/*
* Copyright (C) 2010 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.util.concurrent;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.Beta;
import com.google.common.base.Throwables;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import javax.annotation.concurrent.GuardedBy;
/**
* A synchronization abstraction supporting waiting on arbitrary boolean conditions.
*
* <p>This class is intended as a replacement for {@link ReentrantLock}. Code using {@code Monitor}
* is less error-prone and more readable than code using {@code ReentrantLock}, without significant
* performance loss. {@code Monitor} even has the potential for performance gain by optimizing the
* evaluation and signaling of conditions. Signaling is entirely
* <a href="http://en.wikipedia.org/wiki/Monitor_(synchronization)#Implicit_signaling">
* implicit</a>.
* By eliminating explicit signaling, this class can guarantee that only one thread is awakened
* when a condition becomes true (no "signaling storms" due to use of {@link
* java.util.concurrent.locks.Condition#signalAll Condition.signalAll}) and that no signals are lost
* (no "hangs" due to incorrect use of {@link java.util.concurrent.locks.Condition#signal
* Condition.signal}).
*
* <p>A thread is said to <i>occupy</i> a monitor if it has <i>entered</i> the monitor but not yet
* <i>left</i>. Only one thread may occupy a given monitor at any moment. A monitor is also
* reentrant, so a thread may enter a monitor any number of times, and then must leave the same
* number of times. The <i>enter</i> and <i>leave</i> operations have the same synchronization
* semantics as the built-in Java language synchronization primitives.
*
* <p>A call to any of the <i>enter</i> methods with <b>void</b> return type should always be
* followed immediately by a <i>try/finally</i> block to ensure that the current thread leaves the
* monitor cleanly: <pre> {@code
*
* monitor.enter();
* try {
* // do things while occupying the monitor
* } finally {
* monitor.leave();
* }}</pre>
*
* <p>A call to any of the <i>enter</i> methods with <b>boolean</b> return type should always
* appear as the condition of an <i>if</i> statement containing a <i>try/finally</i> block to
* ensure that the current thread leaves the monitor cleanly: <pre> {@code
*
* if (monitor.tryEnter()) {
* try {
* // do things while occupying the monitor
* } finally {
* monitor.leave();
* }
* } else {
* // do other things since the monitor was not available
* }}</pre>
*
* <h2>Comparison with {@code synchronized} and {@code ReentrantLock}</h2>
*
* <p>The following examples show a simple threadsafe holder expressed using {@code synchronized},
* {@link ReentrantLock}, and {@code Monitor}.
*
* <h3>{@code synchronized}</h3>
*
* <p>This version is the fewest lines of code, largely because the synchronization mechanism used
* is built into the language and runtime. But the programmer has to remember to avoid a couple of
* common bugs: The {@code wait()} must be inside a {@code while} instead of an {@code if}, and
* {@code notifyAll()} must be used instead of {@code notify()} because there are two different
* logical conditions being awaited. <pre> {@code
*
* public class SafeBox<V> {
* private V value;
*
* public synchronized V get() throws InterruptedException {
* while (value == null) {
* wait();
* }
* V result = value;
* value = null;
* notifyAll();
* return result;
* }
*
* public synchronized void set(V newValue) throws InterruptedException {
* while (value != null) {
* wait();
* }
* value = newValue;
* notifyAll();
* }
* }}</pre>
*
* <h3>{@code ReentrantLock}</h3>
*
* <p>This version is much more verbose than the {@code synchronized} version, and still suffers
* from the need for the programmer to remember to use {@code while} instead of {@code if}.
* However, one advantage is that we can introduce two separate {@code Condition} objects, which
* allows us to use {@code signal()} instead of {@code signalAll()}, which may be a performance
* benefit. <pre> {@code
*
* public class SafeBox<V> {
* private final ReentrantLock lock = new ReentrantLock();
* private final Condition valuePresent = lock.newCondition();
* private final Condition valueAbsent = lock.newCondition();
* private V value;
*
* public V get() throws InterruptedException {
* lock.lock();
* try {
* while (value == null) {
* valuePresent.await();
* }
* V result = value;
* value = null;
* valueAbsent.signal();
* return result;
* } finally {
* lock.unlock();
* }
* }
*
* public void set(V newValue) throws InterruptedException {
* lock.lock();
* try {
* while (value != null) {
* valueAbsent.await();
* }
* value = newValue;
* valuePresent.signal();
* } finally {
* lock.unlock();
* }
* }
* }}</pre>
*
* <h3>{@code Monitor}</h3>
*
* <p>This version adds some verbosity around the {@code Guard} objects, but removes that same
* verbosity, and more, from the {@code get} and {@code set} methods. {@code Monitor} implements the
* same efficient signaling as we had to hand-code in the {@code ReentrantLock} version above.
* Finally, the programmer no longer has to hand-code the wait loop, and therefore doesn't have to
* remember to use {@code while} instead of {@code if}. <pre> {@code
*
* public class SafeBox<V> {
* private final Monitor monitor = new Monitor();
* private final Monitor.Guard valuePresent = new Monitor.Guard(monitor) {
* public boolean isSatisfied() {
* return value != null;
* }
* };
* private final Monitor.Guard valueAbsent = new Monitor.Guard(monitor) {
* public boolean isSatisfied() {
* return value == null;
* }
* };
* private V value;
*
* public V get() throws InterruptedException {
* monitor.enterWhen(valuePresent);
* try {
* V result = value;
* value = null;
* return result;
* } finally {
* monitor.leave();
* }
* }
*
* public void set(V newValue) throws InterruptedException {
* monitor.enterWhen(valueAbsent);
* try {
* value = newValue;
* } finally {
* monitor.leave();
* }
* }
* }}</pre>
*
* @author Justin T. Sampson
* @author Martin Buchholz
* @since 10.0
*/
@Beta
public final class Monitor {
// TODO(user): Use raw LockSupport or AbstractQueuedSynchronizer instead of ReentrantLock.
// TODO(user): "Port" jsr166 tests for ReentrantLock.
//
// TODO(user): Change API to make it impossible to use a Guard with the "wrong" monitor,
// by making the monitor implicit, and to eliminate other sources of IMSE.
// Imagine:
// guard.lock();
// try { /* monitor locked and guard satisfied here */ }
// finally { guard.unlock(); }
// Here are Justin's design notes about this:
//
// This idea has come up from time to time, and I think one of my
// earlier versions of Monitor even did something like this. I ended
// up strongly favoring the current interface.
//
// I probably can't remember all the reasons (it's possible you
// could find them in the code review archives), but here are a few:
//
// 1. What about leaving/unlocking? Are you going to do
// guard.enter() paired with monitor.leave()? That might get
// confusing. It's nice for the finally block to look as close as
// possible to the thing right before the try. You could have
// guard.leave(), but that's a little odd as well because the
// guard doesn't have anything to do with leaving. You can't
// really enforce that the guard you're leaving is the same one
// you entered with, and it doesn't actually matter.
//
// 2. Since you can enter the monitor without a guard at all, some
// places you'll have monitor.enter()/monitor.leave() and other
// places you'll have guard.enter()/guard.leave() even though
// it's the same lock being acquired underneath. Always using
// monitor.enterXXX()/monitor.leave() will make it really clear
// which lock is held at any point in the code.
//
// 3. I think "enterWhen(notEmpty)" reads better than "notEmpty.enter()".
//
// TODO(user): Implement ReentrantLock features:
// - toString() method
// - getOwner() method
// - getQueuedThreads() method
// - getWaitingThreads(Guard) method
// - implement Serializable
// - redo the API to be as close to identical to ReentrantLock as possible,
// since, after all, this class is also a reentrant mutual exclusion lock!?
/*
* One of the key challenges of this class is to prevent lost signals, while trying hard to
* minimize unnecessary signals. One simple and correct algorithm is to signal some other
* waiter with a satisfied guard (if one exists) whenever any thread occupying the monitor
* exits the monitor, either by unlocking all of its held locks, or by starting to wait for a
* guard. This includes exceptional exits, so all control paths involving signalling must be
* protected by a finally block.
*
* Further optimizations of this algorithm become increasingly subtle. A wait that terminates
* without the guard being satisfied (due to timeout, but not interrupt) can then immediately
* exit the monitor without signalling. If it timed out without being signalled, it does not
* need to "pass on" the signal to another thread. If it *was* signalled, then its guard must
* have been satisfied at the time of signal, and has since been modified by some other thread
* to be non-satisfied before reacquiring the lock, and that other thread takes over the
* responsibility of signaling the next waiter.
*
* Unlike the underlying Condition, if we are not careful, an interrupt *can* cause a signal to
* be lost, because the signal may be sent to a condition whose sole waiter has just been
* interrupted.
*
* Imagine a monitor with multiple guards. A thread enters the monitor, satisfies all the
* guards, and leaves, calling signalNextWaiter. With traditional locks and conditions, all
* the conditions need to be signalled because it is not known which if any of them have
* waiters (and hasWaiters can't be used reliably because of a check-then-act race). With our
* Monitor guards, we only signal the first active guard that is satisfied. But the
* corresponding thread may have already been interrupted and is waiting to reacquire the lock
* while still registered in activeGuards, in which case the signal is a no-op, and the
* bigger-picture signal is lost unless interrupted threads take special action by
* participating in the signal-passing game.
*/
/**
* A boolean condition for which a thread may wait. A {@code Guard} is associated with a single
* {@code Monitor}. The monitor may check the guard at arbitrary times from any thread occupying
* the monitor, so code should not be written to rely on how often a guard might or might not be
* checked.
*
* <p>If a {@code Guard} is passed into any method of a {@code Monitor} other than the one it is
* associated with, an {@link IllegalMonitorStateException} is thrown.
*
* @since 10.0
*/
@Beta
public abstract static class Guard {
final Monitor monitor;
final Condition condition;
@GuardedBy("monitor.lock")
int waiterCount = 0;
/** The next active guard */
@GuardedBy("monitor.lock")
Guard next;
protected Guard(Monitor monitor) {
this.monitor = checkNotNull(monitor, "monitor");
this.condition = monitor.lock.newCondition();
}
/**
* Evaluates this guard's boolean condition. This method is always called with the associated
* monitor already occupied. Implementations of this method must depend only on state protected
* by the associated monitor, and must not modify that state.
*/
public abstract boolean isSatisfied();
}
/**
* Whether this monitor is fair.
*/
private final boolean fair;
/**
* The lock underlying this monitor.
*/
private final ReentrantLock lock;
/**
* The guards associated with this monitor that currently have waiters ({@code waiterCount > 0}).
* A linked list threaded through the Guard.next field.
*/
@GuardedBy("lock")
private Guard activeGuards = null;
/**
* Creates a monitor with a non-fair (but fast) ordering policy. Equivalent to {@code
* Monitor(false)}.
*/
public Monitor() {
this(false);
}
/**
* Creates a monitor with the given ordering policy.
*
* @param fair whether this monitor should use a fair ordering policy rather than a non-fair (but
* fast) one
*/
public Monitor(boolean fair) {
this.fair = fair;
this.lock = new ReentrantLock(fair);
}
/**
* Enters this monitor. Blocks indefinitely.
*/
public void enter() {
lock.lock();
}
/**
* Enters this monitor. Blocks indefinitely, but may be interrupted.
*/
public void enterInterruptibly() throws InterruptedException {
lock.lockInterruptibly();
}
/**
* Enters this monitor. Blocks at most the given time.
*
* @return whether the monitor was entered
*/
public boolean enter(long time, TimeUnit unit) {
long timeoutNanos = unit.toNanos(time);
final ReentrantLock lock = this.lock;
if (!fair && lock.tryLock()) {
return true;
}
long deadline = System.nanoTime() + timeoutNanos;
boolean interrupted = Thread.interrupted();
try {
while (true) {
try {
return lock.tryLock(timeoutNanos, TimeUnit.NANOSECONDS);
} catch (InterruptedException interrupt) {
interrupted = true;
timeoutNanos = deadline - System.nanoTime();
}
}
} finally {
if (interrupted) {
Thread.currentThread().interrupt();
}
}
}
/**
* Enters this monitor. Blocks at most the given time, and may be interrupted.
*
* @return whether the monitor was entered
*/
public boolean enterInterruptibly(long time, TimeUnit unit) throws InterruptedException {
return lock.tryLock(time, unit);
}
/**
* Enters this monitor if it is possible to do so immediately. Does not block.
*
* <p><b>Note:</b> This method disregards the fairness setting of this monitor.
*
* @return whether the monitor was entered
*/
public boolean tryEnter() {
return lock.tryLock();
}
/**
* Enters this monitor when the guard is satisfied. Blocks indefinitely, but may be interrupted.
*/
public void enterWhen(Guard guard) throws InterruptedException {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
lock.lockInterruptibly();
boolean satisfied = false;
try {
if (!guard.isSatisfied()) {
await(guard, signalBeforeWaiting);
}
satisfied = true;
} finally {
if (!satisfied) {
leave();
}
}
}
/**
* Enters this monitor when the guard is satisfied. Blocks indefinitely.
*/
public void enterWhenUninterruptibly(Guard guard) {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
lock.lock();
boolean satisfied = false;
try {
if (!guard.isSatisfied()) {
awaitUninterruptibly(guard, signalBeforeWaiting);
}
satisfied = true;
} finally {
if (!satisfied) {
leave();
}
}
}
/**
* Enters this monitor when the guard is satisfied. Blocks at most the given time, including both
* the time to acquire the lock and the time to wait for the guard to be satisfied, and may be
* interrupted.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterWhen(Guard guard, long time, TimeUnit unit) throws InterruptedException {
long timeoutNanos = unit.toNanos(time);
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
boolean reentrant = lock.isHeldByCurrentThread();
if (fair || !lock.tryLock()) {
long deadline = System.nanoTime() + timeoutNanos;
if (!lock.tryLock(time, unit)) {
return false;
}
timeoutNanos = deadline - System.nanoTime();
}
boolean satisfied = false;
boolean threw = true;
try {
satisfied = guard.isSatisfied() || awaitNanos(guard, timeoutNanos, reentrant);
threw = false;
return satisfied;
} finally {
if (!satisfied) {
try {
// Don't need to signal if timed out, but do if interrupted
if (threw && !reentrant) {
signalNextWaiter();
}
} finally {
lock.unlock();
}
}
}
}
/**
* Enters this monitor when the guard is satisfied. Blocks at most the given time, including
* both the time to acquire the lock and the time to wait for the guard to be satisfied.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterWhenUninterruptibly(Guard guard, long time, TimeUnit unit) {
long timeoutNanos = unit.toNanos(time);
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
long deadline = System.nanoTime() + timeoutNanos;
boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
boolean interrupted = Thread.interrupted();
try {
if (fair || !lock.tryLock()) {
boolean locked = false;
do {
try {
locked = lock.tryLock(timeoutNanos, TimeUnit.NANOSECONDS);
if (!locked) {
return false;
}
} catch (InterruptedException interrupt) {
interrupted = true;
}
timeoutNanos = deadline - System.nanoTime();
} while (!locked);
}
boolean satisfied = false;
try {
while (true) {
try {
return satisfied = guard.isSatisfied()
|| awaitNanos(guard, timeoutNanos, signalBeforeWaiting);
} catch (InterruptedException interrupt) {
interrupted = true;
signalBeforeWaiting = false;
timeoutNanos = deadline - System.nanoTime();
}
}
} finally {
if (!satisfied) {
lock.unlock(); // No need to signal if timed out
}
}
} finally {
if (interrupted) {
Thread.currentThread().interrupt();
}
}
}
/**
* Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but
* does not wait for the guard to be satisfied.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterIf(Guard guard) {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
lock.lock();
boolean satisfied = false;
try {
return satisfied = guard.isSatisfied();
} finally {
if (!satisfied) {
lock.unlock();
}
}
}
/**
* Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but does
* not wait for the guard to be satisfied, and may be interrupted.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterIfInterruptibly(Guard guard) throws InterruptedException {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
boolean satisfied = false;
try {
return satisfied = guard.isSatisfied();
} finally {
if (!satisfied) {
lock.unlock();
}
}
}
/**
* Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
* lock, but does not wait for the guard to be satisfied.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterIf(Guard guard, long time, TimeUnit unit) {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
if (!enter(time, unit)) {
return false;
}
boolean satisfied = false;
try {
return satisfied = guard.isSatisfied();
} finally {
if (!satisfied) {
lock.unlock();
}
}
}
/**
* Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
* lock, but does not wait for the guard to be satisfied, and may be interrupted.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean enterIfInterruptibly(Guard guard, long time, TimeUnit unit)
throws InterruptedException {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
if (!lock.tryLock(time, unit)) {
return false;
}
boolean satisfied = false;
try {
return satisfied = guard.isSatisfied();
} finally {
if (!satisfied) {
lock.unlock();
}
}
}
/**
* Enters this monitor if it is possible to do so immediately and the guard is satisfied. Does not
* block acquiring the lock and does not wait for the guard to be satisfied.
*
* <p><b>Note:</b> This method disregards the fairness setting of this monitor.
*
* @return whether the monitor was entered, which guarantees that the guard is now satisfied
*/
public boolean tryEnterIf(Guard guard) {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
final ReentrantLock lock = this.lock;
if (!lock.tryLock()) {
return false;
}
boolean satisfied = false;
try {
return satisfied = guard.isSatisfied();
} finally {
if (!satisfied) {
lock.unlock();
}
}
}
/**
* Waits for the guard to be satisfied. Waits indefinitely, but may be interrupted. May be
* called only by a thread currently occupying this monitor.
*/
public void waitFor(Guard guard) throws InterruptedException {
if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
throw new IllegalMonitorStateException();
}
if (!guard.isSatisfied()) {
await(guard, true);
}
}
/**
* Waits for the guard to be satisfied. Waits indefinitely. May be called only by a thread
* currently occupying this monitor.
*/
public void waitForUninterruptibly(Guard guard) {
if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
throw new IllegalMonitorStateException();
}
if (!guard.isSatisfied()) {
awaitUninterruptibly(guard, true);
}
}
/**
* Waits for the guard to be satisfied. Waits at most the given time, and may be interrupted.
* May be called only by a thread currently occupying this monitor.
*
* @return whether the guard is now satisfied
*/
public boolean waitFor(Guard guard, long time, TimeUnit unit) throws InterruptedException {
long timeoutNanos = unit.toNanos(time);
if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
throw new IllegalMonitorStateException();
}
return guard.isSatisfied() || awaitNanos(guard, timeoutNanos, true);
}
/**
* Waits for the guard to be satisfied. Waits at most the given time. May be called only by a
* thread currently occupying this monitor.
*
* @return whether the guard is now satisfied
*/
public boolean waitForUninterruptibly(Guard guard, long time, TimeUnit unit) {
long timeoutNanos = unit.toNanos(time);
if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
throw new IllegalMonitorStateException();
}
if (guard.isSatisfied()) {
return true;
}
boolean signalBeforeWaiting = true;
long deadline = System.nanoTime() + timeoutNanos;
boolean interrupted = Thread.interrupted();
try {
while (true) {
try {
return awaitNanos(guard, timeoutNanos, signalBeforeWaiting);
} catch (InterruptedException interrupt) {
interrupted = true;
if (guard.isSatisfied()) {
return true;
}
signalBeforeWaiting = false;
timeoutNanos = deadline - System.nanoTime();
}
}
} finally {
if (interrupted) {
Thread.currentThread().interrupt();
}
}
}
/**
* Leaves this monitor. May be called only by a thread currently occupying this monitor.
*/
public void leave() {
final ReentrantLock lock = this.lock;
try {
// No need to signal if we will still be holding the lock when we return
if (lock.getHoldCount() == 1) {
signalNextWaiter();
}
} finally {
lock.unlock(); // Will throw IllegalMonitorStateException if not held
}
}
/**
* Returns whether this monitor is using a fair ordering policy.
*/
public boolean isFair() {
return fair;
}
/**
* Returns whether this monitor is occupied by any thread. This method is designed for use in
* monitoring of the system state, not for synchronization control.
*/
public boolean isOccupied() {
return lock.isLocked();
}
/**
* Returns whether the current thread is occupying this monitor (has entered more times than it
* has left).
*/
public boolean isOccupiedByCurrentThread() {
return lock.isHeldByCurrentThread();
}
/**
* Returns the number of times the current thread has entered this monitor in excess of the number
* of times it has left. Returns 0 if the current thread is not occupying this monitor.
*/
public int getOccupiedDepth() {
return lock.getHoldCount();
}
/**
* Returns an estimate of the number of threads waiting to enter this monitor. The value is only
* an estimate because the number of threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in monitoring of the system state,
* not for synchronization control.
*/
public int getQueueLength() {
return lock.getQueueLength();
}
/**
* Returns whether any threads are waiting to enter this monitor. Note that because cancellations
* may occur at any time, a {@code true} return does not guarantee that any other thread will ever
* enter this monitor. This method is designed primarily for use in monitoring of the system
* state.
*/
public boolean hasQueuedThreads() {
return lock.hasQueuedThreads();
}
/**
* Queries whether the given thread is waiting to enter this monitor. Note that because
* cancellations may occur at any time, a {@code true} return does not guarantee that this thread
* will ever enter this monitor. This method is designed primarily for use in monitoring of the
* system state.
*/
public boolean hasQueuedThread(Thread thread) {
return lock.hasQueuedThread(thread);
}
/**
* Queries whether any threads are waiting for the given guard to become satisfied. Note that
* because timeouts and interrupts may occur at any time, a {@code true} return does not guarantee
* that the guard becoming satisfied in the future will awaken any threads. This method is
* designed primarily for use in monitoring of the system state.
*/
public boolean hasWaiters(Guard guard) {
return getWaitQueueLength(guard) > 0;
}
/**
* Returns an estimate of the number of threads waiting for the given guard to become satisfied.
* Note that because timeouts and interrupts may occur at any time, the estimate serves only as an
* upper bound on the actual number of waiters. This method is designed for use in monitoring of
* the system state, not for synchronization control.
*/
public int getWaitQueueLength(Guard guard) {
if (guard.monitor != this) {
throw new IllegalMonitorStateException();
}
lock.lock();
try {
return guard.waiterCount;
} finally {
lock.unlock();
}
}
/**
* Signals some other thread waiting on a satisfied guard, if one exists.
*
* We manage calls to this method carefully, to signal only when necessary, but never losing a
* signal, which is the classic problem of this kind of concurrency construct. We must signal if
* the current thread is about to relinquish the lock and may have changed the state protected by
* the monitor, thereby causing some guard to be satisfied.
*
* In addition, any thread that has been signalled when its guard was satisfied acquires the
* responsibility of signalling the next thread when it again relinquishes the lock. Unlike a
* normal Condition, there is no guarantee that an interrupted thread has not been signalled,
* since the concurrency control must manage multiple Conditions. So this method must generally
* be called when waits are interrupted.
*
* On the other hand, if a signalled thread wakes up to discover that its guard is still not
* satisfied, it does *not* need to call this method before returning to wait. This can only
* happen due to spurious wakeup (ignorable) or another thread acquiring the lock before the
* current thread can and returning the guard to the unsatisfied state. In the latter case the
* other thread (last thread modifying the state protected by the monitor) takes over the
* responsibility of signalling the next waiter.
*
* This method must not be called from within a beginWaitingFor/endWaitingFor block, or else the
* current thread's guard might be mistakenly signalled, leading to a lost signal.
*/
@GuardedBy("lock")
private void signalNextWaiter() {
for (Guard guard = activeGuards; guard != null; guard = guard.next) {
if (isSatisfied(guard)) {
guard.condition.signal();
break;
}
}
}
/**
* Exactly like signalNextWaiter, but caller guarantees that guardToSkip need not be considered,
* because caller has previously checked that guardToSkip.isSatisfied() returned false.
* An optimization for the case that guardToSkip.isSatisfied() may be expensive.
*
* We decided against using this method, since in practice, isSatisfied() is likely to be very
* cheap (typically one field read). Resurrect this method if you find that not to be true.
*/
// @GuardedBy("lock")
// private void signalNextWaiterSkipping(Guard guardToSkip) {
// for (Guard guard = activeGuards; guard != null; guard = guard.next) {
// if (guard != guardToSkip && isSatisfied(guard)) {
// guard.condition.signal();
// break;
// }
// }
// }
/**
* Exactly like guard.isSatisfied(), but in addition signals all waiting threads in the
* (hopefully unlikely) event that isSatisfied() throws.
*/
@GuardedBy("lock")
private boolean isSatisfied(Guard guard) {
try {
return guard.isSatisfied();
} catch (Throwable throwable) {
signalAllWaiters();
throw Throwables.propagate(throwable);
}
}
/**
* Signals all threads waiting on guards.
*/
@GuardedBy("lock")
private void signalAllWaiters() {
for (Guard guard = activeGuards; guard != null; guard = guard.next) {
guard.condition.signalAll();
}
}
/**
* Records that the current thread is about to wait on the specified guard.
*/
@GuardedBy("lock")
private void beginWaitingFor(Guard guard) {
int waiters = guard.waiterCount++;
if (waiters == 0) {
// push guard onto activeGuards
guard.next = activeGuards;
activeGuards = guard;
}
}
/**
* Records that the current thread is no longer waiting on the specified guard.
*/
@GuardedBy("lock")
private void endWaitingFor(Guard guard) {
int waiters = --guard.waiterCount;
if (waiters == 0) {
// unlink guard from activeGuards
for (Guard p = activeGuards, pred = null;; pred = p, p = p.next) {
if (p == guard) {
if (pred == null) {
activeGuards = p.next;
} else {
pred.next = p.next;
}
p.next = null; // help GC
break;
}
}
}
}
/*
* Methods that loop waiting on a guard's condition until the guard is satisfied, while
* recording this fact so that other threads know to check our guard and signal us.
* It's caller's responsibility to ensure that the guard is *not* currently satisfied.
*/
@GuardedBy("lock")
private void await(Guard guard, boolean signalBeforeWaiting)
throws InterruptedException {
if (signalBeforeWaiting) {
signalNextWaiter();
}
beginWaitingFor(guard);
try {
do {
guard.condition.await();
} while (!guard.isSatisfied());
} finally {
endWaitingFor(guard);
}
}
@GuardedBy("lock")
private void awaitUninterruptibly(Guard guard, boolean signalBeforeWaiting) {
if (signalBeforeWaiting) {
signalNextWaiter();
}
beginWaitingFor(guard);
try {
do {
guard.condition.awaitUninterruptibly();
} while (!guard.isSatisfied());
} finally {
endWaitingFor(guard);
}
}
@GuardedBy("lock")
private boolean awaitNanos(Guard guard, long nanos, boolean signalBeforeWaiting)
throws InterruptedException {
if (signalBeforeWaiting) {
signalNextWaiter();
}
beginWaitingFor(guard);
try {
do {
if (nanos < 0L) {
return false;
}
nanos = guard.condition.awaitNanos(nanos);
} while (!guard.isSatisfied());
return true;
} finally {
endWaitingFor(guard);
}
}
}