wait without notify

[Asad Zubair]

The following code prints 1000 and I am not sure why as there is no call to notify anywhere in the code.
I thought that a call to wait would cause the currently running thread to wait until something calls notify on the job object. Since that never happens I assumed the program would just be stuck. Please help clear my thoughts here.

class Job extends Thread {

    private int number;
    public Job() {        
    }
    
    public void setNumber(int number){
    	this.number = number;
    }
    
    public int getNumber(){
    	return number;
    }
   
    public void run() {
        for (int i = 1; i <= 1000; i++) {
            number++;
        }
    }

}

public class WaitNotify {
    public static void main(String[] args){
        Job job = new Job();
        job.start();
        
        synchronized(job){
        	try {
				job.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
        }
        System.out.println(job.getNumber());
    	
    }
}

[Stephan van Hulst]

wait() doesn't tell the the Thread you're calling it on to wait (indeed, wait() is a method in Object, not in Thread). wait() tells the *current* thread executing it to wait.

Since your job runs separately from the thread calling wait(), the job continues without a problem. Your main thread however would stall forever if it wasn't for spurious wake-ups.

[Asad Zubair]

So, in the example above ...
Is the "current thread" the main thread or the job thread ?
If it is the main thread then how does it move past job.wait() without the notify() being called on the job object somewhere. Is it because of the spurious wake-ups that you talk about ?

[Stephan van Hulst]

Exactly.

You should always wrap a call to wait() in a while loop, to check whether the condition that you are waiting for is indeed true.

A simple example:

private boolean ready;

synchronized void doSomething() {
    try {
        while (!ready) {
            wait();
        }
        System.out.println("Finished doing something!");
    } catch (InterruptedException ex) {
        System.out.println("Interrupted while doing something!");
    }
}

synchronized void finish() {
    ready = true;
    notifyAll();
}

[Henry Wong]

This issue comes up often on JR -- and is basically caused by an implementation detail of the join() method. Please do a search for something like "wait join wakeup". For example, this is one of the first hits.

https://coderanch.com/t/268246/java-programmer-SCJP/certification/Thread-wait-notify

Henry

[Asad Zubair]

Thank you guys for the explanation. I got it now