多线程Condition

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条件变量是Java5线程中很重要的一个概念，顾名思义，条件变量就是表示条件的一种变量。但是必须说明，这里的条件是没有实际含义的，仅仅是个标记而已，并且条件的含义往往通过代码来赋予其含义。

这里的条件和普通意义上的条件表达式有着天壤之别。

条件变量都实现了java.util.concurrent.locks.Condition接口，条件变量的实例化是通过一个Lock对象上调用newCondition()方法来获取的，这样，条件就和一个锁对象绑定起来了。因此，Java中的条件变量只能和锁配合使用，来控制并发程序访问竞争资源的安全。

条件变量的出现是为了更精细控制线程等待与唤醒，在Java5之前，线程的等待与唤醒依靠的是Object对象的wait()和notify()/notifyAll()方法，这样的处理不够精细。

而在Java5中，一个锁可以有多个条件，每个条件上可以有多个线程等待，通过调用await()方法，可以让线程在该条件下等待。当调用signalAll()方法，又可以唤醒该条件下的等待的线程。有关Condition接口的API可以具体参考JavaAPI文档。

条件变量比较抽象，原因是他不是自然语言中的条件概念，而是程序控制的一种手段。

package producerConsumer;import java.util.LinkedList;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public class TestCondition {private LinkedList myList = new LinkedList();private int MAX = 3;private final Lock lock = new ReentrantLock();private final Condition full = lock.newCondition();private final Condition empty = lock.newCondition();public static void main(String[] args) {new TestCondition().run();}public void run(){ExecutorService pool= Executors.newFixedThreadPool(10);Thread c1 = new Consumer();Thread c2 = new Consumer();Thread c3 = new Consumer();Thread c4 = new Consumer();Thread c5 = new Consumer();Thread p1 = new Producer();Thread p2 = new Producer();Thread p3 = new Producer();Thread p4 = new Producer();Thread p5 = new Producer();pool.execute(c1);pool.execute(p1);pool.execute(c2);pool.execute(p2);pool.execute(c3);pool.execute(c4);pool.execute(c5);pool.execute(p3);pool.execute(p4);pool.execute(p5);pool.shutdown();}class Producer extends Thread{public void run(){lock.lock();try {while(myList.size()==MAX){System.out.println("Warning: it's full");full.await();}Product p = new Product("Fruit");if(myList.add(p)){System.out.println("Producer: ");}empty.signal();} catch (InterruptedException e) {System.out.println("Producer is interrupted");}finally{lock.unlock();}}}class Consumer extends Thread{public void run(){lock.lock();try {while(myList.size()==0){System.out.println("Warning: it's empty!");empty.await();}Product p = myList.removeLast();System.out.println("Consumer: "+p);full.signal();} catch (InterruptedException e) {System.out.println("Consumer is interrupted");}finally{lock.unlock();}}}}

运行结果：

Warning: it's empty!
Warning: it's empty!
Warning: it's empty!
Producer:
Consumer: Product [name=Fruit]
Producer:
Producer:
Producer:
Consumer: Product [name=Fruit]
Producer:
Consumer: Product [name=Fruit]
Consumer: Product [name=Fruit]
Consumer: Product [name=Fruit]

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