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waitStatus
LockSupport.unpark(node.thread); //唤醒 return true; }
示例
互斥锁ReentrantLock中的生产者消费者例子
