Java Condition
概述
1.Condition介绍
2.Condition源码分析
3.Condition示例
Condition介绍
在互斥锁ReentrantLock文章中的生产者消费者示例中用到Condition了，分布设置生产者和消费者的Condition，即条件仓库满时，生产者等待，唤醒消费者；条件仓库为空时，消费者等待，唤醒消费者。
Condition的作用是对锁进行更精确的控制。Condition中的await()方法相当于Object的wait()方法，Condition中的sinal()方法相当于Object的notify()方法，Condition中的singalAll()相当于Object的notifyAll()方法。不同的，Object中的wait(),notify(),notifyAll()方法是和synchronized关键字捆绑使用的；而Condition是需要和互斥锁/共享锁捆绑使用的。wait()，notify()方法使用见线程的等待与唤醒。
除此之外，Condition还是对多线程条件进行更精确的控制。比如多个线程对同一块缓冲区操作，对于同一个锁，可以设置多个condition，不同情况下设置不同的锁。
假如多线程读/写同一个缓冲区：当向缓冲区中写入数据之后，唤醒"读线程"；当从缓冲区读出数据之后，唤醒"写线程"；并且当缓冲区满的时候，"写线程"需要等待；当缓冲区为空时，"读线程"需要等待。 如果采用Object类中的wait(), notify(), notifyAll()实现该缓冲区，当向缓冲区写入数据之后需要唤醒"读线程"时，不可能通过notify()或notifyAll()明确的指定唤醒"读线程"，而只能通过notifyAll唤醒所有线程(但是notifyAll无法区分唤醒的线程是读线程，还是写线程)。 但是，通过Condition，就能明确的指定唤醒读线程。
Condition源码分析
主要分析两个常见的方法
await()
Condition是和Lock捆绑的，Lock实现的AQS，所以看下AQS的await()方法：
public final void await() throws InterruptedException {
if (Thread.interrupted()) //线程是否被中断
throw new InterruptedException();
Node node = addConditionWaiter(); //将当前线程加入到Condition队列，非AQS队列
int savedState = fullyRelease(node); //释放锁
int interruptMode = 0;
while (!isOnSyncQueue(node)) { //
LockSupport.park(this); //park/阻塞当前线程
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) //判断是否被中断
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) //acquireQueued自旋获取锁，并从阻塞队列移除
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
private Node addConditionWaiter() { //添加当前线程到队列
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
//等待状态说明/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
signal(),signalAll()
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
/**
* Removes and transfers all nodes.
* @param first (non-null) the first node on condition queue
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) //CAS设置waitStatus
return false;
/*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
Node p = enq(node);
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) //CAS设置waitStatusLockSupport.unpark(node.thread); //唤醒 return true; }
示例
互斥锁ReentrantLock中的生产者消费者例子
