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From Regis <xu.re...@gmail.com>
Subject Re: [class loading] what's the expected behavior when a static method is invoked before its class is initialized?
Date Mon, 05 Jan 2009 06:19:29 GMT


Xiao-Feng Li wrote:
> On Mon, Jan 5, 2009 at 5:12 AM, Ian Rogers <rogers.email@gmail.com> wrote:
>> 2009/1/4 Nathan Beyer <ndbeyer@apache.org>
>>
>>> Could any of the IBM folks on the list get some advice from some class
>>> loader experts?
>>>
>>> -Nathan
>>>
>>  Hi Nathan,
>>
>> from reading the description I can describe how Jikes RVM avoids this
>> problem (I'm not an IBM VME expert and I've not run the test case to check
>> that Jikes RVM passes it). In Jikes RVM we have two variants of all calls,
>> ones to unresolved methods and ones to resolved methods. Unresolved calls
>> are to classes whose initializer hasn't yet been run. If two threads are
>> calling a static method the first will resolve it and in the process acquire
>> a lock, the second thread must wait for the lock before it can attempt to
>> resolve the method (at which point it will discover the method was resolved
>> by the other thread and leave early). Checking for classes being resolved
>> litters all of the class loader code, and we're slightly proactive in
>> resolving in the case of reflected methods so that we needn't check for
>> resolution when performing reflected method invocation (which is now pretty
>> much unnecessary since [1] where we generate bytecodes at runtime to perform
>> reflection). An aside, I wrote a paper where I use the class loader as a
>> test case for optimizations based on stationary/immutable fields specified
>> via constraints [2], this work specialized the class loader to handle the
>> resolved case as a class is normally accessed when it is resolved (figures
>> in the paper).
> 
> Thanks for the explanation.
> 
> Let me try to explain the problem we are meeting:
> 
> 1. A thread invokes a static method createChild of a class Parent. It
> causes class Parent be initialized.
> 
> 2. In Parent's initialization, it creates an array of class Child. It
> leads to class Child be initialized.
if add following code to Child
     static {
         System.err.println("init Child");
     }
the output on RI is:
null
init Child
null

It seems create array of class Child doesn't lead to class Child be 
initialized

if change childCache from array to instance:

     private static final Child childCache = new Child();

     public static Child createChild(){
         return childCache;
     }
the output on RI is:
init Child
Child@affc70
null

It seems when initialize Child, Parent.createChild() is called, and read 
the value of childCache, it's not initialized, so null is returned (it's 
already in initialize process, so just return null?).
> 
> 3. In Child's initialization, it invokes Parent.createChild() to
> initialize a static field ROOT. This leads to re-entrance of Parent
> initialization.
> 
> 4. The thread finds Parent is under initialization by itself, it quits
> the initialization process and invokes Parent.createChild().
> 
> 5. This invocation should not be permitted by the VM spec, and we
> don't know how to deal with it. It can't wait for the initialization
> process finished, because it is in the path of the initialization. It
> has to do something to proceed with the initialization.
> 
> Below is the code of the micro test. Chunrong, please correct me if my
> understanding is inaccurate.
> 
> So my suggestion is to ignore the static method invocation for the
> class under initialization...
> 
> 
> public class Main {
>     public static void main(String[] args) {
>         Child c = Parent.createChild();
>         System.err.println(c);
>         System.err.println(Child.ROOT);
>     }
> }
> 
> class Parent {
>     private static final Child[] childCache = new Child[5];
> 
>     public static Child createChild(){
>         return childCache[0];
>     }
> }
> 
> class Child {
>     public static final Child ROOT = Parent.createChild();
> }
> 
> Thanks,
> xiaofeng
> 
>> Regards,
>> Ian Rogers
>>
>> [1]
>> http://icooolps.loria.fr/icooolps2008/Papers/ICOOOLPS2008_paper08_Rogers_Zhao_Watson_final.pdf
>> [2] http://portal.acm.org/citation.cfm?id=1411746
>>
> 
> 
> 

-- 
Best Regards,
Regis.

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