为什么该程序总是打印1

Question

I have tested the following code many times and it always prints 1, Is it correct and why?

public class TestThread {
    static int a = 0;
    public static void main(String[] args) {
         Thread thread = new Thread() {
            @Override
            public void run() {
                System.out.println(a);
            }
        };
        thread.start();
        a = 1;
   }
}

Answer 1

This is because the application doesn't wait for the thread to be over to continue its execution. It seems like most of the time the affectation

a = 1

is faster than the thread to be executed. That's why It is important in some cases to wait for your thread to be over.

If you set up a breakpoint to your a = 1 line, you will see 0 is printed.

Now try this :

public class TestThread {
    static int a = 0;
    public static void main(String[] args) throws InterruptedException {
         Thread thread = new Thread() {
            @Override
            public void run() {
                System.out.println(a);
            }
        };
        thread.start();
        thread.join();
        a = 1;
   }
}

Using the join() method, It will wait for thread to be finished.

You can also use the static Thread.sleep function but it is definitely not how I recommend to solve this issue since you can't know for sure how long it takes before your thread is complete.

thread.start();
Thread.sleep(100);
a = 1;

Answer 2

It takes a little while to start the thread that prints the value, so the 'main' thread sets a to 1 before the system.out is reached. If you add a little delay before a := 1; the system.out is reached before a := 1; and then 0 is printed:

    thread.start();
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) { }
    a = 1;