Integrating Java code with pre-existing C application

Question

My team has been tasked to develop wizards for a pre-existing C application on Unix. We don't own the C code so we can make only limited changes to the native code.
The challenge is to call the Java wizard from within the C code, and then to allow the Java code to use services and data in the C code. I can call Java from C, and then call C from Java. But the C code that gets called from Java doesn't seem to have access to the data known by the C code that called the Java.
The only way I've thought of to handle this is to pass function pointers as paramters to the Java program, which can then pass the pointers on to the C code that it calls. Is there a way to pass pointers into Java?

Answer 1

But the C code that gets called from Java doesn't seem to have access to the data known by the C code that called the Java.
I don't know exactly what you mean here, unless you're simply saying that the scope of your C data makes it inaccessible between functions.
Is there a way to pass pointers into Java?
Yep - Just treat the pointers as opaque types and wrap them in something large enough for the platform - for example a jlong. You need to be sure to be careful in managing the lifetime of the pointers with respect to the Java objects that hold onto them - for example, with function pointers that come from a dynamically loaded shared library or any data pointers that you might be passing around. You probably want to read the section on native peers in the JNI programmer's guide.
God bless,
-Toby Reyelts
Check out the free, open-source, JNI toolkit, Jace - http://jace.reyelts.com/jace