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From Andi Vajda <va...@apache.org>
Subject Re: Unresolved external symbol errors when linking example native c++ code that uses jcc
Date Fri, 22 Oct 2010 16:55:43 GMT

On Fri, 22 Oct 2010, Imre AndrĂ¡s wrote:

> Ok, thanks. After resolving this I got the following error:
> jcc.cpp(294) : error C2039: 'fromJString' : is not a member of 'JCCEnv'
>
> As I see this method is present in JCCEnv.h, but an #ifdef PYTHON 
> directive prevents to make it available for the tier code I intended to 
> write which would have been based on the jcc generated stub.
>
> I am evaluating JNI wrappers. Our dev team need a layer that would ease 
> calling java code from native code. It also must have a clear and 
> documented interface, and should work with a trivial minimal example (Only 
> one class, contains a static and a normal method.)
>
> No offense, but my opinion is that jcc is nice stuff, and surely makes 
> available Lucene for Python developers, but in its current state it seems 
> to be not suited for such a general-purpose JNI wrapper. Integration 
> should be staright, easy and straightforward, no matter what IDE or 
> compiler used.
>
> I would suggest to go for setting up a minimal non-Python example that 
> demonstrates an example C++ code which calls java - in a clear and 
> separate environment (simulating the end user's). This will surely bring 
> up the issues - some of them I have tackled, but unfortunately I ran out 
> of time...

No offense taken. If you have the itch to make this work, then I'd be glad 
to help you along the way. What you're trying to do, use jcc for C++ only, 
was an intended use from the beginning. I think you're the first on this 
list to actually try it. The Python support is built on top of the C++ 
support so I expect the C++ layer to be sound. From a compile/link 
perspective, Python makes it easier as it knows how to invoke the 
compiler/linker to produce a Python extension shared library (via 
distutils). When using just C++, it's up to you to add to your project (via 
the IDE, Makefile, whatever you chose), the files it generates and the 
runtime files it needs to then run. There may be a few glitches such as the 
ones you found but I'd expect this use to work without too much extra work.

The reason for JCC runtime for C++'s is pretty much only for tracking the 
Java objects as they escape the JVM. It manages the global references that 
get created when objects escape the JVM (this is something you have to do 
manually if you used the JNI directly instead).

Using JCC in your project is a two-step process. First get JCC to generate 
the C++ wraper code for the Java code you want access to. This is, 
hopefully, well documented on the JCC site [1]. Then take that generated 
code tree and the necessary JCC runtime source files [2] and add them to 
your project's build. Any file that includes <Python.h> unconditionally is 
not part of the code you should include.

The last problem you found has to do with the fact that the env global 
variable is declared in a file only needed by the Python runtime; jcc.cpp 
should not be included in your project and you need to declare that global 
in code of yours instead.

Once you have a build, to then use it, you still need to write code to 
startup and initialize the JVM (as documented in the Java Native Interface 
docs [3]) and, before you call into any Java class, initialize it first via 
that class' initializeClass() method.

Cheers !

Andi..

[1] http://lucene.apache.org/pylucene/jcc/documentation/readme.html
[2] JObject.cpp, JCCEnv.cpp, JCCEnv.h
[3] http://java.sun.com/docs/books/jni/html/invoke.html
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