What is the correct way to write native method signatures in Android NDK?

Question

I have a working implementation of NDK library and corresponding Java-class. But I am not able to add overloaded method to that class. Currently my class contains:

package com.package;  public class MyClass {   public static native String getFileName();   static   {     System.loadLibrary("mylib");   } } 

My jniwrappers.cpp file has the following declaration:

JNIEXPORT jstring JNICALL Java_com_package_MyClass_getFileName(_JNIEnv* env, jobject thiz); 

Up to this point everything is working fine. But next I modify my class:

package com.package;  public class MyClass {   public static native String getFileName();   public static native String getFileName(int index);   ... } 

And add to jniwrappers.cpp another declaration:

JNIEXPORT jstring JNICALL Java_com_package_MyClass_getFileName__I(_JNIEnv* env, jobject thiz, jint index); 

It compiles fine, Android application starts, does not get UnsatisfiedLinkError but when it calls the second method with the argument the first C++ function is being called but not the second. I have other methods with arguments in that class but none of them are overloaded so their respective JNI signatures do not contain arguments.

So, what am I doing wrong?

Answer 1

You should use javah tool to generate those signatures.

To use it, build the class file where you have your native function. You'll get a class file.

Run javah -jni com.organisation.class_with_native_func, it'll generate a header file for you.

It's far cleaner than editing it yourself.

Answer 2

You have to add a __ onto the end of the original getFileName function now that it is overloaded. Your 2 C function prototypes should now look like this:

JNIEXPORT jstring JNICALL Java_com_package_MyClass_getFileName__   (JNIEnv *, jclass);  JNIEXPORT jstring JNICALL Java_com_package_MyClass_getFileName__I   (JNIEnv *, jclass, jint);