上文接Java源码解析之可重入锁ReentrantLock（一）。
接下来是tryLock方法。代码如下。从注释中我们可以理解到，只有当调用tryLock时锁没有被别的线程占用，tryLock才会获取锁。如果锁没有被另一个线程占用，那么就获取锁，并立刻返回true，并把锁计数设置为1. 甚至在锁被设置为公平排序的情况下，若果锁可用，调用tryLock会立刻获取锁，而不管有没有别的线程在等待锁了。从这里我们总结出，不管可重入锁是公平锁还是非公平锁，tryLock方法只会是非公平的。
/**
* 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);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
接下来是释放锁的方法unlock。代码如下。unlock方式的实现，是以参数1来调用sync.release方法。而release方法是如何实现的呢？release方法首先会调用tryRelease方法，如果tryRelease成功，则唤醒后继者线程。而tryRelease的实现过程十分清晰，首先获取锁状态，锁状态减去参数（放锁次数），得到新状态。然后判断持有锁的线程是否为当前线程，如果不是当前线程，则抛出IllegalMonitorStateException。然后判断，如果新状态为0，说明放锁成功，则把持有锁的线程设置为null，并返回true。如果新状态不为0，则返回false。从tryRelease的返回值来看，它返回的true或false，指的是否成功的释放了该锁。成功的释放该锁的意思是彻底释放锁，别的线程就可以获取锁了。这里要认识到，即便tryRelease返回false，它也只是说明了锁没有完全释放，本次调用的这个释放次数值，依然是释放成功的。
/**
* 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);
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
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;
}
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
接下来是newCondition方法。关于Condition这里不展开介绍，只是了解下该方法的作用。如下图。该方法返回一个和这个锁实例一起使用的Condition实例。返回的Condition实例支持和Object的监控方法例如wait-notify和notifyAll相同的用法。
1，如果没有获取锁，调用Condition的await，signal，signalAll方法的任何一个时，会抛出IllegalMonitorStateException异常。
2，调用Condition的await方法时，锁也会释放，在await返回之前，锁会被重新获取，并且锁计数会恢复到调用await方法时的值。
3，如果一个线程在等待的过程中被中断了，那么等待就会结束，并抛出InterruptedException异常，线程的中断标志位会被清理。
4，等待的线程以FIFO的顺序被唤醒。
5，从await方法返回的线程们的获取到锁的顺序，和线程最开始获取锁的顺序相同，这是未指定情况下的默认实现。但是，公平锁更钟爱那些已经等待了最长时间的线程。
/**
* 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();
}可重入锁还有一些其他的方法，这里就不一一介绍了。This is the end.