Question

Possible Duplicate:
How to simulate constructor race conditions?
How to demonstrate race conditions around values that aren't published properly?

I got the following code from 《java concurrency in practice》:

public class Holder{

  private int n;
  public Holder(int n){this.n = n;}
  public void assertSanity(){
     if(n != n) throw new AssertionError("This statement is false.");

  }


}

I am just wondering the condition n != n, is this could be true under a certain circumstance？

Answer 1

My guess is that you are asking something similar to these questions:

• How to demonstrate race conditions around values that aren't published properly?
• How to simulate constructor race conditions?

I’m guessing the book is talking about the possibility of sharing a reference to an object before it is completely constructed, an act referred to is improper publishing.

Suppose that n != n is broken down into the following steps:

Access n on the right side of the operand
Access n on the left side of the operand
Compare values

Then it follows that it is not hard to imagine a case in which the value of n is changed between the first two steps. Now I know what you are thinking, “but n is never changed”. Actually, it is, because two threads could share access to an instance of Holder before the constructor for that instance has completely run.