How can i implement two JNI methods with same name but different Params?

Question

I have a java class with two native functions with same names but different parameters.

package com.example;

public class Test {

    static {
        System.loadLibrary("TestLib");
    }

    public static native void doSomething(int a);
    public static native void doSomething(int a, long b);
}

My header file looks like

#include <jni.h>

#ifndef _TEST_LIB_H_
#define _TEST_LIB_H_

#ifdef __cplusplus
extern "C" {
#endif

JNIEXPORT void JNICALL Java_com_example_Test_doSomething
    (JNIEnv *env, jclass clazz, jint a);

JNIEXPORT void JNICALL Java_com_example_Test_doSomething
    (JNIEnv *env, jclass clazz, jint a, jlong b);

#ifdef __cplusplus
}
#endif

#endif //_TEST_LIB_H_

and my cpp file

#include "TestLib.h"

JNIEXPORT void JNICALL Java_com_example_Test_doSomething
    (JNIEnv *env, jclass clazz, jint a){
    Java_com_example_Test_doSomething(env, clazz, a, -1);
}

JNIEXPORT void JNICALL Java_com_example_Test_doSomething
    (JNIEnv *env, jclass clazz, jint a, jlong b) {
    // implementation
}

with only one function it works, but after add a second one with same name and different parameters I get an error:

error: conflicting declaration of C function 'void Java_com_example_Test_doSomething(JNIEnv*, jclass, jint, jlong)'
note: previous declaration 'void Java_com_example_Test_doSomething(JNIEnv*, jclass, jint)'

I'm using android studio and experimental gradle plugin. What am I doing wrong?

Answer 1

First of all, if your native methods really take a jclass as their second argument then they should be declared static on the Java side. Otherwise they should take a jobject (the instance that they're invoked on) rather than a jclass.

---

Here's what Oracle's documentation says about naming of overloaded native methods:

A native method name is concatenated from the following components: ... for overloaded native methods, two underscores (“__”) followed by the mangled argument signature

So the name of your second function should be Java_com_example_Test_doSomething__IJ. You might also have to change the first function's name to Java_com_example_Test_doSomething__I.

Answer 2

Use javah tool to generate header files, below is what this tool will generate for your class.

to use it first compile .java file, then run javah on .class file.

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_Test */

#ifndef _Included_com_example_Test
#define _Included_com_example_Test
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_Test
 * Method:    doSomething
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_com_example_Test_doSomething__I
  (JNIEnv *, jobject, jint);

/*
 * Class:     com_example_Test
 * Method:    doSomething
 * Signature: (IJ)V
 */
JNIEXPORT void JNICALL Java_com_example_Test_doSomething__IJ
  (JNIEnv *, jobject, jint, jlong);

#ifdef __cplusplus
}
#endif
#endif