Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent.locks; import java.util.Collection; import java.util.concurrent.TimeUnit; import jdk.internal.vm.annotation.ReservedStackAccess; /** * A reentrant mutual exclusion {@link Lock} with the same basic * behavior and semantics as the implicit monitor lock accessed using * {@code synchronized} methods and statements, but with extended * capabilities. * * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last * successfully locking, but not yet unlocking it. A thread invoking * {@code lock} will return, successfully acquiring the lock, when * the lock is not owned by another thread. The method will return * immediately if the current thread already owns the lock. This can * be checked using methods {@link #isHeldByCurrentThread}, and {@link * #getHoldCount}. * * <p>The constructor for this class accepts an optional * <em>fairness</em> parameter. When set {@code true}, under * contention, locks favor granting access to the longest-waiting * thread. Otherwise this lock does not guarantee any particular * access order. Programs using fair locks accessed by many threads * may display lower overall throughput (i.e., are slower; often much * slower) than those using the default setting, but have smaller * variances in times to obtain locks and guarantee lack of * starvation. Note however, that fairness of locks does not guarantee * fairness of thread scheduling. Thus, one of many threads using a * fair lock may obtain it multiple times in succession while other * active threads are not progressing and not currently holding the * lock. * Also note that the untimed {@link #tryLock()} method does not * honor the fairness setting. It will succeed if the lock * is available even if other threads are waiting. * * <p>It is recommended practice to <em>always</em> immediately * follow a call to {@code lock} with a {@code try} block, most * typically in a before/after construction such as: * * <pre> {@code * class X { * private final ReentrantLock lock = new ReentrantLock(); * // ... * * public void m() { * lock.lock(); // block until condition holds * try { * // ... method body * } finally { * lock.unlock() * } * } * }}</pre> * * <p>In addition to implementing the {@link Lock} interface, this * class defines a number of {@code public} and {@code protected} * methods for inspecting the state of the lock. Some of these * methods are only useful for instrumentation and monitoring. * * <p>Serialization of this class behaves in the same way as built-in * locks: a deserialized lock is in the unlocked state, regardless of * its state when serialized. * * <p>This lock supports a maximum of 2147483647 recursive locks by * the same thread. Attempts to exceed this limit result in * {@link Error} throws from locking methods. * * @since 1.5 * @author Doug Lea */ public class ReentrantLock implements Lock, java.io.Serializable { private static final long serialVersionUID = 7373984872572414699L; /** Synchronizer providing all implementation mechanics */ private final Sync sync; /** * Base of synchronization control for this lock. Subclassed * into fair and nonfair versions below. Uses AQS state to * represent the number of holds on the lock. */ abstract static class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = -5179523762034025860L; /** * Performs non-fair tryLock. tryAcquire is implemented in * subclasses, but both need nonfair try for trylock method. */ @ReservedStackAccess final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) // overflow throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; } @ReservedStackAccess protected final boolean tryRelease(int releases) { int c = getState() - releases; if (Thread.currentThread() != getExclusiveOwnerThread()) throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) { free = true; setExclusiveOwnerThread(null); } setState(c); return free; } protected final boolean isHeldExclusively() { // While we must in general read state before owner, // we don't need to do so to check if current thread is owner return getExclusiveOwnerThread() == Thread.currentThread(); } final ConditionObject newCondition() { return new ConditionObject(); } // Methods relayed from outer class final Thread getOwner() { return getState() == 0 ? null : getExclusiveOwnerThread(); } final int getHoldCount() { return isHeldExclusively() ? getState() : 0; } final boolean isLocked() { return getState() != 0; } /** * Reconstitutes the instance from a stream (that is, deserializes it). */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { s.defaultReadObject(); setState(0); // reset to unlocked state } } /** * Sync object for non-fair locks */ static final class NonfairSync extends Sync { private static final long serialVersionUID = 7316153563782823691L; protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); } } /** * Sync object for fair locks */ static final class FairSync extends Sync { private static final long serialVersionUID = -3000897897090466540L; /** * Fair version of tryAcquire. Don't grant access unless * recursive call or no waiters or is first. */ @ReservedStackAccess protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; } } /** * Creates an instance of {@code ReentrantLock}. * This is equivalent to using {@code ReentrantLock(false)}. */ public ReentrantLock() { sync = new NonfairSync(); } /** * Creates an instance of {@code ReentrantLock} with the * given fairness policy. * * @param fair {@code true} if this lock should use a fair ordering policy */ public ReentrantLock(boolean fair) { sync = fair ? new FairSync() : new NonfairSync(); } /** * Acquires the lock. * * <p>Acquires the lock if it is not held by another thread and returns * immediately, setting the lock hold count to one. * * <p>If the current thread already holds the lock then the hold * count is incremented by one and the method returns immediately. * * <p>If the lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until the lock has been acquired, * at which time the lock hold count is set to one. */ public void lock() { sync.acquire(1); } /** * Acquires the lock unless the current thread is * {@linkplain Thread#interrupt interrupted}. * * <p>Acquires the lock if it is not held by another thread and returns * immediately, setting the lock hold count to one. * * <p>If the current thread already holds this lock then the hold count * is incremented by one and the method returns immediately. * * <p>If the lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until one of two things happens: * * <ul> * * <li>The lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} the * current thread. * * </ul> * * <p>If the lock is acquired by the current thread then the lock hold * count is set to one. * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; or * * <li>is {@linkplain Thread#interrupt interrupted} while acquiring * the lock, * * </ul> * * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to the * interrupt over normal or reentrant acquisition of the lock. * * @throws InterruptedException if the current thread is interrupted */ public void lockInterruptibly() throws InterruptedException { sync.acquireInterruptibly(1); } /** * Acquires the lock only if it is not held by another thread at the time * of invocation. * * <p>Acquires the lock if it is not held by another thread and * returns immediately with the value {@code true}, setting the * lock hold count to one. Even when this lock has been set to use a * fair ordering policy, a call to {@code tryLock()} <em>will</em> * immediately acquire the lock if it is available, whether or not * other threads are currently waiting for the lock. * This "barging" behavior can be useful in certain * circumstances, even though it breaks fairness. If you want to honor * the fairness setting for this lock, then use * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS)} * which is almost equivalent (it also detects interruption). * * <p>If the current thread already holds this lock then the hold * count is incremented by one and the method returns {@code true}. * * <p>If the lock is held by another thread then this method will return * immediately with the value {@code false}. * * @return {@code true} if the lock was free and was acquired by the * current thread, or the lock was already held by the current * thread; and {@code false} otherwise */ public boolean tryLock() { return sync.nonfairTryAcquire(1); } /** * Acquires the lock if it is not held by another thread within the given * waiting time and the current thread has not been * {@linkplain Thread#interrupt interrupted}. * * <p>Acquires the lock if it is not held by another thread and returns * immediately with the value {@code true}, setting the lock hold count * to one. If this lock has been set to use a fair ordering policy then * an available lock <em>will not</em> be acquired if any other threads * are waiting for the lock. This is in contrast to the {@link #tryLock()} * method. If you want a timed {@code tryLock} that does permit barging on * a fair lock then combine the timed and un-timed forms together: * * <pre> {@code * if (lock.tryLock() || * lock.tryLock(timeout, unit)) { * ... * }}</pre> * * <p>If the current thread * already holds this lock then the hold count is incremented by one and * the method returns {@code true}. * * <p>If the lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until one of three things happens: * * <ul> * * <li>The lock is acquired by the current thread; or * * <li>Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * * <li>The specified waiting time elapses * * </ul> * * <p>If the lock is acquired then the value {@code true} is returned and * the lock hold count is set to one. * * <p>If the current thread: * * <ul> * * <li>has its interrupted status set on entry to this method; or * * <li>is {@linkplain Thread#interrupt interrupted} while * acquiring the lock, * * </ul> * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * * <p>If the specified waiting time elapses then the value {@code false} * is returned. If the time is less than or equal to zero, the method * will not wait at all. * * <p>In this implementation, as this method is an explicit * interruption point, preference is given to responding to the * interrupt over normal or reentrant acquisition of the lock, and * over reporting the elapse of the waiting time. * * @param timeout the time to wait for the lock * @param unit the time unit of the timeout argument * @return {@code true} if the lock was free and was acquired by the * current thread, or the lock was already held by the current * thread; and {@code false} if the waiting time elapsed before * the lock could be acquired * @throws InterruptedException if the current thread is interrupted * @throws NullPointerException if the time unit is null */ public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireNanos(1, unit.toNanos(timeout)); } /** * Attempts to release this lock. * * <p>If the current thread is the holder of this lock then the hold * count is decremented. If the hold count is now zero then the lock * is released. If the current thread is not the holder of this * lock then {@link IllegalMonitorStateException} is thrown. * * @throws IllegalMonitorStateException if the current thread does not * hold this lock */ public void unlock() { sync.release(1); } /** * Returns a {@link Condition} instance for use with this * {@link Lock} instance. * * <p>The returned {@link Condition} instance supports the same * usages as do the {@link Object} monitor methods ({@link * Object#wait() wait}, {@link Object#notify notify}, and {@link * Object#notifyAll notifyAll}) when used with the built-in * monitor lock. * * <ul> * * <li>If this lock is not held when any of the {@link Condition} * {@linkplain Condition#await() waiting} or {@linkplain * Condition#signal signalling} methods are called, then an {@link * IllegalMonitorStateException} is thrown. * * <li>When the condition {@linkplain Condition#await() waiting} * methods are called the lock is released and, before they * return, the lock is reacquired and the lock hold count restored * to what it was when the method was called. * * <li>If a thread is {@linkplain Thread#interrupt interrupted} * while waiting then the wait will terminate, an {@link * InterruptedException} will be thrown, and the thread's * interrupted status will be cleared. * * <li>Waiting threads are signalled in FIFO order. * * <li>The ordering of lock reacquisition for threads returning * from waiting methods is the same as for threads initially * acquiring the lock, which is in the default case not specified, * but for <em>fair</em> locks favors those threads that have been * waiting the longest. * * </ul> * * @return the Condition object */ public Condition newCondition() { return sync.newCondition(); } /** * Queries the number of holds on this lock by the current thread. * * <p>A thread has a hold on a lock for each lock action that is not * matched by an unlock action. * * <p>The hold count information is typically only used for testing and * debugging purposes. For example, if a certain section of code should * not be entered with the lock already held then we can assert that * fact: * * <pre> {@code * class X { * ReentrantLock lock = new ReentrantLock(); * // ... * public void m() { * assert lock.getHoldCount() == 0; * lock.lock(); * try { * // ... method body * } finally { * lock.unlock(); * } * } * }}</pre> * * @return the number of holds on this lock by the current thread, * or zero if this lock is not held by the current thread */ public int getHoldCount() { return sync.getHoldCount(); } /** * Queries if this lock is held by the current thread. * * <p>Analogous to the {@link Thread#holdsLock(Object)} method for * built-in monitor locks, this method is typically used for * debugging and testing. For example, a method that should only be * called while a lock is held can assert that this is the case: * * <pre> {@code * class X { * ReentrantLock lock = new ReentrantLock(); * // ... * * public void m() { * assert lock.isHeldByCurrentThread(); * // ... method body * } * }}</pre> * * <p>It can also be used to ensure that a reentrant lock is used * in a non-reentrant manner, for example: * * <pre> {@code * class X { * ReentrantLock lock = new ReentrantLock(); * // ... * * public void m() { * assert !lock.isHeldByCurrentThread(); * lock.lock(); * try { * // ... method body * } finally { * lock.unlock(); * } * } * }}</pre> * * @return {@code true} if current thread holds this lock and * {@code false} otherwise */ public boolean isHeldByCurrentThread() { return sync.isHeldExclusively(); } /** * Queries if this lock is held by any thread. This method is * designed for use in monitoring of the system state, * not for synchronization control. * * @return {@code true} if any thread holds this lock and * {@code false} otherwise */ public boolean isLocked() { return sync.isLocked(); } /** * Returns {@code true} if this lock has fairness set true. * * @return {@code true} if this lock has fairness set true */ public final boolean isFair() { return sync instanceof FairSync; } /** * Returns the thread that currently owns this lock, or * {@code null} if not owned. When this method is called by a * thread that is not the owner, the return value reflects a * best-effort approximation of current lock status. For example, * the owner may be momentarily {@code null} even if there are * threads trying to acquire the lock but have not yet done so. * This method is designed to facilitate construction of * subclasses that provide more extensive lock monitoring * facilities. * * @return the owner, or {@code null} if not owned */ protected Thread getOwner() { return sync.getOwner(); } /** * Queries whether any threads are waiting to acquire this lock. Note that * because cancellations may occur at any time, a {@code true} * return does not guarantee that any other thread will ever * acquire this lock. This method is designed primarily for use in * monitoring of the system state. * * @return {@code true} if there may be other threads waiting to * acquire the lock */ public final boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } /** * Queries whether the given thread is waiting to acquire this * lock. Note that because cancellations may occur at any time, a * {@code true} return does not guarantee that this thread * will ever acquire this lock. This method is designed primarily for use * in monitoring of the system state. * * @param thread the thread * @return {@code true} if the given thread is queued waiting for this lock * @throws NullPointerException if the thread is null */ public final boolean hasQueuedThread(Thread thread) { return sync.isQueued(thread); } /** * Returns an estimate of the number of threads waiting to acquire * this lock. 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 system state, not for synchronization control. * * @return the estimated number of threads waiting for this lock */ public final int getQueueLength() { return sync.getQueueLength(); } /** * Returns a collection containing threads that may be waiting to * acquire this lock. Because the actual set of threads may change * dynamically while constructing this result, the returned * collection is only a best-effort estimate. The elements of the * returned collection are in no particular order. This method is * designed to facilitate construction of subclasses that provide * more extensive monitoring facilities. * * @return the collection of threads */ protected Collection<Thread> getQueuedThreads() { return sync.getQueuedThreads(); } /** * Queries whether any threads are waiting on the given condition * associated with this lock. Note that because timeouts and * interrupts may occur at any time, a {@code true} return does * not guarantee that a future {@code signal} will awaken any * threads. This method is designed primarily for use in * monitoring of the system state. * * @param condition the condition * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ public boolean hasWaiters(Condition condition) { if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject) condition); } /** * Returns an estimate of the number of threads waiting on the * given condition associated with this lock. 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. * * @param condition the condition * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ public int getWaitQueueLength(Condition condition) { if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject) condition); } /** * Returns a collection containing those threads that may be * waiting on the given condition associated with this lock. * Because the actual set of threads may change dynamically while * constructing this result, the returned collection is only a * best-effort estimate. The elements of the returned collection * are in no particular order. This method is designed to * facilitate construction of subclasses that provide more * extensive condition monitoring facilities. * * @param condition the condition * @return the collection of threads * @throws IllegalMonitorStateException if this lock is not held * @throws IllegalArgumentException if the given condition is * not associated with this lock * @throws NullPointerException if the condition is null */ protected Collection<Thread> getWaitingThreads(Condition condition) { if (condition == null) throw new NullPointerException(); if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) throw new IllegalArgumentException("not owner"); return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject) condition); } /** * Returns a string identifying this lock, as well as its lock state. * The state, in brackets, includes either the String {@code "Unlocked"} * or the String {@code "Locked by"} followed by the * {@linkplain Thread#getName name} of the owning thread. * * @return a string identifying this lock, as well as its lock state */ public String toString() { Thread o = sync.getOwner(); return super.toString() + ((o == null) ? "[Unlocked]" : "[Locked by thread " + o.getName() + "]"); } }