Hi,
I'm currently evaluating Joda for use in a project (total newbie :) and
it appears to contain just what I need.
However, I've been taking a look at the source code (version 1.4) and
came across what I think might be a potential thread safety issue in
org.joda.time.PeriodType. Maybe I've just misunderstood the
implementation - if so, I'm happy to be enlightened. :)
The various static methods which return the singleton PeriodType
instances (e.g. months(), years(), etc.) all use lazy initialization to
set the underlying static field values on first use. However, they just
use a simple guard against null to check for first use, e.g.
private static PeriodType cMonths;
...
public static PeriodType months() {
PeriodType type = cMonths;
if (type == null) {
type = new PeriodType( ... );
cMonths = type;
}
return type;
}
Two threads could simultaneously call the method, both could see (type
== null) and attempt to initialise the static field. Assigning a field
value to an object reference is not guaranteed to be atomic, so I think
there's a small chance of it causing problems. As I understand it, even
using double-checked locking isn't guaranteed to work:
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
Wouldn't it be safer to just initialise all the static fields in a
static initializer block, which is guaranteed to be called just once?
You'd lose the lazy initialization, but that's perhaps not such a big
deal.
Alastair.
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