How to Call Java Methods from C Applications?

Invoking Java Methods from C Applications

In the realm of cross-platform development, it may arise that a C application requires access to functionalities provided by Java methods. This query explores the feasibility and nuances of achieving this integration.

Feasibility and Mechanism

Yes, it is possible to call Java methods from within C/C code. However, the procedure is somewhat intricate and adheres to a reflective and non-type-safe approach.

The C Application Programming Interface (API) provides a cleaner method for achieving this integration. The approach involves instantiating a Java Virtual Machine (JVM) within the C code. In scenarios where the native code is invoked from Java, constructing a VM instance is unnecessary.

Implementation

The provided code snippet demonstrates how to call Java methods from a C application:

#include <jni.h>
#include <stdio.h>

int main() {
    JavaVM *vm;  // Pointer to JVM
    JNIEnv *env;  // JNI interface to interact with JVM
    JavaVMInitArgs vm_args;  // JVM initialization arguments

    // Initialize JVM arguments
    vm_args.version = JNI_VERSION_1_2;
    vm_args.nOptions = 0;
    vm_args.ignoreUnrecognized = 1;

    // Construct a Java Virtual Machine
    jint res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);

    // Create a Java string object
    jstring jstr = env->NewStringUTF("Hello World");

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

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

    // Invoke the "toLowerCase()" method on the string object
    jobject result = env->CallObjectMethod(jstr, to_lower);

    // Convert the Java string to a C-style string
    const char* str = env->GetStringUTFChars((jstring) result, NULL);

    // Print the converted string
    printf("%s\n", str);

    // Release the C-style string
    env->ReleaseStringUTFChars(jstr, str);

    // Destroy the Java Virtual Machine
    vm->DestroyJavaVM();

    return EXIT_SUCCESS;
}

Cross-Platform Compilation

For cross-platform compilation on Ubuntu, execute the following command:

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

Error Handling

It is crucial to verify the return codes of all methods used for proper error handling. For instance, the following code snippet checks for potential memory allocation issues when retrieving the C-style string:

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

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