How can I call Java methods from C applications?

Interfacing Java and C : Calling Java Methods from C

The ability to call Java functions from within C applications is indeed possible but requires a somewhat intricate approach. This reflective and non-type-safe mechanism involves creating a Java Virtual Machine (JVM) instance from the C code.

Creating a JVM Instance

The code snippet below demonstrates how to set up a JVM instance:

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);

Interacting with Java Objects

Once the JVM instance is created, you can interact with Java objects. Here's an example of creating a Java String object:

jstring jstr = env->NewStringUTF("Hello World");

Accessing Java Methods

To access a Java method, you need to first get the class that contains the method:

jclass clazz = env->FindClass("java/lang/String");

Then, obtain the method ID:

jmethodID to_lower = env->GetMethodID(clazz, "toLowerCase", "()Ljava/lang/String;");

Calling the Java Method

Finally, call the method on the object:

jobject result = env->CallObjectMethod(jstr, to_lower);

Compilation

On Ubuntu, compile the code using:

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

Conclusion

This approach allows you to access Java methods from C applications, enabling interoperability between the two languages. However, it's crucial to implement proper error handling for robustness.

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