多线程-冲突与同步代码

Posted 2021-01-21 zhrb

1. 使用synchronized(互斥)解决代码冲突问题

public class TestSynchronizedThread {

    /**
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        // TODO Auto-generated method stub

        Thread t1 = new Thread(new Adder());
        Thread t2 = new Thread(new Adder());
        Thread t3 = new Thread(new Adder());

        Thread t4 = new Thread(new Subtracter());
        Thread t5 = new Thread(new Subtracter());
        Thread t6 = new Thread(new Subtracter());

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
        t6.start();

        t1.join();
        t2.join();
        t3.join();
        t4.join();
        t5.join();
        t6.join();

        System.out.println(Counter.getId());
        System.out.println("main end");
    }
}

class Adder implements Runnable {

    @Override
    public void  run() {
        for (int i = 0; i < 10000; i++)
            Counter.addId();
        System.out.println(Thread.currentThread().getName() + " end");
    }
}

class Subtracter implements Runnable {

    @Override
    public void run() {
        for (int i = 0; i < 10000; i++)
            Counter.subtractId();
        System.out.println(Thread.currentThread().getName() + " end");
    }
}

class Counter {
    private static int id = 0;

    public static  void addId() {
        synchronized (Counter.class) {
            id++;
        }
        
    }

    public static synchronized void subtractId() {
        id--;
    }

    public static int getId() {
        return id;
    }
}

2. 使用synchronized、wait、notify解决线程间的合作(同步)问题

主程序代码
1个生产者与3个消费者，同时运行，从而产生了线程的同步问题。

public class MyProducerConsumerTestUpdate {

    /**
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        Repository repo = new Repository();
        Thread producer = new Thread(new Producer(repo,1500));
        Thread consumer1 = new Thread(new Consumer(repo,500));
        Thread consumer2 = new Thread(new Consumer(repo,500));
        Thread consumer3 = new Thread(new Consumer(repo,500));
        producer.start();
        consumer1.start();
        consumer2.start();
        consumer3.start();
        producer.join();
        consumer1.join();
        consumer2.join();
        consumer3.join();
        
        System.out.println("main end! repo中还有 "+repo.size()+"个货物!");
    }

}

生产者与消费者代码

class Consumer implements Runnable {
    private Repository repo;
    private int count;//让Consumer取count次

    public Consumer(Repository repo,int count) {
        this.repo = repo;
        this.count = count;
    }

    @Override
    public void run() {
        for (int i = 0; i < count; i++) {
            repo.remove();//每回都从仓库中取出一个货物
        }
        System.out.format("取出%d个货物完毕!",count);

    }

}
class Producer implements Runnable {

    private Repository repo;
    private int count;//让Producer放入count次

    public Producer(Repository repo,int count) {
        this.repo = repo;
        this.count = count;
    }

    @Override
    public void run() {
        for (int i = 0; i < count; i++) {
            repo.add(new String("sth"));// 每回都放入一个新的货物(字符串对象)
        }
        System.out.format("放入%d个货物完毕!",count);
        
    }

}

仓库代码（重点）注意其中的wait、notify的时机

import java.util.ArrayList;
import java.util.List;

class Repository {// 存放字符串的仓库
    private int capacity = 10;//仓库容量默认为10
    private List<String> repo = new ArrayList<String>();// repo(仓库)，最多只能放10个

    public synchronized void add(String t) {
        try {
            if (size() == capacity){
                System.out.println("仓库已满,请稍等");
                wait();
            }
            repo.add(t);
            System.out.println("放一件东西到repo");
            notify();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void remove() {
        try {
            if (size() == 0) {
                System.out.println("仓库中无货物，请稍候");
                wait();
            } 
            repo.remove(0);
            System.out.println("从repo取出一件货物");
            notify();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    public int getCapacity(){//返回仓库容量
        return capacity;
    }
    public synchronized int size(){
        return repo.size();
    }
}

注意:该代码并未完全解决问题。