多线程如何在Java中实现同步？

Question

我有一个关于多线程的作业，我需要一些帮助

我有一个无法更改的Ressource类

public class Ressource {
    public int val;
    public void incr() {
        val++;

    }
    public void decr() {
        val--;

    }

我有我的主课

public class TwoThreads {
    public static Ressource res = new Ressource();
    public static void main(String[] args)  {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i = 0; i < 100; i++){
                    res.incr();
                }
                System.out.println(res.val);
            }
        });
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i = 0; i < 100; i++){
                    res.decr();
                }
                System.out.println(res.val);
            }
        });
        t1.start();
        t2.start();

    }
}

我尝试在重写方法中使用synchronized，但没有成功。我知道如果我用

 public synchronized void incr() {
        val++;

    }

它会起作用，但我不应该在Resources类中更改任何内容。有什么想法吗

Answer 1

i want to increase to 100 and then decrease to 0.First t1 should run and when its finished should t2 start.But i have to do it in the main method.

您可以按如下方式操作：

t1.start();
t1.join();
t2.start();

演示：

class Ressource {
    public int val;

    public void incr() {
        val++;

    }

    public void decr() {
        val ;

    }
}

public class Main {
    public static Ressource res = new Ressource();

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    System.out.println(res.val);
                    res.incr();
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 100; i++) {
                    System.out.println(res.val);
                    res.decr();
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(res.val);
            }
        });
        t1.start();
        t1.join();
        t2.start();
    }
}