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ReentrantLock是JUC包提供的一种可重入独占锁，它实现了Lock接口。与Semaphore类似，ReentrantLock也提供了两种工作模式：公平模式&非公平模式，也是通过自定义两种同步器FairSync&NonfairSync来实现的。
源代码：
/* * 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.*; 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 * {@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() 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> * 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 {@code isLocked} and * {@code getLockQueueLength}, as well as some associated * {@code protected} 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 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 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 */ 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>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> * 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 {@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() + "]"); } }
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一、lock 不响应中断获取锁
public void lock() { sync.lock(); }
lock方法通过调用自定义同步器的同名方法来获取锁。注意：ReentrantLock自定义了两种同步器：FairSync&NonfairSync，分别对应公平模式&非公平模式。
我们先来看一下非公平模式下的lock方法：
final void lock() { if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread()); else acquire(1); }
lock方法并没有直接调用AQS提供的acquire方法，而是先试探地获取了一下锁，CAS操作失败再去调用acquire方法。我的理解是为了提升性能。因为可能很多时候我们能在第一次试探获取时成功，而不需要经过acquire->tryAcquire->nonfairAcquire的调用过程：
public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) selfInterrupt(); }
AQS提供的acquire方法首先调用了我们自定义同步器重写的tryAcquire方法试图获取锁，如果失败的话先调用addWaiter方法将当前线程加入等待队列，然后对奥用acquireQueued方法进行自旋、检测获取锁的操作，直到成功获取锁。在自旋、检测的过程中如果被中断（注意：acquireQueued延迟处理中断），要在成功获取锁之后调用selfInterrupt方法“补上”这次中断。addWaiter&acquireQueued方法已经在笔者的另一篇博文AQS源码学习笔记中详细介绍过了，不再赘述。这里我们主要关注ReentrantLock重写的tryAcquire方法：
protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); }
nonfairSync的tryAcquire方法通过调用其父类Sync的nonfairTryAcquire方法实现：
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; }
nonfairTryAcquire方法首先判断锁是否被占用，如果锁可用，通过调用CAS操作试图获取锁，如果失败直接返回false；但如果锁被占用（state==0），并不代表没有机会，因为有可能占用锁的正是当前线程。如果正是当前线程占用了锁，让state做+1操作，然后返回true：这正是可重入的概念，一个已经获取锁的线程可以重复获取锁。
我们再来看一下公平模式下的lock方法：
final void lock() { acquire(1); }
fairSync的lock方法直接调用acquire，而没有想NonfairSync一样先试图获取，因为这样可能导致违反“公平”的语义：在已等待在队列中的线程之前获取了锁。
由上面的分析可知，AQS的acquire方法调用了fairSync重写的tryAcquire方法：
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; }
这与nonfairTryAcquire方法大同小异，主要区别在于，当发现锁未被占用的时候，还要判断一下等待队列中是否有先到的线程正在等待锁，如果有，直接返回false。这保证了公平性：线程按照申请锁的顺序获取锁。acquire方法的后续操作同样可以参考笔者的另一篇博文AQS源码学习笔记，这里不再赘述。
二、lockInterruptibly 可响应中断获取锁
public void lockInterruptibly() throws InterruptedException { sync.acquireInterruptibly(1); }
lockInterruptibly方法通过调用AQS提供的acquireInterruptibly方法实现：
public final void acquireInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (!tryAcquire(arg)) doAcquireInterruptibly(arg); }
acquireInterruptibly方法首先检测一下中断，然后调用重写的tryAcquire方法试图获取锁，如果失败，调用doAcquireInterruptibly方法进行自旋、检测获取锁操作：
private void doAcquireInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.EXCLUSIVE); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
doAcquireInterruptibly方法与acquireQueued方法的区别在于：①doAcquireInterruptibly方法将addWaiter的调用写在了方法里，而acquireQueued方法没有；②doAcquireInterruptibly在当前线程从park中被中断唤醒时，直接抛出中断异常，而acquireQueued方法则是用一个局部变量记录下这次中断，但不立即处理，等到成功获取锁/共享资源之后，反馈给上层，由上层调用selfInterrupt方法“补上”这次中断。
这些区别与doAcquireSharedInterruptibly&doAcquireShared方法之间的区别一致。
三、tryLock & tryLock(Timeout) 尝试获取锁
public boolean tryLock() { return sync.nonfairTryAcquire(1); } public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireNanos(1, unit.toNanos(timeout)); }
ReentrantLock提供了两种tryLock方法：限时&不限时。我们注意到，不限时（立即返回）的tryLock方法，不管在公平还是非公平模式下，调用的都是Sync中的nonfairTryAcquire方法。因此，如果在公平模式下调用tryLock，即使队列中有等待线程，也可能获取成功。
而限时（不立即返回）的tryLock(Timeout)方法则公国tryAcquireNanos提供了公平&非公平两种模式的tryLock(Timeout)操作：
public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout); }
可以看到，tryAcquireNanos方法通过调用不同的重写的tryAcquire方法提供了两种模式下的不同操作。tryAcquire方法已经分析过，不再赘述。这里重点关注doAcquireNanos方法：
private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { if (nanosTimeout <= 0L) return false; final long deadline = System.nanoTime() + nanosTimeout; final Node node = addWaiter(Node.EXCLUSIVE); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return true; } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) return false; if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if (Thread.interrupted()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
可以看到，doAcquireNanos方法是立即响应中断的（事实上doAcquireSharedNanos方法也是立即响应中断的），即限时（不立即返回）的尝试获取的方法都是及时响应中断的，没有延迟处理中断的版本。
还有一点需要注意，当线程从park中被唤醒时，我们无法确定唤醒原因是被中断还是超时，因此需要检测一下中断标志。还要注意spinForTimeoutThreshold阈值的应用，这在笔者的另一篇博文Semaphore源码学习笔记中已经分析过，主要目的是为了提高短时长Timeout时的相应效率。
四、unlock 释放锁
公平&非公平模式的unlock操作是一致的：
public void unlock() { sync.release(1); }
通过调用AQS提供的release方法实现：
public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; }
release方法首先调用我们重写的tryRelease方法尝试释放锁。注意，这里tryRelease的返回值并不代表是否成功释放，而是释放后锁是否可用。还记得可重入的概念吗，如果一个线程重复获取了锁，那么在他没有释放到底时，release操作之后，锁仍然是不可使用的（state>0）。如果释放之后锁可用，查看队列中是否有需要唤醒的等待线程，有则调用unparkSuccessor方法唤醒。这在笔者的另一篇博文AQS源码学习笔记中已经分析过了，这里重点关注我们重写的tryRelease方法：
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; }
tryRelease是在FairSync和NonfairSync的父类Sync中定义的，因此公平&非公平模式下的release操作是统一的。tryRelease方法首先检测当前线程是否持有锁，然后计算一下释放之后锁是否可用（计数值state是否等于0），如果可用，释放&设置持有锁线程为null&返回true，如果不可用，释放&返回返回false。
五、Condition
关于Condition的内容请参考笔者的另一篇博文Condition源码学习笔记。
