# Condition

public interface **Condition**

|---|---|---|
| Known Indirect Subclasses [AbstractQueuedLongSynchronizer.ConditionObject](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.ConditionObject.html), [AbstractQueuedSynchronizer.ConditionObject](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedSynchronizer.ConditionObject.html) |---|---| | [AbstractQueuedLongSynchronizer.ConditionObject](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.ConditionObject.html) | Condition implementation for a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.html` serving as the basis of a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html` implementation. | | [AbstractQueuedSynchronizer.ConditionObject](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedSynchronizer.ConditionObject.html) | Condition implementation for a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/AbstractQueuedSynchronizer.html` serving as the basis of a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html` implementation. | |||

`Condition` factors out the `Object` monitor
methods (`https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait()`, `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify()`
and `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notifyAll()`) into distinct objects to
give the effect of having multiple wait-sets per object, by
combining them with the use of arbitrary `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html`
instance use its `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html#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
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ArrayBlockingQueue.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait()` and
`https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify()` methods invoked.
Acquiring the monitor lock of a `Condition` instance, or using its
monitor methods, has no specified relationship with acquiring the
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html` associated with that `Condition` or the use of its
{@linkplain #await waiting} and {@linkplain #signal signalling} 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html` 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 that may have
unblocked the thread. An implementation should document this behavior.

### Public Method Summary

|---|---|
| abstract void | [await](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#await())() Causes the current thread to wait until it is signalled or {@linkplain Thread#interrupt interrupted}. |
| abstract boolean | [await](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#await(long,%20java.util.concurrent.TimeUnit))(long time, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit) Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses. |
| abstract long | [awaitNanos](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#awaitNanos(long))(long nanosTimeout) Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses. |
| abstract void | [awaitUninterruptibly](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#awaitUninterruptibly())() Causes the current thread to wait until it is signalled. |
| abstract boolean | [awaitUntil](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#awaitUntil(java.util.Date))([Date](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Date.html) deadline) Causes the current thread to wait until it is signalled or interrupted, or the specified deadline elapses. |
| abstract void | [signal](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signal())() Wakes up one waiting thread. |
| abstract void | [signalAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signalAll())() Wakes up all waiting threads. |

## Public Methods

#### public abstract void
**await**
()

Causes the current thread to wait until it is signalled or
{@linkplain Thread#interrupt interrupted}.

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:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signal()` method for this `Condition` and the current thread happens to be chosen as the thread to be awakened; or
- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signalAll()` method for this `Condition`; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread, and interruption of thread suspension is supported; or
- A "*spurious wakeup*" occurs.

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:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting and interruption of thread suspension is supported,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html`) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted (and interruption of thread suspension is supported) |
|---|---|

#### public abstract boolean
**await**
(long time, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit)

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

<br />

##### 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted (and interruption of thread suspension is supported) |
|---|---|

#### public abstract long
**awaitNanos**
(long nanosTimeout)

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:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signal()` method for this `Condition` and the current thread happens to be chosen as the thread to be awakened; or
- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signalAll()` method for this `Condition`; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread, and interruption of thread suspension is supported; or
- The specified waiting time elapses; or
- A "*spurious wakeup*" occurs.

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:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting and interruption of thread suspension is supported,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` 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:

     boolean aMethod(long timeout, TimeUnit unit) {
       long nanos = unit.toNanos(timeout);
       lock.lock();
       try {
         while (!conditionBeingWaitedFor()) {
           if (nanos <= 0L)
             return false;
           nanos = theCondition.awaitNanos(nanos);
         }
         // ...
       } finally {
         lock.unlock();
       }
     }

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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html`) 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

- an estimate of the `nanosTimeout` value minus the time spent waiting upon return from this method. A positive value may be used as the argument to a subsequent call to this method to finish waiting out the desired time. A value less than or equal to zero indicates that no time remains.

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted (and interruption of thread suspension is supported) |
|---|---|

#### public abstract 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:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signal()` method for this `Condition` and the current thread happens to be chosen as the thread to be awakened; or
- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signalAll()` method for this `Condition`; or
- A "*spurious wakeup*" occurs.

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 {@linkplain Thread#interrupt interrupted}
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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html`) and the
implementation must document that fact.

#### public abstract boolean
**awaitUntil**
([Date](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Date.html) deadline)

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:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signal()` method for this `Condition` and the current thread happens to be chosen as the thread to be awakened; or
- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Condition.html#signalAll()` method for this `Condition`; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread, and interruption of thread suspension is supported; or
- The specified deadline elapses; or
- A "*spurious wakeup*" occurs.

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:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting and interruption of thread suspension is supported,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` 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:

     boolean aMethod(Date deadline) {
       boolean stillWaiting = true;
       lock.lock();
       try {
         while (!conditionBeingWaitedFor()) {
           if (!stillWaiting)
             return false;
           stillWaiting = theCondition.awaitUntil(deadline);
         }
         // ...
       } finally {
         lock.unlock();
       }
     }

**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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html`) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted (and interruption of thread suspension is supported) |
|---|---|

#### public abstract 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`.

**Implementation Considerations**

An implementation may (and typically does) require that the
current thread hold the lock associated with this `Condition` when this method is called. Implementations must
document this precondition and any actions taken if the lock is
not held. Typically, an exception such as `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html` will be thrown.

#### public abstract 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`.

**Implementation Considerations**

An implementation may (and typically does) require that the
current thread hold the lock associated with this `Condition` when this method is called. Implementations must
document this precondition and any actions taken if the lock is
not held. Typically, an exception such as `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalMonitorStateException.html` will be thrown.