使用 volatile int 的多线程中的兴趣案例

interest case in multithreading with volatile int

好的。我编写了代码并得到了意想不到的结果,我不知道如何解释这个结果。有人可以帮我解决这个问题吗?

public class JMM {
static volatile Boolean ready = false;
static volatile int data = 0;

public static void main() {
    Log.d("JMM", "start");
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            data = 1;
            ready = true;
        }
    }).start();

    for(int i = 0; i < 100; i++) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (!ready)
                    Log.d("JMM", "second thread data " + data);
            }
        }).start();
    }
}
}

我在 Nexus 5 上执行它(它有 4 个内核):

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MM.main();
}

结果:

D/JMM: second thread data 0

...

D/JMM: second thread data 0

D/JMM: second thread data 0

D/JMM: second thread data 0

D/JMM: second thread data 0

D/JMM: second thread data 0

D/JMM: second thread data 1

D/JMM: second thread data 1

D/JMM: second thread data 1

D/JMM: second thread data 0

D/JMM: second thread data 0

D/JMM: second thread data 1

D/JMM: second thread data 1

D/JMM: second thread data 1

D/JMM: second thread data 1

我期待什么?默认情况下 int 是原子类型(但是我之前写过 volatile )并且它不缓存它的值。但是我看到不同的线程在同一时刻从一个字段读取不同的值。谁能给我解释一下?

考虑一下这一行发生了什么:

Log.d("JMM", "second thread data " + data);
  1. 阅读data
  2. 将其转换为字符串并与 "second thread data "
  3. 连接
  4. 将两个参数传递给 Log.d
  5. 它最终打印出消息

第一步之后会发生很多事情,一个线程很可能会一个接一个地开始第 1 步,但在它之前到达第 4 步。例如:

Thread 1               | Thread 2
-----------------------+-----------------------
1. read "data"         |
2. concat string:      |
  "...data 0"          |
     <<< third thread updates data = 1 >>>
                       | 1. read "data"
                       | 2. concat string:
                       |   "... data 1"
                       | 3. invoke Log.d(...)
                       | 4. print message
                       |   with "... data 1"
3. invoke Log.d(...)   | 
4. print message       |
  with "data 0"        |