Question

Does the out variable in the MyThread class need to be declared volatile in this code or will the “volatility” of the stdout variable in the ThreadTest class carry over?

import java.io.PrintStream;

class MyThread implements Runnable
{
    int id;
    PrintStream out; // should this be declared volatile?

    MyThread(int id, PrintStream out) {
        this.id = id;
        this.out = out;
    }

    public void run() {
        try {
            Thread.currentThread().sleep((int)(1000 * Math.random()));
            out.println("Thread " + id);
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class ThreadTest
{
    static volatile PrintStream stdout = System.out;

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(new MyThread(i, stdout)).start();
        }
    }
}

Answer 1

The volatile qualifier won’t carry through, and it serves no purpose in the above code. Volatile establishes a memory barrier on reads and writes to the variable, which is never modified once initialised in the constructor. For the same reason, the volatile qualifier in ThreadTest serves no purpose either.

Just to be clear, volatile applies to the variable, not to the referenced object.