21 April 2016

Bad String

This program has two threads printing messages alternatively. However if you change a String from "cC" to "cc" it doesn’t print anything, WHY?

public class PingPongMain {
    public static void main(String[] args) throws InterruptedException {
        new Thread(() -> {
            try {
                synchronized ("bb") {
                    while (true) {
                        "bb".wait();
                        "bb".notifyAll();
                        System.out.println("b");
                    }
                }
            } catch (InterruptedException e) {
                throw new AssertionError(e);
            }
        }).start();
        Thread.sleep(100);
        setString("bb", 'c', 'C');
        new Thread(() -> {
            try {
                // change "cC" to "cc" and this program prints nothing.
                synchronized ("cC") {
                    while (true) {
                        "cC".notifyAll();
                        "cC".wait();
                        System.out.println("c");
                    }
                }
            } catch (InterruptedException e) {
                throw new AssertionError(e);
            }
        }).start();

    }

    public static void setString(String s, char... chars) {
        try {
            Field value = String.class.getDeclaredField("value");
            value.setAccessible(true);
            value.set(s, chars);
        } catch (Exception e) {
            throw new AssertionError(e);
        }
    }
}

The solution is below

A combination of strangeness

There is a couple of elements to this puzzle

using a String literal as a Object to lock on.
corrupting a String in the literal pool.
replacing one String with another with the same hashCode so the replacement is found. "bb".hashCode() == "cC".hashCode()