多生产者多消费者问题

1单生产者单消费者

package example;


class Resource{
    private String name;
    private int num=1;
    private boolean flag=false;
    public synchronized void set(String name){
        if(flag){
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        this.name=name+num;
        num++;
        System.out.println(Thread.currentThread().getName()+"生产"+this.name);
        flag=true;
        notify();
        
    }
    public synchronized void out(){
        if(!flag){
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName()+"消费"+this.name);
        flag=false;
        notify();
    }
}
class Producer implements Runnable{
    private Resource r;
    Producer(Resource r){
        this.r=r;
    }
    public void run() {
        while(true){
            r.set("面包");
        }
    }
    
}
class Consumer implements Runnable{
    private Resource r;
    Consumer(Resource r){
        this.r=r;
    }
    public void run() {
        while(true){
            r.out();
        }
    }
    
}
public class Test{
    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(con);
        t1.start();
        t2.start();
    }
}

Thread-0生产面包1
Thread-1消费面包1
Thread-0生产面包2
Thread-1消费面包2
Thread-0生产面包3
Thread-1消费面包3
Thread-0生产面包4
Thread-1消费面包4
Thread-0生产面包5
Thread-1消费面包5
Thread-0生产面包6
Thread-1消费面包6
Thread-0生产面包7
Thread-1消费面包7
Thread-0生产面包8
Thread-1消费面包8
Thread-0生产面包9
Thread-1消费面包9
Thread-0生产面包10
Thread-1消费面包10
Thread-0生产面包11

.........................

2多生产者与多消费者模式

package example;


class Resource{
    private String name;
    private int num=1;
    private boolean flag=false;
    public synchronized void set(String name){
        while(flag){
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        this.name=name+num;
        num++;
        System.out.println(Thread.currentThread().getName()+"生产"+this.name);
        flag=true;
        notifyAll();
        
    }
    public synchronized void out(){
        while(!flag){                     //改if为while
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName()+"消费"+this.name);
        flag=false;
        notifyAll();                    //变为notifyAll()
    }
}
class Producer implements Runnable{
    private Resource r;
    Producer(Resource r){
        this.r=r;
    }
    public void run() {
        while(true){
            r.set("面包");
        }
    }
    
}
class Consumer implements Runnable{
    private Resource r;
    Consumer(Resource r){
        this.r=r;
    }
    public void run() {
        while(true){
            r.out();
        }
    }
    
}
public class Test{
    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();
    }
}

在这里我们做了两处改动，原因主要有2点。

1.如果任然用if和notify()会存在安全问题

public synchronized void set(String name){
        if(flag){            //2，t0再次取得执行权，判断后wait(），接着生产线程t1获得执行权，判断后wait（）　　　　　　　　　　　
            try {
                wait();                     //6，T0拿到执行权，并生产面包2，唤醒t2，释放锁
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        this.name=name+num;                         
        num++;
        System.out.println(Thread.currentThread().getName()+"生产"+this.name); //1,生产线程t0生产面包1,置flag为true
        flag=true;
        notify();
        
    }
    public synchronized void out(){
        if(!flag){                     //5消费线程t3取得执行权和锁，wait(),释放锁
            try {
                wait();                           //7,t2醒来，并消费面包2，唤醒t3，t3又消费面包2
            } catch (InterruptedException e) {
                // TODO 自动生成的 catch 块
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName()+"消费"+this.name); //3消费线程t2取得执行权，消费t1
        flag=false;
        notify();                                      //4唤醒t0，但t0还没有锁，flag=false;
    }

 最大问题是用if（），线程醒来后不用再判断flag了所以该用while

2如果用while加notify（），可能会造成死锁。所以用notifyAll（）