线程的通信-生产者消费者

当对于一个生产者和一个消费者时

public static void main(String[] agrs) {
	Resource res = new Resource();
	Producer p = new Producer(res);
	Consumer c = new Consumer(res);
	
	Thread t1 = new Thread(p);
	Thread t2 = new Thread(c);
	t1.start();
	t2.start();
}

class Resource {
	private String name;
	private int cont = 1;
	private boolean flag = false;
	public synchronized void set(String name) {
		if(flag) {
			try{
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		this.name = name + "--" + cont++;
		System.out.println(Thread.currentThread().getName()+"- producer -"+this.name);
		flag = true;
		this.notify();
	}
	public synchronized void out() {
		if(!flag) {
			try {
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println(Thread.currentThread().getName()+"---- consumer ----"+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("wares");
		}
	}
}

class Consumer implements Runnable {
	private Resource res;
	Consumer(Resource res) {
		this.res = res;
	}
	public void run() {
		while(true) {
			res.out();
		}
	}
} 

当对于多个生产者和消费者时###

public static void main(String[] agrs) {
		Resource res = new Resource();
		Producer p = new Producer(res);
		Consumer c = new Consumer(res);
		
		Thread t1 = new Thread(p);
		Thread t2 = new Thread(p);
		Thread t3 = new Thread(c);
		Thread t4 = new Thread(c);
		
		t1.start();
		t2.start();
		t3.start();
		t4.start();
	}

会出现生产一个商品，消费出两个商品，或者生产两个而消费一个

这是因为if语句只判断一次flag标记，消费者的线程生产完会唤醒本方的另一条生产线程，而不判断flag标记，导致生产两次而消费一次

然后我们可以把if换成while
原因：让被唤醒的线程再一次的判断

while(flag) {
	try{
		this.wait();
	} catch (InterruptedException e) {
		e.printStackTrace();
	}
}

但是，新的问题又产生了，线程会全部停止
原因：线程唤醒往往是唤醒线程池的第一个，所以会唤醒本方的线程，会导致数据错乱，加入while后，会导致全部的线程都在等待。我们应该唤醒的是消费者的线程，所以我们可以用notifyAll，唤醒所有的线程，因为会while会判断flag标记，所以不会唤醒生产者的线程，而是唤醒消费者的线程
notify，容易出现只唤醒本方线程的情况，导致程序中的所有线程等待

while(flag) {
	try{
		this.wait();
	} catch (InterruptedException e) {
		e.printStackTrace();
	}
}
this.name = name + "--" + cont++;
System.out.println(Thread.currentThread().getName()+"- producer -"+this.name);
flag = true;
this.notifyALL();

最后改进

public class Dealer {
	public static void main(String[] agrs) {
		Resource res = new Resource();
		Producer p = new Producer(res);
		Consumer c = new Consumer(res);
		
		Thread t1 = new Thread(p);
		Thread t2 = new Thread(p);
		Thread t3 = new Thread(c);
		Thread t4 = new Thread(c);
		
		t1.start();
		t2.start();
		t3.start();
		t4.start();
	}
}

class Resource {
	private String name;
	private int cont = 1;
	private boolean flag = false;
	public synchronized void set(String name) {
		while(flag) {
			try{
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		this.name = name + "--" + cont++;
		System.out.println(Thread.currentThread().getName()+"- producer -"+this.name);
		flag = true;
		this.notifyALL();
	}
	public synchronized void out() {
		while(!flag) {
			try {
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println(Thread.currentThread().getName()+"---- consumer ----"+this.name);
		flag = false;
		this.notifyAll();
	}
}

class Producer implements Runnable {
	private Resource res;
	Producer(Resource res) {
		this.res = res;
	}
	public void run() {
		while(true) {
			res.set("wares");
		}
	}
}

class Consumer implements Runnable {
	private Resource res;
	Consumer(Resource res) {
		this.res = res;
	}
	public void run() {
		while(true) {
			res.out();
		}
	}
}