Hi,
at the beginning of this year I had an exchange with Peter Lavart about
JDK-8136353 (see
http://mail.openjdk.java.net/pipermail/mlvm-dev/2016-January/006563.html),
and that is probably based on wrong assumptions. But I must confess I
still have trouble understanding ClassValue semantics. And since the
ClassValue problem in Groovy came up again, I though I make another try
based on a list of assumptions and asking if they are wrong or right
1) ClassValue can be basically understood as a strong reference of a
class to a class value
2) a ClassValue associated with a system class (for example
Integer.TYPE) is never garbage collected
3) a ClassValue from a different loader than the system loader,
associated with a system class, will prevent that loader to unload
4) a ClassValue referencing to the class it is associated with, does not
prevent the collection of that class
Point 2 and 3 are kind of problematic for me and I wish them wrong, but
they would follow from 1. The exchange with Peter makes me think
assumption 4 is wrong... just I don't understand why.
If those assumptions are right, then I actually wonder in what cases I
should use ClassValue without causing memory leaks. What I wanted to use
it for is to associate a meta class with every class I need a meta class
for. This includes system classes. If 3 is right, then doing so would
prevent the Groovy runtime from being unloaded. Even if the meta classes
are able to unload, the implementation of the ClassValue would still be
there. And since that comes from the same loader, that loaded the
runtime, that loader will stay. Now loading and (trying to) unload the
Groovy runtime countless times would end up in a OOME at some point
(permgen problem in older JDKs). And even if I would do something else
for class from the standard loaders, I would still get into trouble on
for example Tomcat. Not to forget that having two parallel structures
for this raises the question as of why to use ClassValue at all.
I think what it boils down to in the end is: When (under what
conditions) for what to use ClassValue at all.
bye Jochen
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