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javac++java-native-interfacelibrariesjniwrapper

Wrapping a Java library with C++ (JNI)

I have a large programme written in C++ that needs to use a specific Java library. Ideally I would like to create an equivalent C++ library that wraps this existing Java code. As such I have been looking into the JNI invocation API.

Since I am not a very experienced programmer, and I am also inexperienced with JNI and multi-language programming, I would greatly appreciate some general pointers/tips/advice as to how to tackle this problem.

Things I would be especially interested to know:

  1. Should each Java class in the Java lib map to to an associated C++ class? I.e. in my C++ library, will I have a class each invoking a JVM for a particular Java class? Or will I have a singular JVM through which everything is accessed? What is the best way to do this and why?
  2. What will be the basic process and architecture for doing this?
  3. Are there any specific resources for creating a C++ lib from Java lib using the invocation API?

Thanks a lot!

Solution

  • I've done this before, but it's not for the faint-hearted especially if your interface between the 2 languages is hard. Debugging can also be a pain in this situation.

    To answer your points:

    1. You should start by deciding on what functionality from the Java library you need to access in your C++ program. Is it just a few tasks? Try making a very simple interface from C++ to Java in this case. Is it complicated? Then you're gonna have to start mapping Java classes to C++, and the more you need then the more work it's gonna be.

    2. The end of q1 is sorta q2 really. Your C++ program will start a single JVM which will run as part of your program. When you make calls across the C++ data will be transferred into the JVM, and then the Java code executed, and then the return values transferred back. This incurs a performance cost so calling small functions like add(int,int) through JNI would be more expensive than just doing it in C++.

    3. There's a lot of basic guides you can Google to get started. Just managing to start a basic JVM from C++ and making a call is actually a bit of work since you need to get the paths to the JVM libs correct or it doesn't work (unless they've improved this, it's been years since I tried). So you might want to check that out first before asking more specific questions about JNI and mapping functions.

    An alternative option (which may or may-not be possible depending on your library and use-case) is to just write some kind of wrapper service around your library, actually in Java. And then send requests to it via JSON-HTTP or some messaging system.

    An even-more alternative option, rewrite whatever the library is doing in C++.