ReentrantLock (Java SE 9 & JDK 9 ) (original) (raw)

• • Constructor Detail

     * #### ReentrantLock  
     public ReentrantLock​()  
     Creates an instance of `ReentrantLock`. This is equivalent to using `ReentrantLock(false)`.  
     * #### ReentrantLock  
     public ReentrantLock​(boolean fair)  
     Creates an instance of `ReentrantLock` with the given fairness policy.  
     Parameters:  
     `fair` \- `true` if this lock should use a fair ordering policy  

  • Method Detail

    * #### lock  
    public void lock​()  
    Acquires the lock.  
    Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.  
    If the current thread already holds the lock then the hold count is incremented by one and the method returns immediately.  
    If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired, at which time the lock hold count is set to one.  
    Specified by:  
    `[lock](../../../../java/util/concurrent/locks/Lock.html#lock--)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    * #### lockInterruptibly  
    public void lockInterruptibly​()  
                           throws [InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang")  
    Acquires the lock unless the current thread is[interrupted](../../../../java/lang/Thread.html#interrupt--).  
    Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.  
    If the current thread already holds this lock then the hold count is incremented by one and the method returns immediately.  
    If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:  
             * The lock is acquired by the current thread; or  
             * Some other thread [interrupts](../../../../java/lang/Thread.html#interrupt--) the current thread.  
    If the lock is acquired by the current thread then the lock hold count is set to one.  
    If the current thread:  
             * has its interrupted status set on entry to this method; or  
             * is [interrupted](../../../../java/lang/Thread.html#interrupt--) while acquiring the lock,  
     then [InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang") is thrown and the current thread's interrupted status is cleared.  
    In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock.  
    Specified by:  
    `[lockInterruptibly](../../../../java/util/concurrent/locks/Lock.html#lockInterruptibly--)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    Throws:  
    `[InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang")` \- if the current thread is interrupted  
    * #### tryLock  
    public boolean tryLock​()  
    Acquires the lock only if it is not held by another thread at the time of invocation.  
    Acquires the lock if it is not held by another thread and returns immediately with the value `true`, setting the lock hold count to one. Even when this lock has been set to use a fair ordering policy, a call to `tryLock()` _will_ immediately acquire the lock if it is available, whether or not other threads are currently waiting for the lock. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting for this lock, then use[tryLock(0, TimeUnit.SECONDS) ](../../../../java/util/concurrent/locks/ReentrantLock.html#tryLock-long-java.util.concurrent.TimeUnit-) which is almost equivalent (it also detects interruption).  
    If the current thread already holds this lock then the hold count is incremented by one and the method returns `true`.  
    If the lock is held by another thread then this method will return immediately with the value `false`.  
    Specified by:  
    `[tryLock](../../../../java/util/concurrent/locks/Lock.html#tryLock--)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    Returns:  
    `true` if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and `false` otherwise  
    * #### tryLock  
    public boolean tryLock​(long timeout,  
                           [TimeUnit](../../../../java/util/concurrent/TimeUnit.html "enum in java.util.concurrent") unit)  
                    throws [InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang")  
    Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been[interrupted](../../../../java/lang/Thread.html#interrupt--).  
    Acquires the lock if it is not held by another thread and returns immediately with the value `true`, setting the lock hold count to one. If this lock has been set to use a fair ordering policy then an available lock _will not_ be acquired if any other threads are waiting for the lock. This is in contrast to the [tryLock()](../../../../java/util/concurrent/locks/ReentrantLock.html#tryLock--) method. If you want a timed `tryLock` that does permit barging on a fair lock then combine the timed and un-timed forms together:  
     `  
     if (lock.tryLock() ||  
         lock.tryLock(timeout, unit)) {  
       ...  
     }`  
    If the current thread already holds this lock then the hold count is incremented by one and the method returns `true`.  
    If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:  
             * The lock is acquired by the current thread; or  
             * Some other thread [interrupts](../../../../java/lang/Thread.html#interrupt--) the current thread; or  
             * The specified waiting time elapses  
    If the lock is acquired then the value `true` is returned and the lock hold count is set to one.  
    If the current thread:  
             * has its interrupted status set on entry to this method; or  
             * is [interrupted](../../../../java/lang/Thread.html#interrupt--) while acquiring the lock,  
     then [InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang") is thrown and the current thread's interrupted status is cleared.  
    If the specified waiting time elapses then the value `false` is returned. If the time is less than or equal to zero, the method will not wait at all.  
    In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock, and over reporting the elapse of the waiting time.  
    Specified by:  
    `[tryLock](../../../../java/util/concurrent/locks/Lock.html#tryLock-long-java.util.concurrent.TimeUnit-)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    Parameters:  
    `timeout` \- the time to wait for the lock  
    `unit` \- the time unit of the timeout argument  
    Returns:  
    `true` if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and `false` if the waiting time elapsed before the lock could be acquired  
    Throws:  
    `[InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang")` \- if the current thread is interrupted  
    `[NullPointerException](../../../../java/lang/NullPointerException.html "class in java.lang")` \- if the time unit is null  
    * #### unlock  
    public void unlock​()  
    Attempts to release this lock.  
    If the current thread is the holder of this lock then the hold count is decremented. If the hold count is now zero then the lock is released. If the current thread is not the holder of this lock then [IllegalMonitorStateException](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang") is thrown.  
    Specified by:  
    `[unlock](../../../../java/util/concurrent/locks/Lock.html#unlock--)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    Throws:  
    `[IllegalMonitorStateException](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang")` \- if the current thread does not hold this lock  
    * #### newCondition  
    public [Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") newCondition​()  
    Returns a [Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") instance for use with this[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks") instance.  
    The returned [Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") instance supports the same usages as do the [Object](../../../../java/lang/Object.html "class in java.lang") monitor methods ([wait](../../../../java/lang/Object.html#wait--), [notify](../../../../java/lang/Object.html#notify--), and [notifyAll](../../../../java/lang/Object.html#notifyAll--)) when used with the built-in monitor lock.  
             * If this lock is not held when any of the [Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") [waiting](../../../../java/util/concurrent/locks/Condition.html#await--) or [signalling](../../../../java/util/concurrent/locks/Condition.html#signal--) methods are called, then an [IllegalMonitorStateException](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang") is thrown.  
             * When the condition [waiting](../../../../java/util/concurrent/locks/Condition.html#await--) methods are called the lock is released and, before they return, the lock is reacquired and the lock hold count restored to what it was when the method was called.  
             * If a thread is [interrupted](../../../../java/lang/Thread.html#interrupt--) while waiting then the wait will terminate, an [InterruptedException](../../../../java/lang/InterruptedException.html "class in java.lang") will be thrown, and the thread's interrupted status will be cleared.  
             * Waiting threads are signalled in FIFO order.  
             * The ordering of lock reacquisition for threads returning from waiting methods is the same as for threads initially acquiring the lock, which is in the default case not specified, but for _fair_ locks favors those threads that have been waiting the longest.  
    Specified by:  
    `[newCondition](../../../../java/util/concurrent/locks/Lock.html#newCondition--)` in interface `[Lock](../../../../java/util/concurrent/locks/Lock.html "interface in java.util.concurrent.locks")`  
    Returns:  
    the Condition object  
    * #### getHoldCount  
    public int getHoldCount​()  
    Queries the number of holds on this lock by the current thread.  
    A thread has a hold on a lock for each lock action that is not matched by an unlock action.  
    The hold count information is typically only used for testing and debugging purposes. For example, if a certain section of code should not be entered with the lock already held then we can assert that fact:  
     `  
     class X {  
       ReentrantLock lock = new ReentrantLock();  
       // ...  
       public void m() {  
         assert lock.getHoldCount() == 0;  
         lock.lock();  
         try {  
           // ... method body  
         } finally {  
           lock.unlock();  
         }  
       }  
     }`  
    Returns:  
    the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread  
    * #### isHeldByCurrentThread  
    public boolean isHeldByCurrentThread​()  
    Queries if this lock is held by the current thread.  
    Analogous to the [Thread.holdsLock(Object)](../../../../java/lang/Thread.html#holdsLock-java.lang.Object-) method for built-in monitor locks, this method is typically used for debugging and testing. For example, a method that should only be called while a lock is held can assert that this is the case:  
     `  
     class X {  
       ReentrantLock lock = new ReentrantLock();  
       // ...  
       public void m() {  
           assert lock.isHeldByCurrentThread();  
           // ... method body  
       }  
     }`  
    It can also be used to ensure that a reentrant lock is used in a non-reentrant manner, for example:  
     `  
     class X {  
       ReentrantLock lock = new ReentrantLock();  
       // ...  
       public void m() {  
           assert !lock.isHeldByCurrentThread();  
           lock.lock();  
           try {  
               // ... method body  
           } finally {  
               lock.unlock();  
           }  
       }  
     }`  
    Returns:  
    `true` if current thread holds this lock and`false` otherwise  
    * #### isLocked  
    public boolean isLocked​()  
    Queries if this lock is held by any thread. This method is designed for use in monitoring of the system state, not for synchronization control.  
    Returns:  
    `true` if any thread holds this lock and`false` otherwise  
    * #### 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 [Thread](../../../../java/lang/Thread.html "class in java.lang") getOwner​()  
    Returns the thread that currently owns this 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  
    * #### hasQueuedThreads  
    public final boolean hasQueuedThreads​()  
    Queries whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, a `true` return does not guarantee that any other thread will ever acquire this 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  
    * #### hasQueuedThread  
    public final boolean hasQueuedThread​([Thread](../../../../java/lang/Thread.html "class in java.lang") thread)  
    Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, a`true` return does not guarantee that this thread will ever acquire this 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](../../../../java/lang/NullPointerException.html "class in java.lang")` \- if the thread is null  
    * #### getQueueLength  
    public final int getQueueLength​()  
    Returns an estimate of the number of threads waiting to acquire this 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:  
    the estimated number of threads waiting for this lock  
    * #### getQueuedThreads  
    protected [Collection](../../../../java/util/Collection.html "interface in java.util")<[Thread](../../../../java/lang/Thread.html "class in java.lang")> getQueuedThreads​()  
    Returns a collection containing threads that may be waiting to acquire this 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  
    * #### hasWaiters  
    public boolean hasWaiters​([Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") condition)  
    Queries whether any threads are waiting on the given condition associated with this 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](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang")` \- if this lock is not held  
    `[IllegalArgumentException](../../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if the given condition is not associated with this lock  
    `[NullPointerException](../../../../java/lang/NullPointerException.html "class in java.lang")` \- if the condition is null  
    * #### getWaitQueueLength  
    public int getWaitQueueLength​([Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") condition)  
    Returns an estimate of the number of threads waiting on the given condition associated with this 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](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang")` \- if this lock is not held  
    `[IllegalArgumentException](../../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if the given condition is not associated with this lock  
    `[NullPointerException](../../../../java/lang/NullPointerException.html "class in java.lang")` \- if the condition is null  
    * #### getWaitingThreads  
    protected [Collection](../../../../java/util/Collection.html "interface in java.util")<[Thread](../../../../java/lang/Thread.html "class in java.lang")> getWaitingThreads​([Condition](../../../../java/util/concurrent/locks/Condition.html "interface in java.util.concurrent.locks") condition)  
    Returns a collection containing those threads that may be waiting on the given condition associated with this 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](../../../../java/lang/IllegalMonitorStateException.html "class in java.lang")` \- if this lock is not held  
    `[IllegalArgumentException](../../../../java/lang/IllegalArgumentException.html "class in java.lang")` \- if the given condition is not associated with this lock  
    `[NullPointerException](../../../../java/lang/NullPointerException.html "class in java.lang")` \- if the condition is null  
    * #### toString  
    public [String](../../../../java/lang/String.html "class in java.lang") toString​()  
    Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes either the String `"Unlocked"` or the String `"Locked by"` followed by the[name](../../../../java/lang/Thread.html#getName--) of the owning thread.  
    Overrides:  
    `[toString](../../../../java/lang/Object.html#toString--)` in class `[Object](../../../../java/lang/Object.html "class in java.lang")`  
    Returns:  
    a string identifying this lock, as well as its lock state