A ReentrantLock is owned by the thread last * successfully locking, but not yet unlocking it. A thread invoking * 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 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 lock with a 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 isLocked and * getLockQueueLength, as well as some associated * 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 2147483648 recursive locks by * the same thread. * * @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. */ static abstract class Sync extends AbstractQueuedSynchronizer { /** Current owner thread */ transient Thread owner; /** * Perform {@link Lock#lock}. The main reason for subclassing * is to allow fast path for nonfair version. */ abstract void lock(); /** * Perform non-fair tryLock. tryAcquire is * implemented in subclasses, but both need nonfair * try for trylock method */ final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (compareAndSetState(0, acquires)) { owner = current; return true; } } else if (current == owner) { setState(c+acquires); return true; } return false; } protected final boolean tryRelease(int releases) { int c = getState() - releases; if (Thread.currentThread() != owner) throw new IllegalMonitorStateException(); boolean free = false; if (c == 0) { free = true; owner = null; } setState(c); return free; } protected final boolean isHeldExclusively() { return getState() != 0 && owner == Thread.currentThread(); } final ConditionObject newCondition() { return new ConditionObject(); } // Methods relayed from outer class final Thread getOwner() { int c = getState(); Thread o = owner; return (c == 0)? null : o; } final int getHoldCount() { int c = getState(); Thread o = owner; return (o == Thread.currentThread())? c : 0; } final boolean isLocked() { return getState() != 0; } /** * Reconstitute this lock instance from a stream * @param s the stream */ 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 */ final static class NonfairSync extends Sync { /** * Perform lock. Try immediate barge, backing up to normal * acquire on failure. */ final void lock() { if (compareAndSetState(0, 1)) owner = Thread.currentThread(); else acquire(1); } protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); } } /** * Sync object for fair locks */ final static class FairSync extends Sync { final void lock() { acquire(1); } /** * Fair version of tryAcquire. Don't grant access unless * recursive call or no waiters or is first. */ protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { Thread first = getFirstQueuedThread(); if ((first == null || first == current) && compareAndSetState(0, acquires)) { owner = current; return true; } } else if (current == owner) { setState(c+acquires); return true; } return false; } } /** * Creates an instance of ReentrantLock. * This is equivalent to using ReentrantLock(false). */ public ReentrantLock() { sync = new NonfairSync(); } /** * Creates an instance of ReentrantLock with the * given fairness policy. * @param fair true if this lock will be fair; else false */ public ReentrantLock(boolean fair) { sync = (fair)? 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 * {@link 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: * *
If the lock is acquired by the current thread then the lock hold * count is set to one. * *
If the current thread: * *
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. * *
Acquires the lock if it is not held by another thread and * returns immediately with the value true, setting the * lock hold count to one. Even when this lock has been set to use a * fair ordering policy, a call to 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 true. * *
If the lock is held by another thread then this method will return * immediately with the value false. * * @return true if the lock was free and was acquired by the * current thread, or the lock was already held by the current thread; and * 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 * {@link Thread#interrupt interrupted}. * *
Acquires the lock if it is not held by another thread and returns * immediately with the value 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 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 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: * *
If the lock is acquired then the value true is returned and * the lock hold count is set to one. * *
If the current thread: * *
If the specified waiting time elapses then the value 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 true if the lock was free and was acquired by the * current thread, or the lock was already held by the current thread; and * false if the waiting time elapsed before the lock could be * acquired. * * @throws InterruptedException if the current thread is interrupted * @throws NullPointerException if unit is null * */ public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireNanos(1, 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.release(1); } /** * 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. * *
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 true if current thread holds this lock and
* 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 true if any thread holds this lock and
* false otherwise.
*/
public boolean isLocked() {
return sync.isLocked();
}
/**
* Returns true if this lock has fairness set true.
* @return 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
* null if not owned. Note that the owner may be
* momentarily 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 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 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 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
* 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 true if the given thread is queued waiting for this lock.
* @throws NullPointerException if 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