Can I know the name of the class that calls a JNI C method?

Question

Is there any way I can know the name of the class that called a method in JNI C code ? I can obtain a reference to the class using the following statement :

jclass cls = (*env)->GetObjectClass(env,obj);

But is there any way I can know the name of the class ? .

Answer 1

This code will give you the calling class name:

jclass cls = env->GetObjectClass(obj);

// First get the class object
jmethodID mid = env->GetMethodID(cls, "getClass", "()Ljava/lang/Class;");
jobject clsObj = env->CallObjectMethod(obj, mid);

// Now get the class object's class descriptor
cls = env->GetObjectClass(clsObj);

// Find the getName() method on the class object
mid = env->GetMethodID(cls, "getName", "()Ljava/lang/String;");

// Call the getName() to get a jstring object back
jstring strObj = (jstring)env->CallObjectMethod(clsObj, mid);

// Now get the c string from the java jstring object
const char* str = env->GetStringUTFChars(strObj, NULL);

// Print the class name
printf("\nCalling class is: %s\n", str);

// Release the memory pinned char array
env->ReleaseStringUTFChars(strObj, str);

Note that I haven't taken any actions to check for errors. This is just a small code snippet describing how it could be done.

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Alternatively you could do this instead of using the GetStringUTFChars/ReleaseStringUTFChars:

// Make sure that the buffer is large enough
char str[128];
jint strlen = env->GetStringUTFLength(strObj);
env->GetStringUTFRegion(strObj, 0, strlen, str);
printf("\nCalling class is: %s\n", str);

No need to release since the string is copied to local buffer.

Answer 2

In my case, I haven't had the object to get the class. Instead, I wanted to get the name of a given class, based on its signature.

So, this worked for me. I hope it can help:

// Find the class by its JNI signature
jclass cls = env->FindClass(expectedType);

// Get the class object's class descriptor
jclass clsClazz = env->GetObjectClass(cls);

// Find the getSimpleName() method in the class object
jmethodID methodId = env->GetMethodID(clsClazz, "getSimpleName", "()Ljava/lang/String;");
jstring className = (jstring) env->CallObjectMethod(cls, methodId);

// And finally, don't forget to release the JNI objects after usage!!!!
env->DeleteLocalRef(clsClazz);
env->DeleteLocalRef(cls);