David Holmes - Sun Microsystems
David.Holmes at Sun.COM
Thu Aug 9 04:02:08 PDT 2007
There was past on work on hosting multiple VM's within a single VM (with
both in process and out-of-process variants). This was based around
Isolates and the experimental MVM system.
I don't know the end result of this work, nor whether some aspect of it
is still being developed. (The sunlabs site seems to be having problems
Markus KARG said the following on 9/08/07 07:12 PM:
> Steve Bohne wrote:
>> Very interesting project.
> Thank you. In fact, the target of the project is to provide a general
> solution that allows to write Shell Extensions in pure Java (without
> further knowledge of the native desktop API), and add it to either JDIC
> and / or OpenJDK. The Java peer of that shall be a simple API that can
> not only be implemented for Windows (as I do it right now) but hopefully
> will be implemented by others (or Sun?) for KDE, Gnome, MacOS etc.
> Integrating Java deeply into the desktop is one of my key interests and
> I wonder why there is not yet something like that already.
>> When your DLL gets unloaded, does jvm.dll also get unloaded? As you
>> mentioned, if jvm.dll is getting loaded twice, even sequentially, this
>> might cause problems. (Yes, it's a known limitation of embedding a
>> JVM in another application. Don't know of any current activity to work
>> on a solution for this issue, but submitted patches will be carefully
>> considered. :))
> Unfortunately I am not understanding the jvm.dll source code well enough
> to provide a patch, but that limitation really is a huge problem for
> embedding projects like this one (which, BTW, are needed for better
> acceptance of Java by the end users of the applications).
> I tried three things: (a) Keep the JavaVM* pointer and do not load a
> second time, (b) Always LoadLibrary but never unload, (c) Always
> LoadLibrary and FreeLibrary even in the same single method. In fact, all
> three ways of handling procude the same problem: Everything works well
> (since Windows kindly maps the DLL to the same memory region ever as it
> seems), but AttachCurrentThread crashs down.
>> Did you try using explicit linking (use LoadLibrary, GetProcAddress,
>> etc) for jvm.dll so it won't automatically get unloaded when your DLL
> Yes, see above.
>> Are you able to catch the crash in a debugger and provide a symbolic
>> stack trace? I have a feeling it will reflect the limitation mentioned
>> above, so it may not lead to a solution, but it might be educational.
> Unfortunately I do not know how. I have no full C++ development
> environment but just work with the free Microsoft Compiler plus a text
>> P.S. Be careful embedding Java (or any single instance runtime) in a
>> shell extension:
>> http://blogs.msdn.com/junfeng/archive/2005/11/18/494572.aspx. Many of
>> the limitations mentioned here for CLR will also apply to Java.
> Yes I thought about that right from the start. Maybe I will change my
> architecture to not use JNI in the Shell Extension, but to instead call
> an out-of-process COM server implementation (a server EXE) that drives
> JNI instead, but I did not yet do it because I feared for the
> performance penalties of crossing process borders.
> It is really a pity that JNI does not allow to create more than one VM
> actually, and I think a lot of JNI users would be really happy when
> fixing that. Maybe one day some kind programmer provides that. :-)
> Thanks a lot
>> Markus KARG wrote:
>>> Hello Hotspot Community,
>>> I experienced a problem when using JNI and since nobody else was able
>>> to help me since months, and since it looks like a bug in the
>>> jvm.dll, you are my last chance. :-)
>>> In fact I am an experienced JNI user so my question is not about JNI
>>> itself but about a very complex case that makes jvm.dll crash down.
>>> So I want to discuss with you how we can solve it -- either by
>>> working around it in my code, or by adding a fix to the Hotspot VM
>>> (which seems to be the cause of the problems).
>>> I am writing a DLL in Windows XP that itself is a plugin to the
>>> desktop, a.k.a "Shell Extension". The DLL itself runs really stable
>>> (in fact I have drilled down so far that it actually does nothing
>>> anymore now, so I am pretty sure there is no bug in it since there is
>>> no more "real" code in it besides creating the Java VM). explorer.exe
>>> (i. e. the desktop process of Windows) loads the DLL, executes my
>>> code, and unloads the DLL then. So far, so good, works pretty well.
>>> But the same Windows process then loads my DLL once more, and
>>> executes it again (just as it did before). My DLL, you can imagine,
>>> is using JNI to call some Java code. Certainly I was clever enough
>>> not to create a VM twice, since I certainly know that both is
>>> impossible, either creating two VMs in the same process, or to try to
>>> create a VM after the previous VM was deleted (which I think is a
>>> bug, but anyways, that is not the point here).
>>> So I am checking first wheter there is a VM already using
>>> JNI_GetCreatedJavaVMs which returns the number of 1 and a pointer to
>>> the JavaVM. Great. That works pretty well. But if I now try to call
>>> AttachCurrentThread to get a pointer to the Env, then the desktop
>>> totally crashs down, telling me that I tried to execute code that
>>> actually is data (I switched off Data Execution Prevention then, but
>>> now it crashs down without ANY further note, so it is not a real
>>> help). So why did that happen?
>>> I looked at the source code of the JVM and it seems as if it uses
>>> global variables very heavily in the JNI source files. I could
>>> imagine that in fact one of those pointers now (after explorer
>>> unloaded and reloaded my DLL in the same process) point to a memory
>>> page that is marked as "data" but not as "code", so when using the
>>> function pointer, windows crashs down (since the memory pointed to is
>>> presumably no more part of the process's memory map).
>>> Please help me. I do not have any idea how to fix that. As a
>>> workaround I told Windows not to ever unload my DLL (what is
>>> working), but that certainly is just a bad hack. The real solution
>>> must be to convince jvm.dll to attach the thread without crashing down.
>>> If needed, I can provide an "at most simple" project that proofs the
>>> behaviour on any Windows machine (without seriously damaging it).
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