How the main thread sleep/interrupt works in following code

Question

How the second loop actually interrupts the sleeping main thread, and first does not?? My understanding is after Thread.sleep(3000), the code Thread.currentThread().interrupt() will be executed after 3 seconds. Can anyone explain how it actually works

for (int i = 0; i < 2; i++) {
            try {
                System.out.println("loop : " + i);
                Thread.sleep(3000);
                System.out.println("Woke up");
                Thread.currentThread().interrupt();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

loop : 0
Woke up
loop : 1
java.lang.InterruptedException: sleep interrupted
exception loop:1
    at java.base/java.lang.Thread.sleep(Native Method)
    at multithreadings.Mainclass.main(Mainclass.java:13)

Answer 1

Interruption is a polite request to stop: a Thread is under no obligation to stop.

It's like the Robin Williams joke about what police in the UK say when you commit a crime:

Stop! Or I'll say stop again!

Also, interrupting a thread doesn't cause an InterruptedException to be thrown: it merely sets a flag on the thread. If something (like Thread.sleep) checks this flag, and finds that it is set, it may then throw an InterruptedException; but the flag and exception are two orthogonal ways of indicating interruption.

As such:

• On the first execution, you sleep for 3 seconds, then set the interrupted flag, and the loop body finishes normally.
• On the second execution, you ask to sleep for 3 seconds, but Thread.sleep detects the interrupted flag, and throws the exception.