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/*
 * 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/licenses/publicdomain
 */

package scala.actors.threadpool.locks;

import java.util.Collection;
import scala.actors.threadpool.*;
import scala.actors.threadpool.helpers.*;

/**
 * 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.
 *
 * 

A {@code ReentrantLock} is owned 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}. * *

The constructor for this class accepts an optional * fairness 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() tryLock} method does not * honor the fairness setting. It will succeed if the lock * is available even if other threads are waiting. * *

It is recommended practice to always immediately * follow a call to {@code lock} with a {@code try} block, most * typically in a before/after construction such as: * *

 * class X {
 *   private final ReentrantLock lock = new ReentrantLock();
 *   // ...
 *
 *   public void m() {
 *     lock.lock();  // block until condition holds
 *     try {
 *       // ... method body
 *     } finally {
 *       lock.unlock()
 *     }
 *   }
 * }
 * 
* *

In addition to implementing the {@link Lock} interface, this * class defines methods {@code isLocked} and * {@code getLockQueueLength}, as well as some associated * {@code protected} access methods that may be useful for * instrumentation and monitoring. * *

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. * *

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 * @author Dawid Kurzyniec */ public class ReentrantLock implements Lock, java.io.Serializable, CondVar.ExclusiveLock { private static final long serialVersionUID = 7373984872572414699L; private final Sync sync; /** * Base of synchronization control for this lock. Subclassed * into fair and nonfair versions below. */ static abstract class Sync implements java.io.Serializable { private static final long serialVersionUID = -5179523762034025860L; protected transient Thread owner_ = null; protected transient int holds_ = 0; protected Sync() {} /** * Performs {@link Lock#lock}. The main reason for subclassing * is to allow fast path for nonfair version. */ public abstract void lock(); public abstract void lockInterruptibly() throws InterruptedException; final void incHolds() { int nextHolds = ++holds_; if (nextHolds < 0) throw new Error("Maximum lock count exceeded"); holds_ = nextHolds; } public boolean tryLock() { Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return true; } else if (caller == owner_) { incHolds(); return true; } } return false; } public abstract boolean tryLock(long nanos) throws InterruptedException; public abstract void unlock(); public synchronized int getHoldCount() { return isHeldByCurrentThread() ? holds_ : 0; } public synchronized boolean isHeldByCurrentThread() { return holds_ > 0 && Thread.currentThread() == owner_; } public synchronized boolean isLocked() { return owner_ != null; } public abstract boolean isFair(); protected synchronized Thread getOwner() { return owner_; } public boolean hasQueuedThreads() { throw new UnsupportedOperationException("Use FAIR version"); } public int getQueueLength() { throw new UnsupportedOperationException("Use FAIR version"); } public Collection getQueuedThreads() { throw new UnsupportedOperationException("Use FAIR version"); } public boolean isQueued(Thread thread) { throw new UnsupportedOperationException("Use FAIR version"); } } /** * Sync object for non-fair locks */ final static class NonfairSync extends Sync { private static final long serialVersionUID = 7316153563782823691L; NonfairSync() {} /** * Performs lock. Try immediate barge, backing up to normal * acquire on failure. */ public void lock() { Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return; } else if (caller == owner_) { incHolds(); return; } else { boolean wasInterrupted = Thread.interrupted(); try { while (true) { try { wait(); } catch (InterruptedException e) { wasInterrupted = true; // no need to notify; if we were signalled, we // will act as signalled, ignoring the // interruption } if (owner_ == null) { owner_ = caller; holds_ = 1; return; } } } finally { if (wasInterrupted) Thread.currentThread().interrupt(); } } } } public void lockInterruptibly() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return; } else if (caller == owner_) { incHolds(); return; } else { try { do { wait(); } while (owner_ != null); owner_ = caller; holds_ = 1; return; } catch (InterruptedException ex) { if (owner_ == null) notify(); throw ex; } } } } public boolean tryLock(long nanos) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return true; } else if (caller == owner_) { incHolds(); return true; } else if (nanos <= 0) return false; else { long deadline = Utils.nanoTime() + nanos; try { for (; ; ) { TimeUnit.NANOSECONDS.timedWait(this, nanos); if (caller == owner_) { incHolds(); return true; } else if (owner_ == null) { owner_ = caller; holds_ = 1; return true; } else { nanos = deadline - Utils.nanoTime(); if (nanos <= 0) return false; } } } catch (InterruptedException ex) { if (owner_ == null) notify(); throw ex; } } } } public synchronized void unlock() { if (Thread.currentThread() != owner_) throw new IllegalMonitorStateException("Not owner"); if (--holds_ == 0) { owner_ = null; notify(); } } public final boolean isFair() { return false; } } /** * Sync object for fair locks */ final static class FairSync extends Sync implements WaitQueue.QueuedSync { private static final long serialVersionUID = -3000897897090466540L; private transient WaitQueue wq_ = new FIFOWaitQueue(); FairSync() {} public synchronized boolean recheck(WaitQueue.WaitNode node) { Thread caller = Thread.currentThread(); if (owner_ == null) { owner_ = caller; holds_ = 1; return true; } else if (caller == owner_) { incHolds(); return true; } wq_.insert(node); return false; } public synchronized void takeOver(WaitQueue.WaitNode node) { // assert (holds_ == 1 && owner_ == Thread.currentThread() owner_ = node.getOwner(); } public void lock() { Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return; } else if (caller == owner_) { incHolds(); return; } } WaitQueue.WaitNode n = new WaitQueue.WaitNode(); n.doWaitUninterruptibly(this); } public void lockInterruptibly() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return; } else if (caller == owner_) { incHolds(); return; } } WaitQueue.WaitNode n = new WaitQueue.WaitNode(); n.doWait(this); } public boolean tryLock(long nanos) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Thread caller = Thread.currentThread(); synchronized (this) { if (owner_ == null) { owner_ = caller; holds_ = 1; return true; } else if (caller == owner_) { incHolds(); return true; } } WaitQueue.WaitNode n = new WaitQueue.WaitNode(); return n.doTimedWait(this, nanos); } protected synchronized WaitQueue.WaitNode getSignallee(Thread caller) { if (caller != owner_) throw new IllegalMonitorStateException("Not owner"); // assert (holds_ > 0) if (holds_ >= 2) { // current thread will keep the lock --holds_; return null; } // assert (holds_ == 1) WaitQueue.WaitNode w = wq_.extract(); if (w == null) { // if none, clear for new arrivals owner_ = null; holds_ = 0; } return w; } public void unlock() { Thread caller = Thread.currentThread(); for (;;) { WaitQueue.WaitNode w = getSignallee(caller); if (w == null) return; // no one to signal if (w.signal(this)) return; // notify if still waiting, else skip } } public final boolean isFair() { return true; } public synchronized boolean hasQueuedThreads() { return wq_.hasNodes(); } public synchronized int getQueueLength() { return wq_.getLength(); } public synchronized Collection getQueuedThreads() { return wq_.getWaitingThreads(); } public synchronized boolean isQueued(Thread thread) { return wq_.isWaiting(thread); } private void readObject(java.io.ObjectInputStream in) throws java.io.IOException, ClassNotFoundException { in.defaultReadObject(); synchronized (this) { wq_ = new FIFOWaitQueue(); } } } /** * 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)? (Sync)new FairSync() : new NonfairSync(); } /** * Acquires the lock. * *

Acquires the lock if it is not held by another thread and returns * immediately, setting the lock hold count to one. * *

If the current thread already holds the lock then the hold * count is incremented by one and the method returns immediately. * *

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.lock(); } /** * Acquires the lock unless the current thread is * {@linkplain Thread#interrupt interrupted}. * *

Acquires the lock if it is not held by another thread and returns * immediately, setting the lock hold count to one. * *

If the current thread already holds this lock then the hold count * is incremented by one and the method returns immediately. * *

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: * *

    * *
  • The lock is acquired by the current thread; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} the * current thread. * *
* *

If the lock is acquired by the current thread then the lock hold * count is set to one. * *

If the current thread: * *

    * *
  • has its interrupted status set on entry to this method; or * *
  • is {@linkplain Thread#interrupt interrupted} while acquiring * the lock, * *
* * then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

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.lockInterruptibly(); } /** * Acquires the lock only if it is not held by another thread at the time * of invocation. * *

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()} will * 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). * *

If the current thread already holds this lock then the hold * count is incremented by one and the method returns {@code true}. * *

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.tryLock(); } /** * 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}. * *

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 will not 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: * *

if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
     * 
* *

If the current thread * already holds this lock then the hold count is incremented by one and * the method returns {@code true}. * *

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: * *

    * *
  • The lock is acquired by the current thread; or * *
  • Some other thread {@linkplain Thread#interrupt interrupts} * the current thread; or * *
  • The specified waiting time elapses * *
* *

If the lock is acquired then the value {@code true} is returned and * the lock hold count is set to one. * *

If the current thread: * *

    * *
  • has its interrupted status set on entry to this method; or * *
  • is {@linkplain Thread#interrupt interrupted} while * acquiring the lock, * *
* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

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. * *

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.tryLock(unit.toNanos(timeout)); } /** * Attempts to release this lock. * *

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.unlock(); } /** * Returns a {@link Condition} instance for use with this * {@link Lock} instance. * *

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. * *

    * *
  • 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. * *
  • 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. * *
  • 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. * *
  • Waiting threads are signalled in FIFO order. * *
  • 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 fair locks favors those threads that have been * waiting the longest. * *
* * @return the Condition object */ public Condition newCondition() { return isFair() ? (Condition)new FIFOCondVar(this) : new CondVar(this); } /** * Queries the number of holds on this lock by the current thread. * *

A thread has a hold on a lock for each lock action that is not * matched by an unlock action. * *

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: * *

     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *   public void m() {
     *     assert lock.getHoldCount() == 0;
     *     lock.lock();
     *     try {
     *       // ... method body
     *     } finally {
     *       lock.unlock();
     *     }
     *   }
     * }
     * 
* * @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. * *

Analogous to the {@link Thread#holdsLock} 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: * *

     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert lock.isHeldByCurrentThread();
     *       // ... method body
     *   }
     * }
     * 
* *

It can also be used to ensure that a reentrant lock is used * in a non-reentrant manner, for example: * *

     * class X {
     *   ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert !lock.isHeldByCurrentThread();
     *       lock.lock();
     *       try {
     *           // ... method body
     *       } finally {
     *           lock.unlock();
     *       }
     *   }
     * }
     * 
* * @return {@code true} if current thread holds this lock and * {@code false} otherwise */ public boolean isHeldByCurrentThread() { return sync.isHeldByCurrentThread(); } /** * 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.isFair(); } /** * 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 of the 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 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) { return asCondVar(condition).hasWaiters(); } /** * 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) { return asCondVar(condition).getWaitQueueLength(); } /** * 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 getWaitingThreads(Condition condition) { return asCondVar(condition).getWaitingThreads(); } /** * 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 = getOwner(); return super.toString() + ((o == null) ? "[Unlocked]" : "[Locked by thread " + o.getName() + "]"); } private CondVar asCondVar(Condition condition) { if (condition == null) throw new NullPointerException(); if (!(condition instanceof CondVar)) throw new IllegalArgumentException("not owner"); CondVar condVar = (CondVar)condition; if (condVar.lock != this) throw new IllegalArgumentException("not owner"); return condVar; } }




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