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From Oliver Deakin <oliver.dea...@googlemail.com>
Subject Re: [classlib] Is it OK for VM kernel class to call internal classlib API?
Date Tue, 11 Jul 2006 12:10:54 GMT
Andrey Chernyshev wrote:
> On 7/10/06, Oliver Deakin <oliver.deakin@googlemail.com> wrote:
>> Andrey Chernyshev wrote:
>> <snip>
>> >
>> > Thanks Oliver, this explanation sounds reasonable.
>> > If the issue is just in the shutdown hooks order, then, still may be
>> > it makes sense to add VM.addClasslibShutdownHook(Runnable) method or
>> > something like that, which can be:
>> > - used by the classlib to do whatever resource cleanup / shutdown
>> > works they need;
>> > - guaranteed by VM to be executed always after Runtime's shutdown
>> > hooks are done (can be specified in the contract)?
>> > This approach looks more universal than having two specific methods
>> > cloaseJars() and deletOnExit(). It will also allow VM to do not call
>> > internal classlib API explicitly.
>>
>> I think the same problem exists with this approach. You still need to
>> guarantee that closeCachedFiles() and deleteOnExit() are the last 
>> classlib
>> shutdown hooks that are called, otherwise they may cause problems
>
> I think class lib could add just one shutdown hook which is guaranteed
> by VM to be run after all other (e.g. application's) shutdown hooks
> are completed.

How will the VM know which shutdown hook is the right one?

> Within this shutdown hook, classlib are free to choose whatever order
> they like, e.g. run all others hooks (whatever may appear in classlib
> in the future) and then finally call closeCachedFiles() and
> deleteOnExit().
>
> My point was that, if classlib do really care about the shutdown
> sequence of it's internal resources, it should be classlib who defines
> the shutdown hooks execution order, not VM. IMHO there is no need to
> bring this up to the classlib<->VM contract.

Isn't adding a special shutdown hook that the classlib requires and
the VM runs last another classlib<->VM contract?

>
>> with any classlib shutdown hooks called after them. Once you put
>> a restriction on the classlib shutdown hooks that closeCachedFiles() and
>> deleteOnExit() will be that last ones called, you are basically
>> in a similar situation to just calling the methods explicitly after all
>> shutdown hooks complete.
>
> I agree the situation is exactly the same in terms of the code being
> executed, the only question is where this code is located - VM.java
> (which is a part of VM, right?) or classlib. I think the good
> interface between VM and classlib would minimize VM's knowledge about
> classlib and vice versa. In the ideal situation, I would limit it to a
> subset of J2SE API.

But remember that the kernel classes are the Java part of the interface 
between
the VM and classlib, not wholly a part of the VM. They are implemented 
by the
VM vendor because they require internal knowledge of the VM (or vice versa)
 - this does not mean that they cannot have internal knowledge of the class
libraries also.

>
>>
>> I think it would be fine to add an addClasslibShutdownHook() method
>> to VM if it was needed, but at the moment I don't believe it is. You are
>
> well, to be precise, I'm suggesting to replace existing VM.closeJars()
> and VM.deletOnExit() with VM.addClasslibShutdownHook(), not just to
> add :)
>
>> always going to be in the position of requiring closeCachedFiles()
>> and deleteOnExit() to run last, and adding an extra shutdown hook
>> mechanism will not change that.
>
> I thought of:
> - VM could guarantee that the classlib shutdown hook is run last;
> - Classlib could guarantee that closeCachedFiles() and deleteOnExit()
> are called last within the classlib shutdown hook.

This seems to me like saying "the VM will only know of one special
method to run instead of two". The difference is small IMHO, and
I cannot see much advantage. I can understand what you are trying to
do, but I don't think the effort is necessary.

Also, these are not the only instances of the kernel classes
having knowledge of classlib internals (Runtime class uses
org.apache.harmony.luni.internal.process.SystemProcess.create()
for example), so making this change will not resolve all the
VM-classlib contracts.

Regards,
Oliver

>
>>
>> If you feel uncomfortable with the vm directly calling
>> JarURLConnection.closeCachedFiles() and
>> DeleteOnExit.deleteOnExit(), then perhaps those calls could
>> be moved into VM.java? So, for example, you could add a private
>> method to DRLVM's VM.java which is called by the vm after running
>> all shutdown hooks, and would be responsible for calling 
>> closeCachedFiles()
>> and deleteOnExit(). This way the vm is only calling methods within 
>> the VM
>> class, and not directly into classlib internals.
>
> Yes, of course the dependencies could be localized within VM java.
> However, I don't think it really matters where these dependencies are
> coming from - VM.java or any other VM file, we either have them
> between VM and classlib or not :)
>
>>
>>
>> >
>> >>
>> >> >
>> >> > May be it makes sense just to move VM.closeJars() and
>> >> > VM.deleteOnExit() methods and their drlvm implementation to the 
>> luni
>> >> > classlib component, under assumption that they do not really 
>> contain
>> >> > any VM-specific code?
>> >>
>> >> The version of VM.java currently under luni-kernel does not contain
>> >> any VM specific code either, as all its methods simply return null :)
>> >> It does, however, give hints as to how to implement the methods
>> >> in the javadoc comments (perhaps it should clarify the reason
>> >> for not using shutdown hooks).
>> >>
>> >> As described above, I think there is a problem with this 
>> implementation,
>> >> so I would not like to see it used as an example for other VM
>> >> developers.
>> >>
>> >>
>> >> I have noticed that the Javadoc comments for the DRLVM implementation
>> >> of deleteOnExit() and closeJars() both say:
>> >>
>> >>  /**
>> >>     * 1) This is temporary implementation
>> >>     * 2) We've proposed another approach to perform shutdown actions.
>> >>     */
>> >>
>> >> Have I missed the proposal of this other approach, or are the 
>> comments
>> >> inaccurate?
>> >
>> > I guess there were no one on the mailing list, this seems to be a
>> > history of the past internal discussions within drlvm development
>> > team.
>> >
>> >>
>> >> > I guess it will simplify a bit the org.apache.harmony.kernel.vm.VM
>> >> > interface as well as would allow to avoid extra dependencies 
>> between
>> >> > VM and classlib.
>> >> >
>> >>
>> >> IMHO there is no problem for kernel classes to have dependencies on
>> >> classlib - they are, after all, just another part of the class
>> >> library that
>> >> happens to get developed separately due to their nature.
>> >
>> > Actually I thought of the kernel classes being part of VM since they
>> > are tied to VM-specific functionality. If this is true, then it may be
>> > good if we can keep the interface between VM and classlib "clean",
>> > e.g. without  dependencies on the API which doesn't appear in J2SE.
>>
>> Although we want to keep the classlib VM-independent, I don't think 
>> there
>> is a need to keep the vm kernel totally classlib-independent (since the
>> Harmony project currently aims to only have one classlib but multiple 
>> VMs).
>> In other words, I don't see a problem with any kernel implementation 
>> having
>> knowledge of classlib internals, especially when they are contained
>> within the
>> same module (ie luni-kernel has deps on luni).
>
> One possible inconvenience could be, if the shutdown hooks are changed
> somehow in classlib (e.g. more hooks are added, or existing ones are
> changed/removed), shutdown hooks interface will be changed and VM will
> have to be updated synchronously. Isn't this a modularity issue?
> However,  I completely agree this works OK as long as we have one
> classlib and as long as it's internal implementation (e.g resource
> cleanup) isn't changed.
> I think this is just a matter of archiving the good modularity level,
> it is true the code can always work without it.
>
> Thanks,
> Andrey.
>
>>
>> Regards,
>> Oliver
>>
>> >
>> > Thanks,
>> > Andrey.
>> >
>> >>
>> >> Regards,
>> >> Oliver
>> >>
>> >> --
>> >> Oliver Deakin
>> >> IBM United Kingdom Limited
>> >>
>> >>
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>> >>
>> >>
>> >
>> >
>>
>> -- 
>> Oliver Deakin
>> IBM United Kingdom Limited
>>
>>
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>>
>>
>
>

-- 
Oliver Deakin
IBM United Kingdom Limited


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