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/*
 * 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 jersey.repackaged.com.google.common.util.concurrent;

import static jersey.repackaged.com.google.common.base.Preconditions.checkNotNull;

import jersey.repackaged.com.google.common.annotations.Beta;
import jersey.repackaged.com.google.common.base.Throwables;
import jersey.repackaged.com.google.common.collect.Lists;

import java.util.ArrayList;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;

/**
 * A synchronization abstraction supporting waiting on arbitrary boolean conditions.
 *
 * 

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 * * implicit. * 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}). * *

A thread is said to occupy a monitor if it has entered the monitor but not yet * left. 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 enter and leave operations have the same synchronization * semantics as the built-in Java language synchronization primitives. * *

A call to any of the enter methods with void return type should always be * followed immediately by a try/finally block to ensure that the current thread leaves the * monitor cleanly:

   {@code
 *
 *   monitor.enter();
 *   try {
 *     // do things while occupying the monitor
 *   } finally {
 *     monitor.leave();
 *   }}
* * A call to any of the enter methods with boolean return type should always appear as * the condition of an if statement containing a try/finally block to ensure that the * current thread leaves the monitor cleanly:
   {@code
 *
 *   if (monitor.tryEnter()) {
 *     try {
 *       // do things while occupying the monitor
 *     } finally {
 *       monitor.leave();
 *     }
 *   } else {
 *     // do other things since the monitor was not available
 *   }}
* *

Comparison with {@code synchronized} and {@code ReentrantLock}

* *

The following examples show a simple threadsafe holder expressed using {@code synchronized}, * {@link ReentrantLock}, and {@code Monitor}. * *

{@code synchronized}

* *

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.

   {@code
 *
 *   public class SafeBox {
 *     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();
 *     }
 *   }}
* *

{@code ReentrantLock}

* *

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.

   {@code
 *
 *   public class SafeBox {
 *     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();
 *       }
 *     }
 *   }}
* *

{@code Monitor}

* *

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}.

   {@code
 *
 *   public class SafeBox {
 *     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();
 *       }
 *     }
 *   }}
* * @author Justin T. Sampson * @since 10.0 */ @Beta public final class Monitor { // TODO: Use raw LockSupport or AbstractQueuedSynchronizer instead of ReentrantLock. /** * 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. * *

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; 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(); @Override public final boolean equals(Object other) { // Overridden as final to ensure identity semantics in Monitor.activeGuards. return this == other; } @Override public final int hashCode() { // Overridden as final to ensure identity semantics in Monitor.activeGuards. return super.hashCode(); } } /** * 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}). * This is an ArrayList rather than, say, a HashSet so that iteration and almost all adds don't * incur any object allocation overhead. */ @GuardedBy("lock") private final ArrayList activeGuards = Lists.newArrayListWithCapacity(1); /** * 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) { final ReentrantLock lock = this.lock; if (!fair && lock.tryLock()) { return true; } long startNanos = System.nanoTime(); long timeoutNanos = unit.toNanos(time); long remainingNanos = timeoutNanos; boolean interruptIgnored = false; try { while (true) { try { return lock.tryLock(remainingNanos, TimeUnit.NANOSECONDS); } catch (InterruptedException ignored) { interruptIgnored = true; remainingNanos = (timeoutNanos - (System.nanoTime() - startNanos)); } } } finally { if (interruptIgnored) { 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. * *

Note: 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 reentrant = lock.isHeldByCurrentThread(); boolean success = false; lock.lockInterruptibly(); try { waitInterruptibly(guard, reentrant); success = true; } finally { if (!success) { lock.unlock(); } } } /** * 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 reentrant = lock.isHeldByCurrentThread(); boolean success = false; lock.lock(); try { waitUninterruptibly(guard, reentrant); success = true; } finally { if (!success) { 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, and may be * interrupted. * * @return whether the monitor was entered */ public boolean enterWhen(Guard guard, long time, TimeUnit unit) throws InterruptedException { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } final ReentrantLock lock = this.lock; boolean reentrant = lock.isHeldByCurrentThread(); long remainingNanos; if (!fair && lock.tryLock()) { remainingNanos = unit.toNanos(time); } else { long startNanos = System.nanoTime(); if (!lock.tryLock(time, unit)) { return false; } remainingNanos = unit.toNanos(time) - (System.nanoTime() - startNanos); } boolean satisfied = false; try { satisfied = waitInterruptibly(guard, remainingNanos, reentrant); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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 */ public boolean enterWhenUninterruptibly(Guard guard, long time, TimeUnit unit) { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } final ReentrantLock lock = this.lock; boolean reentrant = lock.isHeldByCurrentThread(); boolean interruptIgnored = false; try { long remainingNanos; if (!fair && lock.tryLock()) { remainingNanos = unit.toNanos(time); } else { long startNanos = System.nanoTime(); long timeoutNanos = unit.toNanos(time); remainingNanos = timeoutNanos; while (true) { try { if (lock.tryLock(remainingNanos, TimeUnit.NANOSECONDS)) { break; } else { return false; } } catch (InterruptedException ignored) { interruptIgnored = true; } finally { remainingNanos = (timeoutNanos - (System.nanoTime() - startNanos)); } } } boolean satisfied = false; try { satisfied = waitUninterruptibly(guard, remainingNanos, reentrant); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } finally { if (interruptIgnored) { 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 */ public boolean enterIf(Guard guard) { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } final ReentrantLock lock = this.lock; lock.lock(); boolean satisfied = false; try { satisfied = guard.isSatisfied(); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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 */ 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 { satisfied = guard.isSatisfied(); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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 */ public boolean enterIf(Guard guard, long time, TimeUnit unit) { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } final ReentrantLock lock = this.lock; if (!enter(time, unit)) { return false; } boolean satisfied = false; try { satisfied = guard.isSatisfied(); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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 */ 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 { satisfied = guard.isSatisfied(); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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. * *

Note: This method disregards the fairness setting of this monitor. * * @return whether the monitor was entered */ 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 { satisfied = guard.isSatisfied(); } finally { if (!satisfied) { lock.unlock(); } } return satisfied; } /** * 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) { throw new IllegalMonitorStateException(); } if (!lock.isHeldByCurrentThread()) { throw new IllegalMonitorStateException(); } waitInterruptibly(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) { throw new IllegalMonitorStateException(); } if (!lock.isHeldByCurrentThread()) { throw new IllegalMonitorStateException(); } waitUninterruptibly(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 { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } if (!lock.isHeldByCurrentThread()) { throw new IllegalMonitorStateException(); } return waitInterruptibly(guard, unit.toNanos(time), 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) { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } if (!lock.isHeldByCurrentThread()) { throw new IllegalMonitorStateException(); } return waitUninterruptibly(guard, unit.toNanos(time), true); } /** * Leaves this monitor. May be called only by a thread currently occupying this monitor. */ public void leave() { final ReentrantLock lock = this.lock; if (!lock.isHeldByCurrentThread()) { throw new IllegalMonitorStateException(); } try { signalConditionsOfSatisfiedGuards(null); } finally { lock.unlock(); } } /** * Returns whether this monitor is using a fair ordering policy. */ public boolean isFair() { return lock.isFair(); } /** * 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) { if (guard.monitor != this) { throw new IllegalMonitorStateException(); } lock.lock(); try { return guard.waiterCount > 0; } finally { lock.unlock(); } } /** * 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(); } } @GuardedBy("lock") private void signalConditionsOfSatisfiedGuards(@Nullable Guard interruptedGuard) { final ArrayList guards = this.activeGuards; final int guardCount = guards.size(); try { for (int i = 0; i < guardCount; i++) { Guard guard = guards.get(i); if ((guard == interruptedGuard) && (guard.waiterCount == 1)) { // That one waiter was just interrupted and is throwing InterruptedException rather than // paying attention to the guard being satisfied, so find another waiter on another guard. continue; } if (guard.isSatisfied()) { guard.condition.signal(); return; } } } catch (Throwable throwable) { for (int i = 0; i < guardCount; i++) { Guard guard = guards.get(i); guard.condition.signalAll(); } throw Throwables.propagate(throwable); } } @GuardedBy("lock") private void incrementWaiters(Guard guard) { int waiters = guard.waiterCount++; if (waiters == 0) { activeGuards.add(guard); } } @GuardedBy("lock") private void decrementWaiters(Guard guard) { int waiters = --guard.waiterCount; if (waiters == 0) { activeGuards.remove(guard); } } @GuardedBy("lock") private void waitInterruptibly(Guard guard, boolean signalBeforeWaiting) throws InterruptedException { if (!guard.isSatisfied()) { if (signalBeforeWaiting) { signalConditionsOfSatisfiedGuards(null); } incrementWaiters(guard); try { final Condition condition = guard.condition; do { try { condition.await(); } catch (InterruptedException interrupt) { try { signalConditionsOfSatisfiedGuards(guard); } catch (Throwable throwable) { Thread.currentThread().interrupt(); throw Throwables.propagate(throwable); } throw interrupt; } } while (!guard.isSatisfied()); } finally { decrementWaiters(guard); } } } @GuardedBy("lock") private void waitUninterruptibly(Guard guard, boolean signalBeforeWaiting) { if (!guard.isSatisfied()) { if (signalBeforeWaiting) { signalConditionsOfSatisfiedGuards(null); } incrementWaiters(guard); try { final Condition condition = guard.condition; do { condition.awaitUninterruptibly(); } while (!guard.isSatisfied()); } finally { decrementWaiters(guard); } } } @GuardedBy("lock") private boolean waitInterruptibly(Guard guard, long remainingNanos, boolean signalBeforeWaiting) throws InterruptedException { if (!guard.isSatisfied()) { if (signalBeforeWaiting) { signalConditionsOfSatisfiedGuards(null); } incrementWaiters(guard); try { final Condition condition = guard.condition; do { if (remainingNanos <= 0) { return false; } try { remainingNanos = condition.awaitNanos(remainingNanos); } catch (InterruptedException interrupt) { try { signalConditionsOfSatisfiedGuards(guard); } catch (Throwable throwable) { Thread.currentThread().interrupt(); throw Throwables.propagate(throwable); } throw interrupt; } } while (!guard.isSatisfied()); } finally { decrementWaiters(guard); } } return true; } @GuardedBy("lock") private boolean waitUninterruptibly(Guard guard, long timeoutNanos, boolean signalBeforeWaiting) { if (!guard.isSatisfied()) { long startNanos = System.nanoTime(); if (signalBeforeWaiting) { signalConditionsOfSatisfiedGuards(null); } boolean interruptIgnored = false; try { incrementWaiters(guard); try { final Condition condition = guard.condition; long remainingNanos = timeoutNanos; do { if (remainingNanos <= 0) { return false; } try { remainingNanos = condition.awaitNanos(remainingNanos); } catch (InterruptedException ignored) { try { signalConditionsOfSatisfiedGuards(guard); } catch (Throwable throwable) { Thread.currentThread().interrupt(); throw Throwables.propagate(throwable); } interruptIgnored = true; remainingNanos = (timeoutNanos - (System.nanoTime() - startNanos)); } } while (!guard.isSatisfied()); } finally { decrementWaiters(guard); } } finally { if (interruptIgnored) { Thread.currentThread().interrupt(); } } } return true; } }





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