Native method not found: Package.methodName:()Ljava/lang/String;

Question

I am having trouble on android ndk development. I am very new to ndk and following a tutorial. On compilation time no error comes but when I uploaded the application in device it gives me an error that Native Method Not Found. I have attached the code and snap shots of my package.

AT ndk-build I got this:

[arm64-v8a] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[arm64-v8a] SharedLibrary  : libcom_testing_ndk_FibLib.so
[arm64-v8a] Install        : libcom_testing_ndk_FibLib.so => libs/arm64-v8a/libcom_testing_ndk_FibLib.so
[x86_64] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[x86_64] SharedLibrary  : libcom_testing_ndk_FibLib.so
[x86_64] Install        : libcom_testing_ndk_FibLib.so => libs/x86_64/libcom_testing_ndk_FibLib.so
[mips64] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[mips64] SharedLibrary  : libcom_testing_ndk_FibLib.so
[mips64] Install        : libcom_testing_ndk_FibLib.so => libs/mips64/libcom_testing_ndk_FibLib.so
[armeabi-v7a] Compile thumb  : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[armeabi-v7a] SharedLibrary  : libcom_testing_ndk_FibLib.so
[armeabi-v7a] Install        : libcom_testing_ndk_FibLib.so => libs/armeabi-v7a/libcom_testing_ndk_FibLib.so
[armeabi] Compile thumb  : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[armeabi] SharedLibrary  : libcom_testing_ndk_FibLib.so
[armeabi] Install        : libcom_testing_ndk_FibLib.so => libs/armeabi/libcom_testing_ndk_FibLib.so
[x86] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[x86] SharedLibrary  : libcom_testing_ndk_FibLib.so
[x86] Install        : libcom_testing_ndk_FibLib.so => libs/x86/libcom_testing_ndk_FibLib.so
[mips] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
[mips] SharedLibrary  : libcom_testing_ndk_FibLib.so
[mips] Install        : libcom_testing_ndk_FibLib.so => libs/mips/libcom_testing_ndk_FibLib.so

FibLib.java

 package com.testing.ndk;

public class FibLib {
       static {
          System.loadLibrary("com_testing_ndk_FibLib"); // Load native library at runtime
                                       // hello.dll (Windows) or libhello.so (Unixes)
       }

       // Declare a native method sayHello() that receives nothing and returns void
       public static native String sayHello();

       // Test Driver
       public static void main(String[] args) {
          new FibLib().sayHello();  // invoke the native method
       }
    }

com_testing_ndk_FibLib.c

  #include <jni.h>
#include <stdio.h>
#include "com_testing_ndk_FibLib.h"
// Implementation of native method sayHello() of HelloJNI class

   JNIEXPORT void JNICALL Java_com_testing_ndk_FibLib_sayHello(JNIEnv *env, jobject thisObj) {
   printf("Hello World!\n");
   return;
}

com_testing_ndk_FibLib.h

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

#ifndef _Included_com_testing_ndk_FibLib
#define _Included_com_testing_ndk_FibLib
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_testing_ndk_FibLib
 * Method:    sayHello
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_testing_ndk_FibLib_sayHello
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

Application.mk

APP_PLATFORM := android-17
APP_ABI :=all

Android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES :=com_testing_ndk_FibLib.c
LOCAL_MODULE :=com_testing_ndk_FibLib
include $(BUILD_SHARED_LIBRARY) 

Error Log:

[arm64-v8a] Compile        : com_testing_ndk_FibLib <= com_testing_ndk_FibLib.c
jni/com_testing_ndk_FibLib.c:6:24: error: conflicting types for 'Java_com_testing_ndk_FibLib_sayHello'
 JNIEXPORT void JNICALL Java_com_testing_ndk_FibLib_sayHello(JNIEnv *env, jobject thisObj) {
                        ^
In file included from jni/com_testing_ndk_FibLib.c:3:0:
jni/com_testing_ndk_FibLib.h:15:27: note: previous declaration of 'Java_com_testing_ndk_FibLib_sayHello' was here
 JNIEXPORT jstring JNICALL Java_com_testing_ndk_FibLib_sayHello
                           ^
make: *** [obj/local/arm64-v8a/objs/com_testing_ndk_FibLib/com_testing_ndk_FibLib.o] Error 1

Answer 1

JNI peforms native method search using its name. To ensure that exported function it found is correct one, methods must be named according following scheme:

Java_package_name_className_methodName

For example, in you case name of native function must be Java_com_testing_ndk_FibLib_sayHello, but in your com_testing_ndk_FibLib.c there are no function with such name.

Following snippet should be fine if you place it in this file.

com_testing_ndk_FibLib.c

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

// Implementation of native method sayHello() of HelloJNI class
JNIEXPORT void JNICALL Java_com_testing_ndk_FibLib_sayHello(JNIEnv *env, jobject thisObj) {
   printf("Hello World!\n");
   return;
}

---

To fix your problem change return type of signature in com_testing_ndk_FibLib.h at line 15

...
JNIEXPORT void JNICALL Java_com_testing_ndk_FibLib_sayHello
  (JNIEnv *, jclass);
...

and in FibLib.java at line 10:

...
public static native String sayHello();
...

Read about JNI here: http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html