java - 线程同步为什么不一样

Question

package com.dome;

public class Thread01 {

private volatile  static int a =10;
Thread td1 = new Thread(){
    
    public void run(){
        
        for(int i=0;i<3;i++){
            
                a = a+1;
            
            System.out.println(i+"td1:="+a);
        }
        
    }
    
};

Thread td2 = new Thread(){
    public void run(){
        
        for(int i=0;i<3;i++){
            a -=1;
            System.out.println(i+"td2:="+a);
        }
    }
    
};


public static void main(String[] args) {
    Thread01 th = new Thread01();
    th.td1.start();
    
    th.td2.start();
    
}

}

0td1:=9
0td2:=9
1td1:=10
1td2:=9
2td1:=10
2td2:=9

Answer 1

a = a + 1， a = a - 1 Such a statement actually involves three operations: read-modify-write:

1. Read the variable to a certain location on the stack

2. Add (subtract) 1 to the value at this position in the stack

3. Write the self-incremented value back to the storage location corresponding to the variable

So while variable a uses volatile 修饰，但并不能使涉及上面三个操作的 a = a + 1，a = a - 1具有原子性。为了保证同步性，需要使用 synchronized:

public class Thread01 {

    private volatile static int a = 10;

    Thread td1 = new Thread() {

        public void run() {

            for (int i = 0; i < 3; i++) {
                synchronized (Thread01.class) {
                    a = a + 1;
                    System.out.println(i + "td1:=" + a);
                }
            }
        }

    };

    Thread td2 = new Thread() {
        public void run() {

            for (int i = 0; i < 3; i++) {
                synchronized (Thread01.class) {
                    a -= 1;
                    System.out.println(i + "td2:=" + a);
                }
            }
        }

    };

    public static void main(String[] args) {
        Thread01 th = new Thread01();
        th.td1.start();

        th.td2.start();

    }
}

The result of a certain operation:

(Where td1 appears, a is +1; where td2 appears, a is -1)