Question

While passing SCJP6 exam simulator I found question like this:

class Clerk implements Runnable {

    private Record A, B;

    public Clerk(Record a, Record b) {
        A = a;
        B = b;
    }

    public void run() {
        while(true) {
            doStuff(A, B);
        }
    }

    public synchronized void doStuff(Record a, Record b) {
        synchronized(a) {
        synchronized(b) {
            a.add(1);
            b.add(-1);
        }}
    }

}

then

Record a = new Record();
Record b = new Record();

new Thread(new Clerk(a, b)).start();
new Thread(new Clerk(a, b)).start();

Answer says that this code can cause deadlock, but I don’t get it – how exactly is that possible? Can someone can help me figure that out?

Answer 1

Apart from the fact that it does not compile, there is no deadlock in that code. This code could definitely create a deadlock:

new Thread(new Clerk(a, b)).start();
new Thread(new Clerk(b, a)).start();

So if the question is: could the Clerk class be the source of the deadlock? Then the answer is yes.

EDIT

Short example that should deadlock fairly fast. If a and b are used like in the original question, the program runs fine.

public class Test1 {

    public static void main(String[] args) {
        Record a = new Record();
        Record b = new Record();

        new Thread(new Clerk(a, b)).start();
        new Thread(new Clerk(b, a)).start();
    }

    static class Record {
    }

    static class Clerk implements Runnable {

        private Record A, B;

        public Clerk(Record a, Record b) {
            A = a;
            B = b;
        }

        public void run() {
            while (true) {
                System.out.println("in thread " + Thread.currentThread());
                for (int i = 0; i < 10000; i++) {
                    doStuff(A, B);
                }
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ex) {
                }
            }
        }

        public synchronized void doStuff(Record a, Record b) {
            synchronized (a) {
                synchronized (b) {
                }
            }
        }
    }
}