Looking for a convenient way to call Java from C++

Question

It seems most documentation or helper libraries relating to JNI (Java Native Interface) are concerned with calling native code from Java. This seems to be the main use of it, ev

Answer 1

Re calling Java from C++.

You can do what you wish but you must let Java be in control. What I mean by this is that you create Java threads that call into Native code and from there they block, kind of waiting for your native code to give it something to do. You create as many Java threads as you need to get enough work / throuhput done.

So your C++ application starts up, it creates a JVM/JavaVM (as per the documented way, example exists in qtjambi codebase see below), this in turn perform the usual JNI initialization and System.loadLibrary() and provides JARs with "native" linkage. You then initialize a bunch of threads and call some JNI code (that you created) where they can block in wait for your C++ code to give them some work to do.

Your C++ code (presumabily from another thread) then sets up and passes all the information needed to one of the blocked and waiting Java Thread workers, it is then given the order to run, it may then go back into pure-Java code to do work and return with a result.

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It is possible to setup and create and contain a JavaVM instance from C++ code. This can be force fed your own CLASSPATH/JARs to setup the contained environment you need encapsulated inside your C++ program.

Outline of that as I'm sure you have found already at http://download.oracle.com/javase/1.5.0/docs/guide/jni/spec/invocation.html

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There is a kind of C++ => Java JNI generator in the QtJambi project (that I work on and help maintain). This is quite bespoke for the Qt toolkit but essentially it translates a bunch of C++ header files into a collection of C++ .cpp/.h files and *.java file to provide linkage and shell containment of the object so that the competing memory allocation schemes play well together. Maybe there is something to be taken from this.

This is certainly a proof in cencept for what you are asking the generator just happens to be contained in the qtjambi project (but could be made independant with some work) and this is LGPL licensed (open-source). The Qt toolkit is not a small API yet it can generated 100s of classes to cover high % of API (>85% and almost 100% of Core/GUI parts).

HTH