Re: final and constructor, a Java Wart

Lew <>
Mon, 04 May 2009 02:58:41 -0400
Lew wrote:

Just to clear up something all of us, including me, are eliding, 'final' by
itself doesn't guarantee read-onliness nor thread safety in all cases.

Not thread safe:
  public class Foo
    private final List <Bar> bars = buildBars();
    private List <Bar> buildBars()
      List <Bar> ret = new ArrayList <Bar> ();
      // add elements to ret
      return ret;

coffeymex wrote:

You'd presumably agree that this is safely *published*, though?

I don't understand what you mean by this. What I would say is that only the
initial values in the List are safely published. Future changes to its
contents might not be.

Although it
obviously doesn't guarantee read-only access, it would in practice be
safe (though bad program design) if either (a) all accessing threads
explicitly synchronized on the list, or (b) all accesses were reads.

Well, yes, but that was my very point. Simply declaring a mutable object
reference as 'final' is not itself enough to guarantee thread safety; you need
further synchronization or to guarantee that all accesses are reads.

I showed the latter upthread by wrapping the List in a
'Collections.unmodifiableList()' call.


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