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 {@code Object.wait}. * *
A {@code Condition} instance is intrinsically bound to a lock. * To obtain a {@code Condition} instance for a particular {@link Lock} * instance use its {@link Lock#newCondition newCondition()} method. * *
As an example, suppose we have a bounded buffer which supports * {@code put} and {@code take} methods. If a * {@code take} is attempted on an empty buffer, then the thread will block * until an item becomes available; if a {@code put} is attempted on a * full buffer, then the thread will block until a space becomes available. * We would like to keep waiting {@code put} threads and {@code 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 * {@link 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 {@link java.util.concurrent.ArrayBlockingQueue} class provides
* this functionality, so there is no reason to implement this
* sample usage class.)
*
* A {@code Condition} implementation can provide behavior and semantics * that is * different from that of the {@code 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 {@code Condition} instances are just normal objects and can * themselves be used as the target in a {@code synchronized} statement, * and can have their own monitor {@link Object#wait wait} and * {@link Object#notify notification} methods invoked. * Acquiring the monitor lock of a {@code Condition} instance, or using its * monitor methods, has no specified relationship with acquiring the * {@link Lock} associated with that {@code Condition} or the use of its * {@linkplain #await waiting} and {@linkplain #signal signalling} methods. * It is recommended that to avoid confusion you never use {@code Condition} * instances in this way, except perhaps within their own implementation. * *
Except where noted, passing a {@code null} value for any parameter * will result in a {@link NullPointerException} being thrown. * *
When waiting upon a {@code 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 * {@code 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. * * @since 1.5 * @author Doug Lea */ public interface Condition { /** * Causes the current thread to wait until it is signalled or * {@linkplain Thread#interrupt interrupted}. * *
The lock associated with this {@code 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: *
Implementation Considerations * *
The current thread is assumed to hold the lock associated with this * {@code 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 {@link 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 if the current thread is interrupted * (and interruption of thread suspension is supported) */ void await() throws InterruptedException; /** * 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 {@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 * {@code 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 {@link IllegalMonitorStateException}) and the * implementation must document that fact. */ void awaitUninterruptibly(); /** * 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: *
The method returns an estimate of the number of nanoseconds * remaining to wait given the supplied {@code 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: * *
{@code
* 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 * {@code 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 {@link 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. * * @param nanosTimeout the maximum time to wait, in nanoseconds * @return an estimate of the {@code 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 if the current thread is interrupted * (and interruption of thread suspension is supported) */ long awaitNanos(long nanosTimeout) throws 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: *
{@code awaitNanos(unit.toNanos(time)) > 0}
*
* @param time the maximum time to wait
* @param unit the time unit of the {@code time} argument
* @return {@code false} if the waiting time detectably elapsed
* before return from the method, else {@code true}
* @throws InterruptedException if the current thread is interrupted
* (and interruption of thread suspension is supported)
*/
boolean await(long time, TimeUnit unit) throws 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: *
The return value indicates whether the deadline has elapsed, * which can be used as follows: *
{@code
* 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 * {@code 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 {@link 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. * * @param deadline the absolute time to wait until * @return {@code false} if the deadline has elapsed upon return, else * {@code true} * @throws InterruptedException if the current thread is interrupted * (and interruption of thread suspension is supported) */ boolean awaitUntil(Date deadline) throws InterruptedException; /** * 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 {@code await}. * *
Implementation Considerations * *
An implementation may (and typically does) require that the * current thread hold the lock associated with this {@code * 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 {@link * IllegalMonitorStateException} will be thrown. */ void signal(); /** * 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 {@code await}. * *
Implementation Considerations * *
An implementation may (and typically does) require that the * current thread hold the lock associated with this {@code * 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 {@link * IllegalMonitorStateException} will be thrown. */ void signalAll(); }