[ImageJ-devel] imagej-loader / dlopen args

Johannes Schindelin johannes.schindelin at gmx.de
Wed Mar 18 14:32:23 CDT 2015

Hi Lee,

On 2015-03-18 14:47, Lee Kamentsky wrote:

> I'm programming "monkey see, monkey do" style. I have very little idea 
> what
> I'm doing, trying to replicate the logic for mac loading dylibs in
> imagej-loader.
> I've gotten pretty far, but at the end, I'm seeing the error,
> "JavaNativeFoundation: GetGlobalVM: JNI_GetCreatedJavaVMs() failed to 
> get
> any VM" when I start an AWT app.

Do you expect a JVM to be running already? If not, you should [create 

> I am guessing that this is why you dlopened libjli here
> <https://github.com/imagej/imagej-launcher/blob/master/src/main/c/ImageJ.c#L264>.

Heh, I have to admit that I did not remember at all why I did that. 
Happily, even I learned to write kick-ass commit messages, and I also 
learned the awesomeness of the `Blame` button on GitHub: 
(I vaguely remember writing that commit message after spending something 
between twenty and thirty super-focused hours on hunting down this bug, 
that was not really fun, that one hunt).

> I do the same, but (so far) with Python's ctypes.CDLL. That doesn't let 
> you
> specify RTLD_LAZY as a dlopen mode. So, I think that's the big 
> difference
> between how you do things and how I do them. My first question is 
> whether
> RTLD_LAZY is needed as the mode. My second question is why you needed 
> to
> load libjli.dylib in the first place and, if you might grant me a third
> wish, my third question is whether you've seen this error before.

The JLI library is only needed for forward compatibility, so that MacOSX 
does not annoy the user by a "Do you want to install Java 6?" popup.

And yes, I have seen an error like that before, but only when trying to 
attach to a running JVM from within Matlab (Matlab's very own GUI is 
written in Java... so there is already a JVM running in a Matlab 
instance although it is a bit tricky to get at it under certain 
circumstances, but it is impossible to run two Oracle JVMs in the same 
process in any case).

> Thanks in advance, Mr. Wizard.

Sorry, oh ye mighty Python God, I fear my input was not really helpful 
to your cause. But maybe you have a project for me to clone, with 
instructions that might let this poor Python pilgrim reproduce the 


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