\file ReentrantLock .d \brief A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as explained at

http:
//creativecommons.org/licenses/publicdomain Ported to D by Ben Hinkle. Email comments and bug reports to ben.hinkle@gmail.com

revision 2.0

  • class ReentrantLock : mango.locks.Lock.Lock;
    • \class ReentrantLock \brief A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

    A ReentrantLock is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking lock will return, successfully acquiring the lock, when the lock is not owned by another thread. The method will return immediately if the current thread already owns the lock. This can be checked using methods isHeldByCurrentThread, and getHoldCount.

    The constructor for this class accepts an optional fairness parameter. When set true, under contention, locks favor granting access to the longest-waiting thread. Otherwise this lock does not guarantee any particular access order. Programs using fair locks accessed by many threads may display lower overall throughput (i.e., are slower; often much slower) than those using the default setting, but have smaller variances in times to obtain locks and guarantee lack of starvation. Note however, that fairness of locks does not guarantee fairness of thread scheduling. Thus, one of many threads using a fair lock may obtain it multiple times in succession while other active threads are not progressing and not currently holding the lock.

    It is recommended practice to always immediately follow a call to lock with a try block, most typically in a before/after construction such as: 

     class X {
   private ReentrantLock lock;
   // ...
   this() {
     lock = new ReentrantLock;
   }
   void m() {
     lock.lock();  // block until condition holds
     try {
       // ... method body
     } finally {
       lock.unlock()
     }
   }
 }

    In addition to implementing the Lock interface, this class defines methods isLocked and getLockQueueLength, as well as some associated protected access methods that may be useful for instrumentation and monitoring.

    This lock supports a maximum of 2147483648 recursive locks by the same thread.

  • this(bool fair = false);
    • Creates an instance of ReentrantLock with the given fairness policy. \param fair true if this lock will be fair; else false

  • 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.

  • bool 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 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.

    \return 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.

  • bool tryLock (long timeout, TimeUnit unit);
    • Acquires the lock if it is not held by another thread within the given waiting time.

    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 () 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 two things happens:

    • The lock is acquired by 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 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.

    \param time the maximum time to wait for the lock \param unit the time unit of the time argument.

    \return 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.

  • 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.

  • Condition newCondition ();
    • Returns a Condition instance for use with this Lock instance.

    • When the condition Condition.wait() 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.

    • 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.



    \return the Condition object

  • 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;
   // ...
   this() {
     lock = new ReentrantLock;
   }
   void m() {
     assert( lock.getHoldCount() == 0 );
     lock.lock();
     try {
       // ... method body
     } finally {
       lock.unlock();
     }
   }
 }
    \return the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread.

  • bool isHeldByCurrentThread ();
    • Queries if this lock is held by the current thread.

    This method is typically used for debugging and testing. \return true if current thread holds this lock and false otherwise.

  • bool 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. \return true if any thread holds this lock and false otherwise.

  • final bool isFair ();
    • Returns true if this lock has fairness set true. @return true if this lock has fairness set true.

  • protected Thread getOwner ();
    • Returns the thread that currently owns the exclusive lock, or null if not owned. Note that 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. 'return the owner, or null if not owned.

  • final bool hasQueuedThreads ();
    • Queries whether any threads are waiting to acquire. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire. This method is designed primarily for use in monitoring of the system state.

    'return true if there may be other threads waiting to acquire the lock.

  • final bool hasQueuedThread (Thread 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 method is designed primarily for use in monitoring of the system state.

    \param thread the thread \return true if the given thread is queued waiting for this lock.

  • final int getQueueLength ();
    • Returns an estimate of the number of threads waiting to acquire. 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. \return the estimated number of threads waiting for this lock

  • protected Thread[] getQueuedThreads ();
    • Returns a collection containing threads that may be waiting to acquire. 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. \return the collection of threads

  • bool hasWaiters (Condition 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. \param condition the condition \return true if there are any waiting threads.

  • int getWaitQueueLength (Condition 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. \param condition the condition \return the estimated number of waiting threads.

  • protected Thread[] getWaitingThreads (Condition 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. \param condition the condition \return the collection of threads

  • char[] 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 Thread. toString of the owning thread. \return a string identifying this lock, as well as its lock state.

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