publishing objects and thread safety

Question

I read in "Java Concurrency In Practice" that "publishing objects before they are fully constructed can compromise thread safety". Could someone explain this?

Answer 1

Consider this code:

public class World{
    public static Point _point;

    public static void main(String[] args){
        new PointMaker().start();
        System.out.println(_point);
    }
}

public class Point{
    private final int _x, _y;

    public Point(int x, int y){
        _x = x;
        World._point = this;//BAD: publish myself before I'm fully constructed
        //some long computation here
        _y = y;
    }

    public void toString(){
        return _x + "," + _y;
    }
}

public class PointMaker extends Thread{
    public void run(){
        new Point(1, 1);
    }
}

Because Point publishes itself before setting the value of _y, the call to println may yield "1,0" instead of the expected "1,1".

(Note that it may also yield "null" if PointMaker + Point.<init> don't get far enough to set the World._point field before the call to println executes.)