1.volatile 針對b=b+1 不是原子操作的

　　理論上來說結果等於1000，實際上很多時候小於1000

package com.sun.util;
class A {
    volatile int b=0;

       void add(int n){
        for (int i = 0; i < n; i++) {
            b+=1;
        }
    }
   
}
public class VolatileTest {
    

    public static void main(String[] args) {
        A aaa 
 
= new A();
        Thread[] t=new Thread[100];
        for (int i = 0; i < t.length; i++) {
            final int x=10;
            t[i]=new Thread(()->aaa.add(10));
        }
        for (int i = 0; i < t.length; i++) {
            t[i].start();
        }
        for (int i = 0; i < t.length; i++) {
            
 
try {
                t[i].join();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        System.out.println(aaa.b);


    }
}

2.用synchronized

package com.sun.util;
class A {
    volatile
 
 int b=0;

    void add(int n){
        for (int i = 0; i < n; i++) {
//            b+=1;
            inc();
        }
    }
    synchronized void inc(){
        b=b+1;
    }

   
    

}
public class VolatileTest {
    

    public static void main(String[] args) {
        A aaa = new A();
        Thread[] t=new Thread[100];
        for (int i = 0; i < t.length; i++) {
            final int x=i+1;
            t[i]=new Thread(()->aaa.add(x));
        }
        for (int i = 0; i < t.length; i++) {
            t[i].start();
        }
        for (int i = 0; i < t.length; i++) {
            try {
                t[i].join();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        System.out.println(aaa.b);



    }
}

或者

package com.sun.util;
class A {
    volatile int b=0;

    synchronized void add(int n){
        for (int i = 0; i < n; i++) {
            b+=1;
//            inc();
        }
    }
//    synchronized void inc(){
//        b=b+1;
//    }

   
    

}
public class VolatileTest {
    

    public static void main(String[] args) {
        A aaa = new A();
        Thread[] t=new Thread[100];
        for (int i = 0; i < t.length; i++) {
            final int x=i+1;
            t[i]=new Thread(()->aaa.add(x));
        }
        for (int i = 0; i < t.length; i++) {
            t[i].start();
        }
        for (int i = 0; i < t.length; i++) {
            try {
                t[i].join();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        System.out.println(aaa.b);

    }
}