On Mon, 22 Oct 2001 11:04, Daniel Rall wrote:
> Peter Donald <[EMAIL PROTECTED]> writes:
> > I noticed you were using double checked locking in one class.
> > Unfortunately due to javas memory model this is not an approach that
> > works. In short DCL is a broken pattern and if you want the full
> > commentry there was discussion of this approach on the Javaworld site (do
> > a search for Brian Goetz + DCL).
> >
> > The probelm essentially comes down to java allowing extensive operation
> > reordering that could result in the value being assigned to value before
> > it is fully initialized and thus a check for null would pass but the
> > actual object would not be ready for use. Heres a patch to correct this
>
> We're aware of DCL issues. Patch was not attached, and I'm not sure
> which class you're referring to.
Ooops - my excuse is that I hadn't yet had my daily dose of caffeine ... err
coffee ;)
--
Cheers,
Pete
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If your life passes before your eyes when you die,
does that include the part where your life passes before
your eyes?
-----------------------------------------------------------
Index: Torque.java
===================================================================
RCS file: /home/cvspublic/jakarta-turbine-torque/src/java/org/apache/torque/Torque.java,v
retrieving revision 1.18
diff -u -r1.18 Torque.java
--- Torque.java 2001/10/21 21:06:52 1.18
+++ Torque.java 2001/10/22 00:24:12
@@ -296,23 +296,19 @@
}
}
- // Quick (non-sync) check for the map we want.
- DatabaseMap map = (DatabaseMap) dbMaps.get(name);
- if (map == null)
+ //DCL is broken !!!!!!!!!!!!!!!
+ //We can not use it in a multithreaded environment.
+ //Thus we are forced to go through synchronizaed section
+ synchronized (dbMaps)
{
- // Map not there...
- synchronized (dbMaps)
+ DatabaseMap map = (DatabaseMap) dbMaps.get(name);
+ if (map == null)
{
- // ... sync and look again to avoid race condition.
- map = (DatabaseMap) dbMaps.get(name);
- if (map == null)
- {
- // Still not there. Create and add.
- map = initDatabaseMap(name);
- }
+ // Still not there. Create and add.
+ map = initDatabaseMap(name);
}
+ return map;
}
- return map;
}
/**
@@ -334,8 +330,7 @@
try
{
DB db = DBFactory.create(getDatabaseProperty(name, "driver"));
- for (int i = 0; i < IDGeneratorFactory.ID_GENERATOR_METHODS.length;
- i++)
+ for (int i = 0; i < IDGeneratorFactory.ID_GENERATOR_METHODS.length; i++)
{
map.addIdGenerator(IDGeneratorFactory.ID_GENERATOR_METHODS[i],
IDGeneratorFactory.create(db));
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