生产者和消费者问题

生产者和消费者问题：假设有一个仓库，生产者生产产品放入仓库中，消费者从仓库中消费产品。生产者和消费者有多个。

class ProducerConsumerDemo{public static void main(String[] args){Resource r = new Resource();Producer pro = new Producer(r);Consumer con = new Consumer(r);Thread t1 = new Thread(pro);Thread t2 = new Thread(pro);Thread t3 = new Thread(con);Thread t4 = new Thread(con);t1.start();t2.start();t3.start();t4.start();}}class Resource{private String name;private int count = 1;private boolean flag = false;public synchronized void set(String name){if(flag)try{wait();}catch(Exception e){}this.name = name+"---"+count++;System.out.println(Thread.currentThread().getName()+"...生产者"+this.name);flag = true;this.notify();}public synchronized void out(){if(!flag)try{wait();}catch(Exception e){}System.out.println(Thread.currentThread().getName()+"...消费者.."+this.name);flag = false;this.notify();}}class Producer implements Runnable{private Resource res;Producer(Resource res){this.res = res;}public void run(){while(true){res.set("+商品+");}}}class Consumer implements Runnable{private Resource res;Consumer(Resource res){this.res = res;}public void run(){while(true){res.out();}}}

输出结果中可能会出现生产了多个产品但是只消费了一个的情况。这是因为生产线程醒来后没有判断flag的原因，所以要把if换成while。但是这样就会出现全部线程都等待的情况，因此还要把notify换成notifyAll。

 

class ProducerConsumerDemo{public static void main(String[] args){Resource r = new Resource();Producer pro = new Producer(r);Consumer con = new Consumer(r);Thread t1 = new Thread(pro);Thread t2 = new Thread(pro);Thread t3 = new Thread(con);Thread t4 = new Thread(con);t1.start();t2.start();t3.start();t4.start();}}class Resource{private String name;private int count = 1;private boolean flag = false;public synchronized void set(String name){<span style="color:#33ff33;">while(flag)</span>try{wait();}catch(Exception e){}this.name = name+"---"+count++;System.out.println(Thread.currentThread().getName()+"...生产者.."+this.name);flag = true;<span style="color:#33cc00;">this.notifyAll();</span>}public synchronized void out(){<span style="color:#33cc00;">while(!flag)</span>try{wait();}catch(Exception e){}System.out.println(Thread.currentThread().getName()+"...消费者.."+this.name);flag = false;<span style="color:#33cc00;">this.notifyAll();</span>}}class Producer implements Runnable{private Resource res;Producer(Resource res){this.res = res;}public void run(){while(true){res.set("+商品+");}}}class Consumer implements Runnable{private Resource res;Consumer(Resource res){this.res = res;}public void run(){while(true){res.out();}}｝

对于多个生产者和消费者，为什么要定义while判断标记？

让被唤醒的线程再一次判断标记。

 

为什么定义notifyAll？

因为需要唤醒对方线程。只用notify，容易出现只唤醒本方线程的情况，导致程序中所有的线程都等待。

JDK1.5中提供了多线程升级解决方案。
将同步synchronized替换成了显式Lock操作，Object中的wait,notify,notifyAll,替换成了Condition对象。该对象可以对
Lock锁进行获取。

class ProducerConsumerDemo{public static void main(String[] args){Resource r = new Resource();Producer pro = new Producer(r);Consumer con = new Consumer(r);Thread t1 = new Thread(pro);Thread t2 = new Thread(pro);Thread t3 = new Thread(con);Thread t4 = new Thread(con);t1.start();t2.start();t3.start();t4.start();}}class Resource{private String name;private int count = 1;private boolean flag = false;private Lock lock =new ReentrantLock();<span style="background-color: rgb(204, 51, 204);">private Condition <span style="color:#33cc00;">condition_pro</span> = lock.newCondition();private Condition <span style="color:#66ffff;">condition_con </span>= lock.newCondition();</span>public void set(String name) throws InterruptedException{<span style="color:#6633ff;">lock.lock();</span>try{while(flag)<span style="color:#33cc00;">condition_pro.await()</span>;//消费者等待this.name = name+"---"+count++;System.out.println(Thread.currentThread().getName()+"...生产者.."+this.name);flag = true;<span style="color:#33ffff;">condition_con.signalAll()</span>;//生产者唤醒}<span style="color:#33ccff;">finally</span>{<span style="color:#6633ff;">lock.unlock();</span>//释放锁的动作一定要执行}}public  void out()throws InterruptedException{lock.lock();try{while(!flag)condition_con.await();System.out.println(Thread.currentThread().getName()+"...消费者.."+this.name);flag = false;condition_pro.signalAll();}finally{lock.unlock();}}}class Producer implements Runnable{private Resource res;Producer(Resource res){this.res = res;}public void run(){while(true){try{res.set("+商品+");}catch(InterruptedException e){}}}}class Consumer implements Runnable{private Resource res;Consumer(Resource res){this.res = res;}public void run(){while(true){try{res.out();}catch(InterruptedException e){}}

在该实例中实现了本方只唤醒对方操作