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From "Xiao-Feng Li" <xiaofeng...@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 04:08:33 GMT
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.

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
>



-- 
Managed Runtime Technology Center, Intel

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