
Throws
NullPointerException if the thread is null

hasQueuedThreads

Added in API level 1
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
boolean true if there may be other threads waiting to acquire the lock

hasWaiters

Added in API level 1
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 Condition: the condition
Returns
boolean 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

Added in API level 1
boolean isFair ()

Returns true if this lock has fairness set true.

Returns
boolean true if this lock has fairness set true

isWriteLocked

Added in API level 1
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
boolean true if any thread holds the write lock and false otherwise

isWriteLockedByCurrentThread

Added in API level 1
boolean isWriteLockedByCurrentThread ()

Queries if the write lock is held by the current thread.

Returns
boolean true if the current thread holds the write lock and false otherwise

readLock

Added in API level 1
ReentrantReadWriteLock.ReadLock readLock ()

Returns the lock used for reading.

Returns
ReentrantReadWriteLock.ReadLock the lock used for reading

toString

Added in API level 1
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
String a string identifying this lock, as well as its lock state

writeLock

Added in API level 1
ReentrantReadWriteLock.WriteLock writeLock ()

Returns the lock used for writing.

Returns
ReentrantReadWriteLock.WriteLock the lock used for writing

Protected methods

getOwner

Added in API level 1
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
Thread the owner, or null if not owned

getQueuedReaderThreads

Added in API level 1
Collection<Thread> 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
Collection<Thread> the collection of threads

getQueuedThreads

Added in API level 1
Collection<Thread> 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
Collection<Thread> the collection of threads

getQueuedWriterThreads

Added in API level 1
Collection<Thread> 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
Collection<Thread> the collection of threads

getWaitingThreads

Added in API level 1
Collection<Thread> 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 Condition: the condition
Returns
Collection<Thread> 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
This site uses cookies to store your preferences for site-specific language and display options.

Hooray!

This class requires API level or higher

This doc is hidden because your selected API level for the documentation is . You can change the documentation API level with the selector above the left navigation.

For more information about specifying the API level your app requires, read Supporting Different Platform Versions.