Synchronizing two threads printing to stderr

Question

It is my understanding that synchronization of two threads using java synchronize and notify() and wait() methods works like this:

public class Tester {
    public static void main(String[] args) throws InterruptedException {
        final Business business = new Business();
        // 子线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 50; i++) {
                    try {
                        business.sonBusiness(i);
                    } catch (InterruptedException e) {
                    }
                }
            }
        }).start();
        for (int i = 0; i < 50; i++) {
            business.mainBusiness(i);
        }
    }
}
class Business {
    public void mainBusiness(int i) throws InterruptedException {
        synchronized (this) {
            for (int j = 1; j <= 20; j++) {
                System.out.println("主线程第" + i + "轮，第" + j + "次");
            }
            this.notify();
            this.wait();
        }
    }
    public void sonBusiness(int i) throws InterruptedException {
        synchronized (this) {
            for (int j = 1; j <= 30; j++) {
                System.err.println("子线程第" + i + "轮，第" + j + "次");
            }
            this.notify();
            this.wait();
        }
    }
}

However the output that I see (see below) tells me that the synchronization is not workign as I expect. I think the main thread and the new thread may run "one by one" and in most cases they do. But I get the following output. I don't know how to explain it , please give me a hand .

What exactly is your question? If two threads can run at once? — UnholySheep, Nov 18 '16 at 08:45
@UnholySheep I think when one thread is printing , the other one must be waiting to be notified . But above output means no . — passion, Nov 18 '16 at 08:47
You can try to print to stderr in main and son Business. Or flush stdout after printing. — Matthias, Nov 18 '16 at 09:02
i think root cause here http://stackoverflow.com/questions/1883321/java-system-out-println-and-system-err-println-out-of-order — Viet, Nov 18 '16 at 09:05
You can't call `wait` until you make sure the thing you are waiting for hasn't already happened. — David Schwartz, Nov 18 '16 at 09:46

score 3 · Accepted Answer · answered Nov 18 '16 at 08:58

3

I guess it's because you are printing them to System.out and System.errat the same time. In IDE they may be the same console, but not synchronized very well.

answered Nov 18 '16 at 08:58

grape_mao

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Likely. System.out is buffered. While System.err is not. – Matthias Nov 18 '16 at 09:01
@Matthias I've never thought about this, but it's logic since error should be seen immediately. : ) – grape_mao Nov 18 '16 at 09:04
You mean the way 2 theads run as expected but the console cheated me ? – passion Nov 18 '16 at 09:09
@passion Exactly. – grape_mao Nov 18 '16 at 09:09
This is it. Try replacing `System.err` with `System.out` and you will get the expected output. – robert Nov 18 '16 at 09:24

Synchronizing two threads printing to stderr

1 Answers1