Analysis of visibility errors between threads in Java

Title Description

In the verification of volatile keyword, we found this scenario: in the code, if the comment line is executed or not, the final result is different.

When the printed information is commented out, the business logic will be executed in a dead loop, which is understandable

When the comment is released, the business logic can read the value modified by the child thread

The source of the topic and my own ideas

Related codes

public class VolatileTest {

    public static void main(String[] args) {
        VolatileThread thread = new VolatileThread();
        thread.start();

        while (true) {
//            System.out.println("thread.isFlag():" + thread.isFlag());
            if (thread.isFlag()) {
                System.out.println("----------------------");
                break;
            }
        }
    }

    public static class VolatileThread extends Thread {

        public boolean flag = false;

        @Override
        public void run() {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            flag = true;

            System.out.println("flag= " + flag);
        }

        public boolean isFlag() {
            return flag;
        }

        public void setFlag(boolean flag) {
            this.flag = flag;
        }
    }
}

What are your expectations? What is the actual error message?

Comment the result of South

Result of execution of South