[Biojava-dev] Thread safety question

[Keith James]

I was looking at lazy instantiation and thread safety issues. There
are a few well-publicised articles about the double checked locking
idiom in Java and how it doesn't quite work e.g.

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

This rang a bell regarding AbstractChangeable which does this

protected ChangeSupport getChangeSupport(ChangeType ct) {
    if(changeSupport != null) {
      return changeSupport;
    }

    synchronized(this) {
      if(changeSupport == null) {
        changeSupport = generateChangeSupport();
      }
    }

    return changeSupport;
  }

The example given in the reference is

// Broken multithreaded version
// "Double-Checked Locking" idiom
class Foo {
  private Helper helper = null;
  public Helper getHelper() {
    if (helper == null)
      synchronized(this) {
        if (helper == null)
          helper = new Helper();
      }
    return helper;
    }
  // other functions and members...
  }

where the initial test is reversed wrt the biojava example. However,
it looks to me like there might be a (theoretical) danger here (of
ChangeSupport being initialised twice). Can someone convince me
otherwise?

cheers,

Keith

-- 

- Keith James <kdj at sanger.ac.uk> Microarray Facility, Team 65 -
- The Wellcome Trust Sanger Institute, Hinxton, Cambridge, UK -