How to access the Java method in a C++ application

Question

Just a simple question: Is it possible to call a java function from c/c++ ?

Answer 1

Yes you can, but it is a little convoluted, and works in a reflective/non type safe way (example uses the C++ api which is a little cleaner than the C version). In this case it creates an instance of the Java VM from within the C code. If your native code is first being called from Java then there is no need to construct a VM instance

#include<jni.h> #include<stdio.h>  int main(int argc, char** argv) {      JavaVM *vm;     JNIEnv *env;     JavaVMInitArgs vm_args;     vm_args.version = JNI_VERSION_1_2;     vm_args.nOptions = 0;     vm_args.ignoreUnrecognized = 1;      // Construct a VM     jint res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);      // Construct a String     jstring jstr = env->NewStringUTF("Hello World");      // First get the class that contains the method you need to call     jclass clazz = env->FindClass("java/lang/String");      // Get the method that you want to call     jmethodID to_lower = env->GetMethodID(clazz, "toLowerCase",                                       "()Ljava/lang/String;");     // Call the method on the object     jobject result = env->CallObjectMethod(jstr, to_lower);      // Get a C-style string     const char* str = env->GetStringUTFChars((jstring) result, NULL);      printf("%s\n", str);      // Clean up     env->ReleaseStringUTFChars(jstr, str);      // Shutdown the VM.     vm->DestroyJavaVM(); } 

To compile (on Ubuntu):

g++ -I/usr/lib/jvm/java-6-sun/include \      -I/usr/lib/jvm/java-6-sun/include/linux \      -L/usr/lib/jvm/java-6-sun/jre/lib/i386/server/ -ljvm jnitest.cc 

Note: that the return code from each of these methods should be checked in order to implement correct error handling (I've ignored this for convenience). E.g.

str = env->GetStringUTFChars(jstr, NULL); if (str == NULL) {     return; /* out of memory */ } 

Answer 2

Yes it is, but you have to do it via JNI: http://java.sun.com/javase/6/docs/technotes/guides/jni/index.html