Viscent/javamtia

老师你好我爱阅读第三章volatile锁的测试书中提供的counter类，不是太能理解

Opened this issue · 2 comments

markIsCat commented

问题描述:
1、测试结果中有两个线程拿到一样的值
2、还有一种情况是线程获取的值顺序不对
count的变量值在增加的时候使用了锁，也就是同时只有一个线程对count变量做修改，读取变量增量了volatile保障读取到最新的值，但是实际反复测试确不是这样，麻烦老师解惑下谢谢
测试的代码如下
package io.github.viscent.mtia.ch3.case03;

import io.github.viscent.mtia.ch3.Counter;
import io.github.viscent.mtia.util.Debug;

public class ThreadTest {
public static void main(String[] args) {
Counter counter = new Counter();

    Thread thread =new Thread(){
        @Override
        public void run() {
            for(int i=0;i<50;i++){
                counter.increment();
                Debug.info(counter.vaule()+"");
            }
        }
    };

    Thread thread1 =new Thread(){
        @Override
        public void run() {
            for(int i=0;i<50;i++){
                counter.increment();
                Debug.info(counter.vaule()+"");
            }
        }
    };
    thread.start();
    thread1.start();
}

}
问题1 的测试结果
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问题2的测试结果
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1Jack2 commented

刚看到这里，碰巧看到了这个问题，说下我的理解（不保证正确），希望能对你有所帮助 :-)

其实每个线程增加50次，输出值中有100，说明counter的增加是符合预期的

1、测试结果中有两个线程拿到一样的值
两个线程并发读取volatile变量count的值，当然可能是一样的

2、还有一种情况是线程获取的值顺序不对

// 增加counter与打印counter并不是一个原子操作
counter.increment();
Debug.info(counter.vaule()+"");
thread1 thread2
counter.increment(); //counter = x
str = counter.vaule()+"" // "x"
counter.increment(); //counter = x + 1
str = counter.vaule()+"" // "x + 1"
Debug.info(str) // "x+1";
Debug.info(str) // "x";

3、count的变量值在增加的时候使用了锁
是CAS，不是锁

4、同时只有一个线程对count变量做修改，读取变量增量了volatile保障读取到最新的值
volatile不保证能读到最新值 ，书上有几次提到。 @markIsCat

markIsCat commented

刚看到这里，碰巧看到了这个问题，说下我的理解（不保证正确），希望能对你有所帮助 :-)

其实每个线程增加50次，输出值中有100，说明counter的增加是符合预期的

1、测试结果中有两个线程拿到一样的值
两个线程并发读取volatile变量count的值，当然可能是一样的

2、还有一种情况是线程获取的值顺序不对

// 增加counter与打印counter并不是一个原子操作
counter.increment();
Debug.info(counter.vaule()+"");

thread1 thread2
counter.increment(); //counter = x
str = counter.vaule()+"" // "x"
counter.increment(); //counter = x + 1
str = counter.vaule()+"" // "x + 1"
Debug.info(str) // "x+1";
Debug.info(str) // "x";
3、count的变量值在增加的时候使用了锁
是CAS，不是锁

4、同时只有一个线程对count变量做修改，读取变量增量了volatile保障读取到最新的值
volatile不保证能读到最新值 ，书上有几次提到。 @markIsCat

仔细看完你的解释差不多明白了，感谢！