Calling C++ functions from Java

Question

I am developing a Java application in which I need to call some C++ functions (from Google Talk library libjingle) . The objective is to run it all on Google App Engine (which only supports Python or Java).

How can I do this?

Solution 1

You need to define native methods in your java code for whatever you want to be implemented in C++ and directly access your native code. Then you run javah on your code and it will generate the C header files for you and you'll need to provide C++ implementations.

The native methods you can call from your Java code like any other methods and still they'll have their implementation written in C++ and talking to whatever other native library directly.

You then need to set the java.library.path system property to include the shared C/C++ libraries that you require: the google library and your own JNI implementation library would be required in this case.

Solution 2

If the library has C bindings through a DLL/SO, I usually prefer writing wrappers in Java using Java Native Access (JNA) rather than writing the bindings in C/C++ using the Java Native Interface (JNI). The former is easier to manipulate as the JNI access to Java objects is a real pain in the neck. However, it's not as obvious to wrap C++ classes using that API.

You might also want to look into the Simplified Wrapper and Interface Generator (SWIG) for automating part of this process!

Solution 3

You can't run native code on App Engine - only JRE code. If there's no avoiding the native code, you'll need to run this part of your app on another system, and call it from your App Engine app - or use the built-in XMPP API, in this case.

Comments

[Karl von Moor]

With JNI, Why dou you already have that in your tags ?!

[Asaf]

do you specifically require libjingle or will any XMPP library do? you can see a list of available libraries here

[Admin]

Thank you all. Do you know if JNI and native code in C++ work fine with Google App Engine?

[ryan]

no, they don't. app engine doesn't support native code or compiled binaries, via JNI or anything else. From that link: "An app cannot provide or directly invoke any native JNI code." asaf's comment above is on the right track. you probably want to use app engine's built in xmpp api instead.