/*
* 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 java.util.concurrent.locks;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
/**
* A reentrant mutual exclusion {@link Lock} with the same basic
* behavior and semantics as the implicit monitor lock accessed using
* <tt>synchronized</tt> methods and statements, but with extended
* capabilities.
*
* <p> A <tt>ReentrantLock</tt> is <em>owned</em> by the thread last
* successfully locking, but not yet unlocking it. A thread invoking
* <tt>lock</tt> 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 <tt>true</tt>, 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.
*
* <p> It is recommended practice to <em>always</em> immediately
* follow a call to <tt>lock</tt> with a <tt>try</tt> block, most
* typically in a before/after construction such as:
*
* <pre>
* 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 methods <tt>isLocked</tt> and
* <tt>getLockQueueLength</tt>, as well as some associated
* <tt>protected</tt> access methods that may be 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 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 <tt>ReentrantLock</tt>.
* This is equivalent to using <tt>ReentrantLock(false)</tt>.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* Creates an instance of <tt>ReentrantLock</tt> 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.
*
* <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.lock();
}
/**
* Acquires the lock unless the current thread is
* {@link 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 {@link 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 {@link 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 <tt>true</tt>, setting the
* lock hold count to one. Even when this lock has been set to use a
* fair ordering policy, a call to <tt>tryLock()</tt> <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 <tt>true</tt>.
*
* <p>If the lock is held by another thread then this method will return
* immediately with the value <tt>false</tt>.
*
* @return <tt>true</tt> if the lock was free and was acquired by the
* current thread, or the lock was already held by the current thread; and
* <tt>false</tt> 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}.
*
* <p>Acquires the lock if it is not held by another thread and returns
* immediately with the value <tt>true</tt>, 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 <tt>tryLock</tt> that does permit barging on
* a fair lock then combine the timed and un-timed forms together:
*
* <pre>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 <tt>true</tt>.
*
* <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 {@link Thread#interrupt interrupts} the current
* thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the lock is acquired then the value <tt>true</tt> 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 {@link 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 <tt>false</tt>
* 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 <tt>true</tt> if the lock was free and was acquired by the
* current thread, or the lock was already held by the current thread; and
* <tt>false</tt> 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.
*
* <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}
* {@link Condition#await() waiting} or {@link Condition#signal
* signalling} methods are called, then an {@link
* IllegalMonitorStateException} is thrown.
*
* <li>When the condition {@link 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 {@link 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>
* 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} 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>
* 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>
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* assert !lock.isHeldByCurrentThread();
* lock.lock();
* try {
* // ... method body
* } finally {
* lock.unlock();
* }
* }
* }
* </pre>
* @return <tt>true</tt> if current thread holds this lock and
* <tt>false</tt> 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 <tt>true</tt> if any thread holds this lock and
* <tt>false</tt> 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 the exclusive lock, or
* <tt>null</tt> if not owned. Note that the owner may be
* momentarily <tt>null</tt> 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 <tt>null</tt> if not owned.
*/
protected Thread getOwner() {
return sync.getOwner();
}
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations may occur at any time, a <tt>true</tt>
* return does not guarantee that any other thread will ever
* acquire. 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
* <tt>true</tt> return does not guarantee that this thread
* will ever acquire. 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. 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. 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 <tt>true</tt> return does
* not guarantee that a future <tt>signal</tt> will awaken any
* threads. This method is designed primarily for use in
* monitoring of the system state.
* @param condition the condition
* @return <tt>true</tt> 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 condition 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 condition 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 condition 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
* "Unlocked" or the String "Locked by"
* followed by the {@link Thread#getName} of the owning thread.
* @return a string identifying this lock, as well as its lock state.
*/
public String toString() {
Thread owner = sync.getOwner();
return super.toString() + ((owner == null) ?
"[Unlocked]" :
"[Locked by thread " + owner.getName() + "]");
}
}
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