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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 Mon, 10 Jul 2006 13:47:09 GMT
Andrey Chernyshev wrote:
> On 7/7/06, Oliver Deakin <oliver.deakin@googlemail.com> wrote:
>> Andrey Chernyshev wrote:
>> > I was trying to compile the kernel classes set from DRLVM 
>> independently
>> > from the classlib and found it difficult because kernel classes set
>> > currently have a dependence on the internal classlib API, e.g.
>> > org.apache.harmony.luni.internal.net.www.protocol.jar.JarURLConnection
>> > and org.apache.harmony.luni.util.DeleteOnExit classes.
>> >
>> > I've found the spec for kernel class org.apache.harmony.kernel.vm.VM
>> > (l'm looking at
>> > 
>> classlib\trunk\modules\luni-kernel\src\main\java\org\apache\harmony\kernel\vm\VM.java)

>> >
>> > assumes that VM is calling excplicitly
>> > JarURLConnection.closeCachedFiles()
>> > and DeleteOnExit.deleteOnExit() during VM shutdown.
>> >
>> > On the other hand, there is standard API in J2SE called
>> > java.lang.Runtime.addShutdownHook(Thread) which can be used to specify
>> > the
>> > tasks that have to be done during VM shutdown.
>> > BTW, this is exactly how DRLVM implements these methods in it's
>> > VM.java. For example, for VM.closeJars() it currently does like:
>> >
>> > public static void closeJars() {
>> >     class CloseJarsHook implements Runnable {
>> >     public void run() {
>> >     JarURLConnection.closeCachedFiles();
>> >     }
>> >     }
>> >        ...
>> >        Runtime.getRuntime().addShutdownHook(new Thread(new
>> > CloseJarsHook()));
>> > }
>> >
>> > Are there any problems with this approach, should the DRLVM (or other
>> > Vm's) implement these methods differently?
>> There is a potential problem with this approach. The code that is run 
>> by the
>> shutdown hooks is not restricted in its behaviour and the order that the
>> shutdown hooks are executed in is not guaranteed. If the deleteOnExit()
>> and/or closeCachedFiles are not the last hooks to be executed, it is 
>> quite
>> possible that a subsequent shutdown hook could try to access files that
>> have already been deleted. The only way to guarantee that this will
>> never happen is to make sure that deleteOnExit() and closeCachedFiles()
>> are called after all shutdown hooks have completed.
>> Of course, how this is implemented is down to the VM developer - but
>> I would strongly recommend not using shutdown hooks for this purpose.
> 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
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 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
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.

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.

>> >
>> > 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).


> Thanks,
> Andrey.
>> Regards,
>> Oliver
>> -- 
>> Oliver Deakin
>> IBM United Kingdom Limited
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Oliver Deakin
IBM United Kingdom Limited

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