Question

I’m writing a listener thread for a server, and at the moment I’m using:

while (true){
    try {
        if (condition){
            //do something
            condition=false;
        }
        sleep(1000);

    } catch (InterruptedException ex){
        Logger.getLogger(server.class.getName()).log(Level.SEVERE, null, ex);
    }
}

With the code above, I’m running into issues with the run function eating all the cpu time looping. The sleep function works, but it seems be a makeshift fix, not a solution.

Is there some function which would block until the variable ‘condition’ became ‘true’?
Or is continual looping the standard method of waiting until a variable’s value changes?

Answer 1

Polling like this is definitely the least preferred solution.

I assume that you have another thread that will do something to make the condition true. There are several ways to synchronize threads. The easiest one in your case would be a notification via an Object:

Main thread:

synchronized(syncObject) {
    try {
        // Calling wait() will block this thread until another thread
        // calls notify() on the object.
        syncObject.wait();
    } catch (InterruptedException e) {
        // Happens if someone interrupts your thread.
    }
}

Other thread:

// Do something
// If the condition is true, do the following:
synchronized(syncObject) {
    syncObject.notify();
}

syncObject itself can be a simple Object.

There are many other ways of inter-thread communication, but which one to use depends on what precisely you’re doing.