JDK5.0新特性系列---11.4线程 Condition

 

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;

 

/**

 *有时候线程取得lock后需要在一定条件下才能做某些工作，比如经典的Producer和Consumer问题

 *在Java 5.0以前，这种功能是由Object类的wait(),notify()和notifyAll()等方法实现的

 *在5.0里面，这些功能集中到了Condition这个接口实现

 */

/**

 * 使用Condition的关键技术点如下:

 * 1.通过Lock的newCondition方法创建Condition的对象

 * 2.Condition的await方法使当前线程进入等待状态,并且释放它占据的Lock,直到有其他的线程唤醒当前线程时,重新占有Lock.

 * 3.Condition的signal方法唤醒其他正在等待该Condition的线程.

 */

public class ConditionTest {

    /**

     * 篮子程序，这里为了简化问题，篮子中最多只能有一个苹果

     * Consumer必须在篮子里有苹果的时候才能吃苹果，否则它必须暂时放弃对篮子的锁定

     * 等到Producer往篮子里放了苹果后再去拿来吃。

     * 否则它也需要暂时解锁等Comsumer把苹果吃了才能往篮子里放苹果。

     */

    public static class Basket{

        //锁

        Lock lock = new ReentrantLock();

        //根据锁产生Condition对象

        Condition produced = lock.newCondition();

        Condition consumed  = lock.newCondition();

        //篮子里的苹果数，最多为1

        int num = 0;

        //生产苹果，往篮子里放

        public void produce() throws InterruptedException{

            //获得锁

            lock.lock();

            System.out.println("Producer get a lock...");

 

                     try{

                //判断是否满足生产条件

                while(num == 1){

                    //如果有苹果，则不生产，放弃锁，进入睡眠

                    //等待消费者消费

                    System.out.println("Producer sleep...");

                    consumed.await();

                    System.out.println("Producer awaked...");

                }

                //生产苹果

                Thread.sleep(500);

                System.out.println("Producer produced an Apple.");

                num = 1;

                //通知等待produced Condition的线程

                produced.signal();

            }finally{

                lock.unlock();

            }

        }

        //消费苹果，从篮子里取

        public void consume() throws InterruptedException{

            //获得锁

            lock.lock();

            System.out.println("Consumer get a lock...");

            try{

                //判断是否满足消费条件

                while(num == 0){

                    //如果没有苹果，无法消费，则放弃锁，进入睡眠

                    //等待生产者生产苹果

                    System.out.println("Consumer sleep...");

                    produced.await();

                    System.out.println("Consumer awaked...");

                }

                //吃苹果

                Thread.sleep(500);

                System.out.println("Consumer consumed an Apple.");

                num = 0;

                //发信号唤醒某个等待consumed Condition的线程

                            consumed.signal();

            } finally {

                lock.unlock();

            }

        }

    }

    //测试Basket程序

    public static void testBasket() throws Exception{

        final Basket basket = new Basket();

        //定义一个producer

        Runnable producer  = new Runnable(){

            public void run() {

                try{

                    basket.produce();

                }catch(InterruptedException ex){

                    ex.printStackTrace();

                }

            }           

        };

        //定义一个consumer

        Runnable consumer = new Runnable(){

            public void run(){

                try{

                    basket.consume();

                }catch(InterruptedException ex){

                    ex.printStackTrace();

                }

            }

        };

        //各生产3个consumer和producer

        ExecutorService service = Executors.newCachedThreadPool();

        for(int i = 0; i <3; i++){

            service.submit(producer);

        }

        for(int i = 0;i<3;i++){

            service.submit(consumer);

        }

        service.shutdown();

    }

    public static void main(String... args)throws Exception{

        ConditionTest.testBasket();

    }

}