java.util.concurrent.locks
Interface Condition

All Known Implementing Classes:
AbstractQueuedSynchronizer.ConditionObject sample code for java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject definition code for java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject

public interface Condition

Condition factors out the Object monitor methods (wait sample code for java.lang.Object.wait() definition code for java.lang.Object.wait() , notify sample code for java.lang.Object.notify() definition code for java.lang.Object.notify() and notifyAll sample code for java.lang.Object.notifyAll() definition code for java.lang.Object.notifyAll() ) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock sample code for java.util.concurrent.locks.Lock definition code for java.util.concurrent.locks.Lock implementations. Where a Lock replaces the use of synchronized methods and statements, a Condition replaces the use of the Object monitor methods.

Conditions (also known as condition queues or condition variables) provide a means for one thread to suspend execution (to "wait") until notified by another thread that some state condition may now be true. Because access to this shared state information occurs in different threads, it must be protected, so a lock of some form is associated with the condition. The key property that waiting for a condition provides is that it atomically releases the associated lock and suspends the current thread, just like Object.wait.

A Condition instance is intrinsically bound to a lock. To obtain a Condition instance for a particular Lock sample code for java.util.concurrent.locks.Lock definition code for java.util.concurrent.locks.Lock instance use its newCondition() sample code for java.util.concurrent.locks.Lock.newCondition() definition code for java.util.concurrent.locks.Lock.newCondition() method.

As an example, suppose we have a bounded buffer which supports put and take methods. If a take is attempted on an empty buffer, then the thread will block until an item becomes available; if a put is attempted on a full buffer, then the thread will block until a space becomes available. We would like to keep waiting put threads and take threads in separate wait-sets so that we can use the optimization of only notifying a single thread at a time when items or spaces become available in the buffer. This can be achieved using two Condition sample code for java.util.concurrent.locks.Condition definition code for java.util.concurrent.locks.Condition instances.

 class BoundedBuffer {
   final Lock lock = new ReentrantLock();
   final Condition notFull  = lock.newCondition(); 
   final Condition notEmpty = lock.newCondition(); 

   final Object[] items = new Object[100];
   int putptr, takeptr, count;

   public void put(Object x) throws InterruptedException {
     lock.lock();
     try {
       while (count == items.length) 
         notFull.await();
       items[putptr] = x; 
       if (++putptr == items.length) putptr = 0;
       ++count;
       notEmpty.signal();
     } finally {
       lock.unlock();
     }
   }

   public Object take() throws InterruptedException {
     lock.lock();
     try {
       while (count == 0) 
         notEmpty.await();
       Object x = items[takeptr]; 
       if (++takeptr == items.length) takeptr = 0;
       --count;
       notFull.signal();
       return x;
     } finally {
       lock.unlock();
     }
   } 
 }
 
(The ArrayBlockingQueue sample code for java.util.concurrent.ArrayBlockingQueue definition code for java.util.concurrent.ArrayBlockingQueue class provides this functionality, so there is no reason to implement this sample usage class.)

A Condition implementation can provide behavior and semantics that is different from that of the Object monitor methods, such as guaranteed ordering for notifications, or not requiring a lock to be held when performing notifications. If an implementation provides such specialized semantics then the implementation must document those semantics.

Note that Condition instances are just normal objects and can themselves be used as the target in a synchronized statement, and can have their own monitor wait sample code for java.lang.Object.wait(long) definition code for java.lang.Object.wait(long) and notification sample code for java.lang.Object.notify() definition code for java.lang.Object.notify() methods invoked. Acquiring the monitor lock of a Condition instance, or using its monitor methods, has no specified relationship with acquiring the Lock sample code for java.util.concurrent.locks.Lock definition code for java.util.concurrent.locks.Lock associated with that Condition or the use of its waiting sample code for java.util.concurrent.locks.Condition.await() definition code for java.util.concurrent.locks.Condition.await() and signalling sample code for java.util.concurrent.locks.Condition.signal() definition code for java.util.concurrent.locks.Condition.signal() methods. It is recommended that to avoid confusion you never use Condition instances in this way, except perhaps within their own implementation.

Except where noted, passing a null value for any parameter will result in a NullPointerException sample code for java.lang.NullPointerException definition code for java.lang.NullPointerException being thrown.

Implementation Considerations

When waiting upon a Condition, a "spurious wakeup" is permitted to occur, in general, as a concession to the underlying platform semantics. This has little practical impact on most application programs as a Condition should always be waited upon in a loop, testing the state predicate that is being waited for. An implementation is free to remove the possibility of spurious wakeups but it is recommended that applications programmers always assume that they can occur and so always wait in a loop.

The three forms of condition waiting (interruptible, non-interruptible, and timed) may differ in their ease of implementation on some platforms and in their performance characteristics. In particular, it may be difficult to provide these features and maintain specific semantics such as ordering guarantees. Further, the ability to interrupt the actual suspension of the thread may not always be feasible to implement on all platforms.

Consequently, an implementation is not required to define exactly the same guarantees or semantics for all three forms of waiting, nor is it required to support interruption of the actual suspension of the thread.

An implementation is required to clearly document the semantics and guarantees provided by each of the waiting methods, and when an implementation does support interruption of thread suspension then it must obey the interruption semantics as defined in this interface.

As interruption generally implies cancellation, and checks for interruption are often infrequent, an implementation can favor responding to an interrupt over normal method return. This is true even if it can be shown that the interrupt occurred after another action may have unblocked the thread. An implementation should document this behavior.

Since:
1.5

Method Summary
 void await sample code for java.util.concurrent.locks.Condition.await() definition code for java.util.concurrent.locks.Condition.await() ()
          Causes the current thread to wait until it is signalled or interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() .
 boolean await sample code for java.util.concurrent.locks.Condition.await(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.locks.Condition.await(long, java.util.concurrent.TimeUnit) (long time, TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
          Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses.
 long awaitNanos sample code for java.util.concurrent.locks.Condition.awaitNanos(long) definition code for java.util.concurrent.locks.Condition.awaitNanos(long) (long nanosTimeout)
          Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses.
 void awaitUninterruptibly sample code for java.util.concurrent.locks.Condition.awaitUninterruptibly() definition code for java.util.concurrent.locks.Condition.awaitUninterruptibly() ()
          Causes the current thread to wait until it is signalled.
 boolean awaitUntil sample code for java.util.concurrent.locks.Condition.awaitUntil(java.util.Date) definition code for java.util.concurrent.locks.Condition.awaitUntil(java.util.Date) (Date sample code for java.util.Date definition code for java.util.Date  deadline)
          Causes the current thread to wait until it is signalled or interrupted, or the specified deadline elapses.
 void signal sample code for java.util.concurrent.locks.Condition.signal() definition code for java.util.concurrent.locks.Condition.signal() ()
          Wakes up one waiting thread.
 void signalAll sample code for java.util.concurrent.locks.Condition.signalAll() definition code for java.util.concurrent.locks.Condition.signalAll() ()
          Wakes up all waiting threads.
 

Method Detail

await sample code for java.util.concurrent.locks.Condition.await() definition code for java.util.concurrent.locks.Condition.await()

void await()
           throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until it is signalled or interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() .

The lock associated with this Condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:

In all cases, before this method can return the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock.

If the current thread:

then InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException is thrown and the current thread's interrupted status is cleared. It is not specified, in the first case, whether or not the test for interruption occurs before the lock is released.

Implementation Considerations

The current thread is assumed to hold the lock associated with this Condition when this method is called. It is up to the implementation to determine if this is the case and if not, how to respond. Typically, an exception will be thrown (such as IllegalMonitorStateException sample code for java.lang.IllegalMonitorStateException definition code for java.lang.IllegalMonitorStateException ) and the implementation must document that fact.

An implementation can favor responding to an interrupt over normal method return in response to a signal. In that case the implementation must ensure that the signal is redirected to another waiting thread, if there is one.

Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted (and interruption of thread suspension is supported).

awaitUninterruptibly sample code for java.util.concurrent.locks.Condition.awaitUninterruptibly() definition code for java.util.concurrent.locks.Condition.awaitUninterruptibly()

void awaitUninterruptibly()
Causes the current thread to wait until it is signalled.

The lock associated with this condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

In all cases, before this method can return the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock.

If the current thread's interrupted status is set when it enters this method, or it is interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() while waiting, it will continue to wait until signalled. When it finally returns from this method its interrupted status will still be set.

Implementation Considerations

The current thread is assumed to hold the lock associated with this Condition when this method is called. It is up to the implementation to determine if this is the case and if not, how to respond. Typically, an exception will be thrown (such as IllegalMonitorStateException sample code for java.lang.IllegalMonitorStateException definition code for java.lang.IllegalMonitorStateException ) and the implementation must document that fact.


awaitNanos sample code for java.util.concurrent.locks.Condition.awaitNanos(long) definition code for java.util.concurrent.locks.Condition.awaitNanos(long)

long awaitNanos(long nanosTimeout)
                throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses.

The lock associated with this condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of five things happens:

In all cases, before this method can return the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock.

If the current thread:

then InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException is thrown and the current thread's interrupted status is cleared. It is not specified, in the first case, whether or not the test for interruption occurs before the lock is released.

The method returns an estimate of the number of nanoseconds remaining to wait given the supplied nanosTimeout value upon return, or a value less than or equal to zero if it timed out. This value can be used to determine whether and how long to re-wait in cases where the wait returns but an awaited condition still does not hold. Typical uses of this method take the following form:

 synchronized boolean aMethod(long timeout, TimeUnit unit) {
   long nanosTimeout = unit.toNanos(timeout);
   while (!conditionBeingWaitedFor) {
     if (nanosTimeout > 0)
         nanosTimeout = theCondition.awaitNanos(nanosTimeout);
      else
        return false;
   }
   // ... 
 }
 

Design note: This method requires a nanosecond argument so as to avoid truncation errors in reporting remaining times. Such precision loss would make it difficult for programmers to ensure that total waiting times are not systematically shorter than specified when re-waits occur.

Implementation Considerations

The current thread is assumed to hold the lock associated with this Condition when this method is called. It is up to the implementation to determine if this is the case and if not, how to respond. Typically, an exception will be thrown (such as IllegalMonitorStateException sample code for java.lang.IllegalMonitorStateException definition code for java.lang.IllegalMonitorStateException ) and the implementation must document that fact.

An implementation can favor responding to an interrupt over normal method return in response to a signal, or over indicating the elapse of the specified waiting time. In either case the implementation must ensure that the signal is redirected to another waiting thread, if there is one.

Parameters:
nanosTimeout - the maximum time to wait, in nanoseconds
Returns:
A value less than or equal to zero if the wait has timed out; otherwise an estimate, that is strictly less than the nanosTimeout argument, of the time still remaining when this method returned.
Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted (and interruption of thread suspension is supported).

await sample code for java.util.concurrent.locks.Condition.await(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.locks.Condition.await(long, java.util.concurrent.TimeUnit)

boolean await(long time,
              TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
              throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses. This method is behaviorally equivalent to:
   awaitNanos(unit.toNanos(time)) > 0
 

Parameters:
time - the maximum time to wait
unit - the time unit of the time argument.
Returns:
false if the waiting time detectably elapsed before return from the method, else true.
Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted (and interruption of thread suspension is supported).

awaitUntil sample code for java.util.concurrent.locks.Condition.awaitUntil(java.util.Date) definition code for java.util.concurrent.locks.Condition.awaitUntil(java.util.Date)

boolean awaitUntil(Date sample code for java.util.Date definition code for java.util.Date  deadline)
                   throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until it is signalled or interrupted, or the specified deadline elapses.

The lock associated with this condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of five things happens:

In all cases, before this method can return the current thread must re-acquire the lock associated with this condition. When the thread returns it is guaranteed to hold this lock.

If the current thread:

then InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException is thrown and the current thread's interrupted status is cleared. It is not specified, in the first case, whether or not the test for interruption occurs before the lock is released.

The return value indicates whether the deadline has elapsed, which can be used as follows:

 synchronized boolean aMethod(Date deadline) {
   boolean stillWaiting = true;
   while (!conditionBeingWaitedFor) {
     if (stillwaiting)
         stillWaiting = theCondition.awaitUntil(deadline);
      else
        return false;
   }
   // ... 
 }
 

Implementation Considerations

The current thread is assumed to hold the lock associated with this Condition when this method is called. It is up to the implementation to determine if this is the case and if not, how to respond. Typically, an exception will be thrown (such as IllegalMonitorStateException sample code for java.lang.IllegalMonitorStateException definition code for java.lang.IllegalMonitorStateException ) and the implementation must document that fact.

An implementation can favor responding to an interrupt over normal method return in response to a signal, or over indicating the passing of the specified deadline. In either case the implementation must ensure that the signal is redirected to another waiting thread, if there is one.

Parameters:
deadline - the absolute time to wait until
Returns:
false if the deadline has elapsed upon return, else true.
Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted (and interruption of thread suspension is supported).

signal sample code for java.util.concurrent.locks.Condition.signal() definition code for java.util.concurrent.locks.Condition.signal()

void signal()
Wakes up one waiting thread.

If any threads are waiting on this condition then one is selected for waking up. That thread must then re-acquire the lock before returning from await.


signalAll sample code for java.util.concurrent.locks.Condition.signalAll() definition code for java.util.concurrent.locks.Condition.signalAll()

void signalAll()
Wakes up all waiting threads.

If any threads are waiting on this condition then they are all woken up. Each thread must re-acquire the lock before it can return from await.