多线程学习笔记五-------------多生产者多消费者问题

线程通信--多生产者多消费者问题
  多生产者,多消费者会导致线程死锁的情况。

public class RoastDuck {
        public static void main(String[] args) {
            Duck d = new Duck();
            ProductProcess pp = new ProductProcess(d);
            ProductProcess pp0 = new ProductProcess(d);
            ConsumeProcess cp = new ConsumeProcess(d);
            ConsumeProcess cp0 = new ConsumeProcess(d);
            Thread t0 = new Thread(pp0);
            Thread t1 = new Thread(pp);
            Thread t2 = new Thread(cp);
            Thread t3 = new Thread(cp0);
            t0.start();
            t1.start();
            t2.start();
            t3.start();
        }
    }
    class ProductProcess implements Runnable {
        Duck d ;
        public ProductProcess(Duck d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                synchronized(d){
                    d.product("好吃的烤鸭");
                }
            }
        }
    }
    class ConsumeProcess implements Runnable{
        Duck d ;
        public ConsumeProcess(Duck d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                synchronized(d){
                    d.consume();
                }
            }
        }
        
    }
    class Duck{
        private String name;
        private int count = 1;
        private boolean flag = false;
        public void product(String name){
            //第一种写法:
            if(flag == false){ 
                this.name = name+count;
                count++;
                System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
                flag = true;
                this.notify();
            }else{
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //第二种写法:
            /*if(flag == true){
                try {
                    this.wait();//如果这样写,程序会在这里进入线程池,下次被唤醒的时候就不会进行判断,直接进入下面的操作,会出现线程安全问题。
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            this.name = name+count;
            count++;
            System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
            flag = true;
            this.notify();*/
            //第二种方法改正:
            /*while(flag){
                try {
                    this.wait();//如果这样写,程序会在这里进入线程池,下次被唤醒的时候就不会进行判断,直接进入下面的操作,会出现线程安全问题。
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            this.name = name+count;
            count++;
            System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
            flag = true;
            this.notify();*/
        }
        public void consume(){
            if(flag == true){
                System.out.println(Thread.currentThread().getName()+" : consume  "+this.name +"  duck");
                flag = false;
                this.notify();
            }else{
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

产生原因:
因为每次唤醒的线程都是任意的,如果唤醒线程的时候总是唤醒本方的线程(例如都是生产者或者都是消费者),就会导致线程死锁。
解决思路:
每次唤醒都要保证有对方的线程被唤醒。
解决方法:
每次都唤醒所有的线程。使用notifyAll();方法。

public class RoastDuck {
        public static void main(String[] args) {
            Duck d = new Duck();
            ProductProcess pp = new ProductProcess(d);
            ProductProcess pp0 = new ProductProcess(d);
            ConsumeProcess cp = new ConsumeProcess(d);
            ConsumeProcess cp0 = new ConsumeProcess(d);
            Thread t0 = new Thread(pp0);
            Thread t1 = new Thread(pp);
            Thread t2 = new Thread(cp);
            Thread t3 = new Thread(cp0);
            t0.start();
            t1.start();
            t2.start();
            t3.start();
        }
    }
    class ProductProcess implements Runnable {
        Duck d ;
        public ProductProcess(Duck d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                synchronized(d){
                    d.product("好吃的烤鸭");
                }
            }
        }
    }
    class ConsumeProcess implements Runnable{
        Duck d ;
        public ConsumeProcess(Duck d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                synchronized(d){
                    d.consume();
                }
            }
        }
        
    }
    class Duck{
        private String name;
        private int count = 1;
        private boolean flag = false;
        public void product(String name){
            //第一种写法:
            if(flag == false){ 
                this.name = name+count;
                count++;
                System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
                flag = true;
                this.notifyAll();
            }else{
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //第二种写法:
            /*if(flag == true){
                try {
                    this.wait();//如果这样写,程序会在这里进入线程池,下次被唤醒的时候就不会进行判断,直接进入下面的操作,会出现线程安全问题。
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            this.name = name+count;
            count++;
            System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
            flag = true;
            this.notifyAll();*/
            //第二种方法改正:
            /*while(flag){
                try {
                    this.wait();//如果这样写,程序会在这里进入线程池,下次被唤醒的时候就不会进行判断,直接进入下面的操作,会出现线程安全问题。
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            this.name = name+count;
            count++;
            System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
            flag = true;
            this.notifyAll();*/
        }
        public void consume(){
            if(flag == true){
                System.out.println(Thread.currentThread().getName()+" : consume  "+this.name +"  duck");
                flag = false;
                this.notifyAll();
            }else{
                try {
                    this.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

notifyAll()方法解决了一定能唤醒对方线程的问题。
以前解决多生产者多消费者的情况就用while循环判断+notifyAll解决。
但是一唤醒就全醒了,本方也醒了,本方还要重新判断标记。
这些问题在JDK1.5给出了解决方案:
1.5以前,我们用的synchronized同步代码块是隐式的,操作起来不灵活。
1.5以后,新增了一个接口---->lock
在java.util.concurrent.locks包中。
  public interface Lock
  Lock 实现提供了比使用 synchronized 方法(同步函数)和语句(同步块)可获得的【更广泛】的锁定操作。
  此实现允许更灵活的结构,可以具有差别很大的属性,可以支持多个相关的 Condition 对象。
  语法形式:
    lock.lock();
    code...
    lock.unlock();
  将同步和锁封装成了对象,并将操作锁的隐式方式定义到了该对象中,将隐式动作变成了显示动作。
  public interface Condition
  Condition 将 Object 监视器方法(wait、notify 和 notifyAll)分解成截然不同的对象,以便通过将这些对象与任意 Lock 实现组合使用,为每个对象提供多个等待 set(wait-set)。
  其中,Lock 替代了 synchronized 方法和语句的使用,Condition 替代了 Object 监视器方法的使用。

1.5之前解决线程安全问题:

class Object{
        public final void wait() throws InterruptedException {
            wait(0);
        }
        public final native void notify();
        public final native void notifyAll();
    }
    class Demo extends Object{
    }
    class MyThread implements Thread{
        Demo d = new Demo();
        public void run(){
            synchronized(d){
                d.wait();
            }
        }
    }

1.5之后解决线程安全问题:

class Condition{
         void await() throws InterruptedException;
         void signal();
         void signalAll();
    }
    Lock lock = new ReentrantLock();
    Condition c1 = lock.newCondition();
    Condition c2 = lock.newCondition();
    lock.lock();
    try{
        code...
    }finally{
        lock.unlock();
    }

1.5以前,一个锁上只能由一组监视器,这组监视器既监视生产者又监视消费者。
1.5以后,一个锁上面弄两组监视器,一组监视器监视生产者,一组监视器监视消费者。
用Lock解决线程死锁问题:

import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;
    
    public class RoastDuckLock {
        public static void main(String[] args) {
            Duck2 d = new Duck2();
            ProductProcess2 pp = new ProductProcess2(d);
            ProductProcess2 pp0 = new ProductProcess2(d);
            ConsumeProcess2 cp = new ConsumeProcess2(d);
            ConsumeProcess2 cp0 = new ConsumeProcess2(d);
            Thread t0 = new Thread(pp0);
            Thread t1 = new Thread(pp);
            Thread t2 = new Thread(cp);
            Thread t3 = new Thread(cp0);
            t0.start();
            t1.start();
            t2.start();
            t3.start();
        }
    }
    class ProductProcess2 implements Runnable {
        Duck2 d ;
        public ProductProcess2(Duck2 d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                d.product("好吃的烤鸭");
            }
        }
    }
    class ConsumeProcess2 implements Runnable{
        Duck2 d ;
        public ConsumeProcess2(Duck2 d) {
            this.d = d;
        }
        @Override
        public void run() {
            while(true){
                d.consume();
            }
        }
        
    }
    class Duck2{
        private String name;
        private int count = 1;
        private boolean flag = false;
        Lock lock = new ReentrantLock();
        Condition product_con = lock.newCondition();
        Condition consume_con = lock.newCondition();
        
        public void product(String name){
            lock.lock();
            try{
                if(flag == false){ 
                    this.name = name+count;
                    count++;
                    System.out.println(Thread.currentThread().getName()+" : product  "+this.name +"  duck");
                    flag = true;
                    consume_con.signal();
                }else{
                    try {
                        product_con.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }finally{
                lock.unlock();
            }
        }
        public void consume(){
            lock.lock();
            try{
                if(flag == true){
                    System.out.println(Thread.currentThread().getName()+" : consume       "+this.name +"  duck");
                    flag = false;
                    product_con.signal();
                }else{
                    try {
                        consume_con.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }finally{
                lock.unlock();
            }
        }
    }

Lock接口:代替了同步块和同步函数,将隐式锁操作变成了显示锁操作。同时更加灵活,可以一个锁上面加多组监视器。
lock():获取锁
unlock():释放锁,需要放在finally当中。
Condition接口:出现了代替wait() notify() notifyAll(),并将这些监视器方法进行了封装,变成了Condition监视器对象。可以与任意锁进行组合。
await();
signal();
signalAll();
JDK文档内的示例:相对来说复杂,但是很经典,真正开发用的也是这种。

class BoundedBuffer {
       final Lock lock = new ReentrantLock();
       final Condition notFull  = lock.newCondition(); 
       final Condition notEmpty = lock.newCondition(); 
    
       final Object[] items = new Object[100];
       int putptr, takeptr, count;
    
       public void put(Object x) throws InterruptedException {
         lock.lock();
         try {
           while (count == items.length) 
             notFull.await();
           items[putptr] = x; 
           if (++putptr == items.length) putptr = 0;
           ++count;
           notEmpty.signal();
         } finally {
           lock.unlock();
         }
       }
    
       public Object take() throws InterruptedException {
         lock.lock();
         try {
           while (count == 0) 
             notEmpty.await();
           Object x = items[takeptr]; 
           if (++takeptr == items.length) takeptr = 0;
           --count;
           notFull.signal();
           return x;
         } finally {
           lock.unlock();
         }
       } 
     }

 

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