How can I use JNI in C++ to use a Java class?

Question

so I'm trying to use JNI to call my Java class in C++ and everything looks well until I try to run it. In Xcode I get the error

Undefined symbols for architecture x86_64: "_JNI_CreateJavaVM", referenced from:

Which I assume has something to do with the architecture, but don't know how to fix this, any help?

The code I use to initialize the Java VM is

JNI_CreateJavaVM(&internal::gJVM, 
(void**)&internal::gEnv, &vm_args);

I'm on a mac, please post a mac solution if you have any ideas, I'm trying to avoid loading libraries at runtime. Thanks

Answer 1

It sounds like you're missing a library. If you're using gcc check all your -l's. If you're using VS, check your "Additional Dependancies" options under Config->Linker->Input.

Also check that you have the x86_64 version of the necessary libraries.

Answer 2

In all the examples I have seen, they avoid the problem by never invoking that function directly, thus it never has to link, thus no error. The trick is to lookup the function at runtime and invoke via function pointer. Here is how to do it on Windows. Don't know the syntax for doing it on other operating systems.

[Disclaimer - I copied/pasted code from several functions and may have introduced compiler errors. This may or may not compile but it should get you started]

First create your own typedef for a pointer to the function

typedef jint (JNICALL* JvmCreateProcTypeDef)(JavaVM **, void **, void);

Look up the JVM dll using LoadLibrary. It's up to your application to figure out where to find the JVM DLL. In our case we distributed a 3rd party JRE and knew where to expect the DLL.

HMODULE jvmDll = LoadLibrary(jvmDllPath);

Next lookup the address of the function from the JVM dll using GetProcAddress

JvmCreateProcTypeDef jvmCreateProc = (JvmCreateProcTypeDef) GetProcAddress(jvmDll,"JNI_CreateJavaVM");

Now replace your code which calls the function directly, to something like the following which calls it via function pointer:

jvmCreateProc(&internal::gJVM, (void**)&internal::gEnv, &vm_args);

This should get you passed all the compile link errors. Now all you have to do is deal with the runtime errors when your code cannot find the DLL :)

Hope this helps!