What's the use of volatile

First, a small program

public class Main {

    /*volatile */static boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        m();
        Thread.sleep(1000);
        running = false;
        System.out.println("set running = false");
        Thread.sleep(1000);
        System.out.println(running);
    }

    public static void m() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("start...");
                while (running) {

                }
                System.out.println("end...");
            }
        }).start();
    }
}

This is a very simple small program , stay m Method if running by true It's a dead cycle , But sleep in the main thread 1 In seconds, the running Set to false Make it jump out of cycle printing end... Next, let's run the program and have a look

D:\dev\openjdk-18.0.2\bin\java.exe -javaagent:D:\dev\ideaIC-2022.1.4.win\lib\idea_rt.jar=60387:D:\dev\ideaIC-2022.1.4.win\bin -Dfile.encoding=UTF-8 -classpath D:\dev\java-code\JavaObjectLayout\JavaObjectLayout\target\classes;C:\Users\Administrator\.m2\repository\org\openjdk\jol\jol-core\0.16\jol-core-0.16.jar org.example.Main
start...
set running = false
false

From the running results, it doesn't jump out whlie Yeah, but running It is indeed set to false 了 , What we're going to do is running Variable plus volatile Modify and then see the operation effect

D:\dev\openjdk-18.0.2\bin\java.exe -javaagent:D:\dev\ideaIC-2022.1.4.win\lib\idea_rt.jar=60397:D:\dev\ideaIC-2022.1.4.win\bin -Dfile.encoding=UTF-8 -classpath D:\dev\java-code\JavaObjectLayout\JavaObjectLayout\target\classes;C:\Users\Administrator\.m2\repository\org\openjdk\jol\jol-core\0.16\jol-core-0.16.jar org.example.Main
start...
set running = false
end...
false

On the plus volatile after while Just jump out of the cycle normally, because if you don't add volatile Words running It is invisible between threads , Every time a thread is used running It is the second thread that takes value directly from the register to modify running The first thread doesn't know the value of, which causes it to get the value directly from the register, and the value in the register is always true It will not jump out of the loop

volatile The function of is to tell the register that the value in your cache line may be incorrect , You need to get values from the main memory ,volatile Another function is to prohibit instruction reordering , When the two instructions are not related to each other cpu Instructions may be reordered, causing problems in multithreading, so we need to use variables shared by multithreads volatile modification