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tim |
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package java.util.concurrent; |
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import java.util.Date; |
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/** |
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* <tt>Conditions</tt> abstract out the <tt>Object</tt> monitor |
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* methods ({@link Object#wait() wait}, {@link Object#notify notify} and |
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* {@link Object#notifyAll notifyAll}) into distinct objects that can be bound |
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* to other objects to give the effect of having multiple wait-sets per |
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* object monitor. They also generalise the monitor methods to allow them to |
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* be used with arbitrary {@link Lock} implementations when needed. |
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* |
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* <p>Conditions (also known as condition queues or condition variables) |
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* provide |
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* a means for one thread to suspend execution (to "wait") until |
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* notified by another thread that some state-condition the first thread is |
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* waiting for may now be true. Because access to this shared state information |
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* occurs in different threads, it must be protected and invariably |
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* a lock, of some form, is always associated with the condition. The key |
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* property that waiting for a condition provides is that it |
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* <em>atomically</em> releases the associated lock and suspends the current |
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* thread. |
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* |
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* <p>A <tt>Condition</tt> instance is intrinsically bound to a lock: either |
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* the in-built monitor lock of an object, or a {@link Lock} instance. |
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* To obtain a <tt>Condition</tt> instance for a particular object's monitor |
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* lock |
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* use the {@link Locks#newConditionFor(Object)} method. |
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* To obtain a <tt>Condition</tt> instance for a particular {@link Lock} |
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* instance use its {@link Lock#newCondition} method. |
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* |
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* <p>As an example, suppose we have a bounded buffer which supports methods |
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* to <tt>put</tt> and <tt>take</tt> items in/from the buffer. If a |
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* <tt>take</tt> is attempted on an empty buffer then the thread will block |
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* until an item becomes available; if a <tt>put</tt> is attempted on a |
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* full buffer, then the thread will block until a space becomes available. |
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* We would like to keep waiting <tt>put</tt> threads and <tt>take</tt> |
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* threads in separate wait-sets so that we can use the optimisation of |
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* only notifying a single thread at a time when items, or spaces, become |
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* available in the buffer. This can be achieved using either two |
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* {@link Condition} instances, or one {@link Condition} instance and the |
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* actual |
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* monitor wait-set. For clarity we'll use two {@link Condition} instances. |
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* <pre><code> |
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* class BoundedBuffer { |
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* <b>final Condition notFull = Locks.newConditionFor(this); |
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* final Condition notEmpty = Locks.newConditionFor(this); </b> |
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* |
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* Object[] items = new Object[100]; |
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* int putptr, takeptr, count; |
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* |
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* public <b>synchronized</b> void put(Object x) |
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* throws InterruptedException { |
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* while (count == items.length) |
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* <b>notFull.await();</b> |
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* items[putptr] = x; |
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* if (++putptr == items.length) putptr = 0; |
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* ++count; |
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* <b>notEmpty.signal();</b> |
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* } |
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* |
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* public <b>synchronized</b> Object take() throws InterruptedException { |
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* while (count == 0) |
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* <b>notEmpty.await();</b> |
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* Object x = items[takeptr]; |
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* if (++takeptr == items.length) takeptr = 0; |
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* --count; |
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* <b>notFull.signal();</b> |
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* return x; |
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* } |
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* } |
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* </code></pre> |
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* |
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* <p>If we were to use a standalone {@link Lock} object, such as a |
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* {@link ReentrantLock} then we would write the example as so: |
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* <pre><code> |
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* class BoundedBuffer { |
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* <b>Lock lock = new ReentrantLock();</b> |
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* final Condition notFull = <b>lock.newCondition(); </b> |
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* final Condition notEmpty = <b>lock.newCondition(); </b> |
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* |
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* Object[] items = new Object[100]; |
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* int putptr, takeptr, count; |
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* |
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* public void put(Object x) throws InterruptedException { |
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* <b>lock.lock(); |
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* try {</b> |
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* while (count == items.length) |
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* <b>notFull.await();</b> |
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* items[putptr] = x; |
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* if (++putptr == items.length) putptr = 0; |
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* ++count; |
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* <b>notEmpty.signal();</b> |
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* <b>} finally { |
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* lock.unlock(); |
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* }</b> |
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* } |
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* |
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* public Object take() throws InterruptedException { |
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* <b>lock.lock(); |
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* try {</b> |
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* while (count == 0) |
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* <b>notEmpty.await();</b> |
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* Object x = items[takeptr]; |
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* if (++takeptr == items.length) takeptr = 0; |
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* --count; |
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* <b>notFull.signal();</b> |
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* return x; |
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* <b>} finally { |
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* lock.unlock(); |
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* }</b> |
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* } |
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* } |
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* </code></pre> |
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* |
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* <p>A <tt>Condition</tt> implementation can provide behavior and semantics |
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* that is |
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* different from that of the <tt>Object</tt> monitor methods, such as |
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* guaranteed ordering for notifications, or not requiring a lock to be held |
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* when performing notifications. |
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* If an implementation provides such specialised semantics then the |
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* implementation must document those semantics. |
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* |
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* <p>Note that <tt>Condition</tt> instances are just normal objects and can |
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* themselves be used as the target in a <tt>synchronized</tt> statement, |
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* and can have their own monitor {@link Object#wait wait} and |
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* {@link Object#notify notification} methods invoked. |
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* Acquiring the monitor lock of a <tt>Condition</tt> instance, or using its |
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* monitor methods has no specified relationship with acquiring the |
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* {@link Lock} associated with that <tt>Condition</tt> or the use of it's |
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* {@link #await waiting} and {@link #signal signalling} methods. |
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* It is recommended that to avoid confusion you never use <tt>Condition</tt> |
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* instances in this way, except perhaps within their own implementation. |
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* |
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* <p>Except where noted, passing a <tt>null</tt> value for any parameter |
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* will result in a {@link NullPointerException} being thrown. |
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* |
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* <h3>Implementation Considerations</h3> |
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* |
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* <p>When waiting upon a <tt>Condition</tt> instance a "<em>spurious |
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* wakeup</em>" is permitted to occur, in |
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* general, as a concession to the underlying platform semantics. |
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* This has little practical impact on most application programs as a |
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* <tt>Condition</tt> should always be waited upon in a loop, testing |
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* the state predicate that is being waited for. An implementation is |
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* free to remove the possibility of spurious wakeups but it is |
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* recommended that applications programmers always assume that they can |
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* occur and so always wait in a loop. |
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* |
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* <p>It is recognised that the three forms of condition waiting |
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* (interruptible, non-interruptible, and timed) may differ in their ease of |
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* implementation on some platforms and in their performance characteristics. |
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* In particular, it may be difficult to provide these features and maintain |
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* specific semantics such as ordering guarantees. |
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* Further, the ability to interrupt the actual suspension of the thread may |
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* not always be feasible to implement on all platforms. |
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* <p>Consequently, an implementation is not required to define exactly the |
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* same guarantees or semantics for all three forms of waiting, nor is it |
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* required to support interruption of the actual suspension of the thread. |
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* <p>An implementation is required to |
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* clearly document the semantics and guarantees provided by each of the |
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* waiting methods, and when an implementation does support interruption of |
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* thread suspension then it must obey the interruption semantics as defined |
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* in this interface. |
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* |
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* |
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* @since 1.5 |
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* @spec JSR-166 |
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* @revised $Date: 2003/01/30 22:13:23 $ |
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* @editor $Author: dholmes $ |
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*/ |
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public interface Condition { |
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/** |
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* Causes the current thread to wait until it is signalled or |
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* {@link Thread#interrupt interrupted}. |
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* |
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* <p>The lock associated with this <tt>Condition</tt> is atomically |
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* released and the current thread becomes disabled for thread scheduling |
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* purposes and lies dormant until <em>one</em> of four things happens: |
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* <ul> |
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* <li>Some other thread invokes the {@link #signal} method for this |
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* <tt>Condition</tt> and the current thread happens to be chosen as the |
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* thread to be awakened; or |
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* <li>Some other thread invokes the {@link #signalAll} method for this |
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* <tt>Condition</tt>; or |
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* <li> Some other thread {@link Thread#interrupt interrupts} the current |
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* thread, and interruption of thread suspension is supported; or |
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* <li>A "<em>spurious wakeup</em>" occurs |
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* </ul> |
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* |
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* <p>In all cases, before this method can return the current thread must |
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* re-acquire the lock associated with this condition. When the |
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* thread returns it is <em>guaranteed</em> to hold this lock. |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@link Thread#interrupt interrupted} while waiting |
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* and interruption of thread suspension is supported, |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. It is not specified, in the first |
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* case, whether or not the test for interruption occurs before the lock |
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* is released. |
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* |
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* <p>The circumstances under which the wait completes are mutually |
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* exclusive. For example, if the thread is signalled then it will |
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* never return by throwing {@link InterruptedException}; conversely |
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* a thread that is interrupted and throws {@link InterruptedException} |
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* will never consume a {@link #signal}. |
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* |
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* <p><b>Implementation Considerations</b> |
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* <p>The current thread is assumed to hold the lock associated with this |
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* <tt>Condition</tt> when this method is called. |
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* It is up to the implementation to determine if this is |
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* the case and if not, how to respond. Typically, an exception will be |
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* thrown (such as {@link IllegalMonitorStateException}) and the |
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* implementation must document that fact. |
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* |
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* @throws InterruptedException if the current thread is interrupted (and |
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* interruption of thread suspension is supported). |
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*/ |
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void await() throws InterruptedException; |
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/** |
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* Causes the current thread to wait until it is signalled. |
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* |
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* <p>The lock associated with this condition is atomically |
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* released and the current thread becomes disabled for thread scheduling |
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* purposes and lies dormant until <em>one</em> of three things happens: |
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* <ul> |
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* <li>Some other thread invokes the {@link #signal} method for this |
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* <tt>Condition</tt> and the current thread happens to be chosen as the |
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* thread to be awakened; or |
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* <li>Some other thread invokes the {@link #signalAll} method for this |
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* <tt>Condition</tt>; or |
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* <li>A "<em>spurious wakeup</em>" occurs |
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* </ul> |
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* |
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* <p>In all cases, before this method can return the current thread must |
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* re-acquire the lock associated with this condition. When the |
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* thread returns it is <em>guaranteed</em> to hold this lock. |
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* |
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* <p>If the current thread's interrupt status is set when it enters |
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* this method, or it is {@link Thread#interrupt interrupted} |
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* while waiting, it will continue to wait until signalled. When it finally |
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* returns from this method it's <em>interrupted status</em> will still |
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* be set. |
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* |
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* <p><b>Implementation Considerations</b> |
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* <p>The current thread is assumed to hold the lock associated with this |
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* <tt>Condition</tt> when this method is called. |
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* It is up to the implementation to determine if this is |
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* the case and if not, how to respond. Typically, an exception will be |
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* thrown (such as {@link IllegalMonitorStateException}) and the |
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* implementation must document that fact. |
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* |
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*/ |
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void awaitUninterruptibly(); |
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/** |
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* Causes the current thread to wait until it is signalled or |
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* {@link Thread#interrupt interrupted}, |
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* or the specified waiting time elapses. |
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* |
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* <p>The lock associated with this condition is atomically |
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* released and the current thread becomes disabled for thread scheduling |
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* purposes and lies dormant until <em>one</em> of five things happens: |
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* <ul> |
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* <li>Some other thread invokes the {@link #signal} method for this |
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* <tt>Condition</tt> and the current thread happens to be chosen as the |
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* thread to be awakened; or |
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* <li>Some other thread invokes the {@link #signalAll} method for this |
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* <tt>Condition</tt>; or |
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* <li> Some other thread {@link Thread#interrupt interrupts} the current |
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* thread, and interruption of thread suspension is supported; or |
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* <li>The specified waiting time elapses; or |
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* <li>A "<em>spurious wakeup</em>" occurs. |
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* </ul> |
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* |
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* <p>In all cases, before this method can return the current thread must |
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* re-acquire the lock associated with this condition. When the |
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* thread returns it is <em>guaranteed</em> to hold this lock. |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@link Thread#interrupt interrupted} while waiting |
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* and interruption of thread suspension is supported, |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. It is not specified, in the first |
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* case, whether or not the test for interruption occurs before the lock |
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* is released. |
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* |
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* <p>If the specified time has elapsed then a value less than or |
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* equal to zero is returned. Otherwise this method returns an estimate |
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* of the number of nanoseconds |
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* remaining to wait given the supplied <tt>nanosTimeout</tt> |
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* value. |
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* This value can be used to determine whether and how |
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* long to re-wait in cases where the wait returns but the |
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* condition still does not hold. Typical uses of this method take |
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* the following form: |
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* |
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* <pre> |
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* synchronized boolean aMethod(long timeout, TimeUnit unit) { |
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* long nanosTimeout = unit.toNanos(timeout); |
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* while (!conditionBeingWaitedFor) { |
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* if (nanosTimeout > 0) |
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* nanosTimeout = theCondition.awaitNanos(nanosTimeout); |
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* else |
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* return false; |
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* } |
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* // ... |
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* return true; |
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* } |
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* </pre> |
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* |
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* <p>The circumstances under which the wait completes are mutually |
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* exclusive. For example, if the thread is signalled then it will |
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* never return by throwing {@link InterruptedException}; conversely |
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* a thread that is interrupted and throws {@link InterruptedException} |
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* will never consume a {@link #signal}. |
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* |
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* <p> Design note: This method requires a nanosecond argument so |
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* as to avoid truncation errors in reporting remaining times. |
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* Such precision loss would make it difficult for programmers to |
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* ensure that total waiting times are not systematically shorter |
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* than specified when re-waits occur. |
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* |
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* <p><b>Implementation Considerations</b> |
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* <p>The current thread is assumed to hold the lock associated with this |
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* <tt>Condition</tt> when this method is called. |
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* It is up to the implementation to determine if this is |
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* the case and if not, how to respond. Typically, an exception will be |
| 338 |
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* thrown (such as {@link IllegalMonitorStateException}) and the |
| 339 |
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* implementation must document that fact. |
| 340 |
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* |
| 341 |
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* @param nanosTimeout the maximum time to wait, in nanoseconds |
| 342 |
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* @return an estimate, that |
| 343 |
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* is strictly less than the <tt>nanosTimeout</tt> argument, |
| 344 |
|
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* of the time still remaining when this method returned. |
| 345 |
|
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* If the specified time has elapsed |
| 346 |
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* then a value less than or equal to zero is returned. |
| 347 |
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* |
| 348 |
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* @throws InterruptedException if the current thread is interrupted (and |
| 349 |
|
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* interruption of thread suspension is supported). |
| 350 |
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*/ |
| 351 |
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long awaitNanos(long nanosTimeout) throws InterruptedException; |
| 352 |
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|
| 353 |
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|
| 354 |
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|
| 355 |
|
|
/** |
| 356 |
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* Causes the current thread to wait until it is signalled or interrupted, |
| 357 |
|
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* or the specified absolute deadline passes. |
| 358 |
|
|
* |
| 359 |
|
|
* <p>The lock associated with this condition is atomically |
| 360 |
|
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* released and the current thread becomes disabled for thread scheduling |
| 361 |
|
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* purposes and lies dormant until <em>one</em> of five things happens: |
| 362 |
|
|
* <ul> |
| 363 |
|
|
* <li>Some other thread invokes the {@link #signal} method for this |
| 364 |
|
|
* <tt>Condition</tt> and the current thread happens to be chosen as the |
| 365 |
|
|
* thread to be awakened; or |
| 366 |
|
|
* <li>Some other thread invokes the {@link #signalAll} method for this |
| 367 |
|
|
* <tt>Condition</tt>; or |
| 368 |
|
|
* <li> Some other thread {@link Thread#interrupt interrupts} the current |
| 369 |
|
|
* thread, and interruption of thread suspension is supported; or |
| 370 |
|
|
* <li>The specified absolute deadline passes; or |
| 371 |
|
|
* <li>A "<em>spurious wakeup</em>" occurs. |
| 372 |
|
|
* </ul> |
| 373 |
|
|
* |
| 374 |
|
|
* <p>In all cases, before this method can return the current thread must |
| 375 |
|
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* re-acquire the lock associated with this condition. When the |
| 376 |
|
|
* thread returns it is <em>guaranteed</em> to hold this lock. |
| 377 |
|
|
* |
| 378 |
|
|
* |
| 379 |
|
|
* <p>If the current thread: |
| 380 |
|
|
* <ul> |
| 381 |
|
|
* <li>has its interrupted status set on entry to this method; or |
| 382 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting |
| 383 |
|
|
* and interruption of thread suspension is supported, |
| 384 |
|
|
* </ul> |
| 385 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
| 386 |
|
|
* interrupted status is cleared. It is not specified, in the first |
| 387 |
|
|
* case, whether or not the test for interruption occurs before the lock |
| 388 |
|
|
* is released. |
| 389 |
|
|
* |
| 390 |
|
|
* <p>If the specified deadline has passed then the value <tt>false</tt> |
| 391 |
|
|
* is returned. This return value |
| 392 |
|
|
* This value can be used to determine whether |
| 393 |
|
|
* to re-wait in cases where the wait returns but the |
| 394 |
|
|
* condition still does not hold. Typical uses of this method take |
| 395 |
|
|
* the following form: |
| 396 |
|
|
* <pre> |
| 397 |
|
|
* synchronized boolean aMethod(Date deadline) { |
| 398 |
|
|
* boolean stillWaiting = true; |
| 399 |
|
|
* while (!conditionBeingWaitedFor) { |
| 400 |
|
|
* if (stillwaiting) |
| 401 |
|
|
* stillWaiting = theCondition.awaitUntil(deadline); |
| 402 |
|
|
* else |
| 403 |
|
|
* return false; |
| 404 |
|
|
* } |
| 405 |
|
|
* // ... |
| 406 |
|
|
* return true; |
| 407 |
|
|
* } |
| 408 |
|
|
* |
| 409 |
|
|
* </pre> |
| 410 |
|
|
* <p>The circumstances under which the wait completes are mutually |
| 411 |
|
|
* exclusive. For example, if the thread is signalled then it will |
| 412 |
|
|
* never return by throwing {@link InterruptedException}; conversely |
| 413 |
|
|
* a thread that is interrupted and throws {@link InterruptedException} |
| 414 |
|
|
* will never consume a {@link #signal}. |
| 415 |
|
|
* |
| 416 |
|
|
* <p><b>Implementation Considerations</b> |
| 417 |
|
|
* <p>The current thread is assumed to hold the lock associated with this |
| 418 |
|
|
* <tt>Condition</tt> when this method is called. |
| 419 |
|
|
* It is up to the implementation to determine if this is |
| 420 |
|
|
* the case and if not, how to respond. Typically, an exception will be |
| 421 |
|
|
* thrown (such as {@link IllegalMonitorStateException}) and the |
| 422 |
|
|
* implementation must document that fact. |
| 423 |
|
|
* |
| 424 |
|
|
* |
| 425 |
|
|
* @param deadline the absolute time to wait until |
| 426 |
|
|
* @return <tt>false</tt> if the deadline passed, and |
| 427 |
|
|
* <tt>true</tt> otherwise. |
| 428 |
|
|
* |
| 429 |
|
|
* @throws InterruptedException if the current thread is interrupted (and |
| 430 |
|
|
* interruption of thread suspension is supported). |
| 431 |
|
|
*/ |
| 432 |
|
|
boolean awaitUntil(Date deadline) throws InterruptedException; |
| 433 |
|
|
|
| 434 |
|
|
/** |
| 435 |
|
|
* Wakes up one waiting thread. |
| 436 |
|
|
* |
| 437 |
|
|
* <p>If any threads are waiting on this condition then one |
| 438 |
|
|
* is selected for waking up. That thread must then re-acquire the |
| 439 |
|
|
* lock before returning from <tt>await</tt>. |
| 440 |
|
|
*/ |
| 441 |
|
|
void signal(); |
| 442 |
|
|
|
| 443 |
|
|
/** |
| 444 |
|
|
* Wakes up all waiting threads. |
| 445 |
|
|
* |
| 446 |
|
|
* <p>If any threads are waiting on this condition then they are |
| 447 |
|
|
* all woken up. Each thread must re-acquire the lock before it can |
| 448 |
|
|
* return from <tt>await</tt>. |
| 449 |
|
|
*/ |
| 450 |
|
|
void signalAll(); |
| 451 |
|
|
|
| 452 |
|
|
} |
| 453 |
|
|
|
| 454 |
|
|
|
| 455 |
|
|
|
| 456 |
|
|
|
