Question

I’ve just found strange behavior in java threads.
Here is a code example:

class Job extends Thread {
    private Integer number = 0;
    public void run() {
        for (int i = 1; i < 1000000; i++) {
            number++;
        }
    }
    public Integer getNumber() {
        return number;
    }
}
public class Test {
    public static void main(String[] args)
            throws InterruptedException {
        Job thread = new Job();
        thread.start();
        synchronized (thread) {
            thread.wait();
        }
        System.out.println(thread.getNumber());
    }
}

Unexpectedly it’ll print out the 999999.
Seems like there is notify() call at the end of start() method logic.
Any ideas?

Answer 1

Seems like there is notify() call at the end of start() method logic.

Yes, this is true. When a thread finishes it does a notify() which is how Thread.join() works. Here’s a sample of the Java1.6 code for Thread.join():

public final synchronized void join(long millis) throws InterruptedException {
    long base = System.currentTimeMillis();
    long now = 0;
    if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
    }
    if (millis == 0) {
        while (isAlive()) {
            wait(0);
        }
    } else {
            ...

That said, this may be implementation dependent and should not be relied on.