生产者和消费者问题

Posted 2022-12-14 薰衣草

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