//多生产者多消费者
//这是一段会产生错误数据的示例

class Resource {
    private String name;
    int count;
    boolean flag = false;

    public synchronized void produce(String name) {
        if (flag) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        this.name = name + count;
        ++count;
        System.out.println(Thread.currentThread().getName()+"----" + name+count + "---"+" produced");
        flag = true;
        notify();
    }

    public synchronized void consume() {
        if (!flag) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName() + name +count+ "consumed");
        flag = false;
        notify();
    }
}

class Producer implements Runnable {
    Resource r;

    public Producer(Resource r) {
        this.r = r;
    }

    @Override
    public void run() {
        while (true) {
            r.produce("烤鸭");
        }

    }
}

class Consumer implements Runnable {
    Resource r;

    public Consumer(Resource r) {
        this.r = r;
    }

    @Override
    public void run() {
        while(true){
            r.consume();
        }
    }
}

public class ProducerConsumer {
    public static void main(String[] args) {
        Resource r=new Resource();
        Producer producer=new Producer(r);
        Consumer consumer=new Consumer(r);
        Thread t0=new Thread(producer);
        Thread t1=new Thread(producer);
        Thread t2=new Thread(producer);
        Thread t3=new Thread(consumer);
        Thread t4=new Thread(consumer);
        Thread t5=new Thread(consumer);

        t0.start();
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();

1.线程安全问题产生的原因：

当有一个生产线程开始执行的同时，有另外的生产线程和消费线程在被wait时，生产线程在run方法末尾随机唤醒一个进程，恰好唤醒了另一个生产线程，导致该生产线程在flag=true的情况下仍然可以生产

解决办法1：

将flag的if判断变成while循环判断，解决了线程获取执行权后是否应该执行的问题。
将notify改成notifyAll，如果本方唤醒了本方的线程，没有意义，而且while判断+notify会导致死锁。

但是这种方法开销很大，可能造成很多次无用的判断，降低效率

解决办法2：使用Lock和Condition

JDK1.5 以后将同步和锁封装成了对象，并将操作锁的隐式方法变成了显式的动作。

//显式锁的使用示例
Lock  lock =new ReentrantLock();

lock.lock();
try(){
.....
需要同步的代码块
....
}
finally{
lock.unlock();
}

condition接口

子类对象可以由lock的方法获得，一个lock可以有多个condition对象。
1. await（）；
2. signal（）；
3. signalAll（）；

//示例
Lock lock=new ReentrantLock();
Condition cond=lock.newCondition();

使用lock和condition解决多生产者多消费者问题

package MultiThread;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Created by lenovo on 2017/3/14.
 */
class Item{
    public int value;

    public Item(int value) {
        this.value = value;
    }
}
class Buffer {
    Lock lock = new ReentrantLock();

    Condition notFull = lock.newCondition();
    Condition notEmpty = lock.newCondition();

    final int Max = 100;
    int putIndex=0;
    int takeIndex=0;
    int count = 0;
    final Item[] items = new Item[Max];

    public void produce() {
        lock.lock();
        try {
            while (count == Max) {
                try {
                    notFull.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            Item x=new Item(putIndex);
            items[putIndex] = x;
            System.out.println(x.value+"has been produced");
            if (++putIndex == Max)
                putIndex = 0;
            ++count;
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
    }

    public Item consume() {
        lock.lock();
        try {
            while (count == 0) {
                try {
                    notEmpty.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            Item x = items[takeIndex];
            System.out.println(x.value+"has been consumed");
            if (++takeIndex == Max)
                takeIndex = 0;
            --count;
            notFull.signal();
            return x;
        } finally {
            lock.unlock();
        }
    }
}

class Producer implements Runnable {
    Buffer r;

    public Producer(Buffer r) {
        this.r = r;
    }

    @Override
    public void run() {
        while (true) {
            r.produce();
        }

    }
}

class Consumer implements Runnable {
    Buffer r;

    public Consumer(Buffer r) {
        this.r = r;
    }

    @Override
    public void run() {
        while (true) {
            r.consume();
        }
    }
}

public class ProducerConsumer {
    public static void main(String[] args) {
        Buffer r = new Buffer();
        Producer producer = new Producer(r);
        Consumer consumer = new Consumer(r);
        Thread t0 = new Thread(producer);
        Thread t1 = new Thread(producer);
        Thread t2 = new Thread(producer);
        Thread t3 = new Thread(consumer);
        Thread t4 = new Thread(consumer);
        Thread t5 = new Thread(consumer);

        t0.start();
        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
    }
}

**注意点：
1. lock.lock()和lock.unlcok()包围同步代码块
2.lock.unlock() 需要写在finally中，保证即使同步代码块中发生异常，unlock（）仍会被执行。
3. 只需唤醒另一个监视器中的一个进程即可，不必signalAll（）。**

java 多线程学习之多生产者多消费者产生的线程安全问题分析与解决：Lock和Condition