What is the difference between wait/notify and wait/interrupt?

Question

synchronized (Foo.class) {
    while (someCondition) {
        try {
            Foo.class.wait();
        } catch (InterruptedException e) {
            e.printStac         


        

Answer 1

When a thread calls notify on some monitor, it wakes up a single thread that's waiting on that monitor, but which thread gets woken is decided by the scheduler. (Alternatively a thread can call notifyAll, which wakes up all the threads waiting for that monitor, then they all contend for the monitor, then the losers go back to waiting.) That's why the target of the call is different, the notification is made to the monitor, which tells the scheduler to pick a thread to wake up.

Unlike notify, interruption targets a specific thread. And interruption does not require that the interrupted thread be waiting on a monitor. For a thread to call wait on a monitor it has to have acquired that monitor first, then wait releases that monitor until the thread is done waiting or is interrupted.

Oracle's recommendation is to use interruption only for cancellation. Also the classes in java.util.concurrent are designed to use interrupt for cancellation.

In your example interruption won't be very effective, because control doesn't leave the while loop, the thread still has to check the condition it's waiting on, and there's no check in the while loop condition for whether the interrupt flag is set. It's likely the thread that's interrupted will go right back to waiting.

In order to make this code quit once it's interrupted, rather then return to waiting, add a check for the interrupted flag status to the loop condition, and have the catch block set the interrupt flag (which gets reset when the exception is thrown):

synchronized (Foo.class) {
    while (someCondition && !Thread.currentThread().isInterrupted()) {
        try {
            Foo.class.wait();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();            
        }
    }
}