ReentrantLock实现原理

同步状态

/**     * The synchronization state.     */    private volatile int state;    /**     * Returns the current value of synchronization state.     * This operation has memory semantics of a {@code volatile} read.     * @return current state value     */    protected final int getState() {        return state;    }    /**     * Sets the value of synchronization state.     * This operation has memory semantics of a {@code volatile} write.     * @param newState the new state value     */    protected final void setState(int newState) {        state = newState;    }    /**     * Atomically sets synchronization state to the given updated     * value if the current state value equals the expected value.     * This operation has memory semantics of a {@code volatile} read     * and write.     *     * @param expect the expected value     * @param update the new value     * @return {@code true} if successful. False return indicates that the actual     *         value was not equal to the expected value.     */    protected final boolean compareAndSetState(int expect, int update) {        // See below for intrinsics setup to support this        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);    }

volatile

unsafe.compareAndSwapInt(this, stateOffset, expect, update)

源码

/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. *//* * 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.concurrent.TimeUnit;import java.util.Collection;/** * 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 {@link Lock#lock}. The main reason for subclassing         * is to allow fast path for nonfair version.         */        abstract void lock();        /**         * Performs 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)) {                    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;        }        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;        /**         * Performs lock.  Try immediate barge, backing up to normal         * acquire on failure.         */        final void lock() {            if (compareAndSetState(0, 1))                setExclusiveOwnerThread(Thread.currentThread());            else                acquire(1);        }        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;        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) {                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.lock();    }    /**     * 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 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<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() + "]");    }}