不满意的链接错误：Android JNI

Question

I'm working on a Android project which uses JNI. After 2 days of debugging, i still get an error: An unsatisfied link error:

Java.lang.UnsatisfiedLinkError: Native method not found: org.opencv.samples.facedetect.Hello.sayHello:()V

There are many people who have had this problem before, so i have read and tried many possible solutions but i still can't get it to work :(

My code:

Hello.java =

public class Hello { 

    public static native void sayHello(); 

    static {
        Log.i("JNI", "Loading hello");
        System.loadLibrary("hello");
    }
} 

Hello_jni.cpp =

#include <jni.h>
#include <Hello_jni.h>
#include <stdio.h>
#include <android/log.h>

#define LOG_TAG "FaceDetection/DetectionBasedTracker"
#define LOGD(...) ((void)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__))

extern "C"  JNIEXPORT void JNICALL Java_org_opencv_samples_facedetect_Hello_sayHello
 (JNIEnv *env, jclass obj)
{
 printf("Hello world!\n");
 LOGD("werkt");
 return;
}

Hello_jni.h =

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

#ifndef _Included_org_opencv_samples_facedetect_Hello
#define _Included_org_opencv_samples_facedetect_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_opencv_samples_facedetect_Hello
 * Method:    sayHello
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_org_opencv_samples_facedetect_Hello_sayHello
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

Android.mk = (first part is for other module from openCV. I'm using the facedetection example from openCV as template for my application)

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

#OPENCV_CAMERA_MODULES:=off
#OPENCV_INSTALL_MODULES:=off
#OPENCV_LIB_TYPE:=SHARED
include ../../sdk/native/jni/OpenCV.mk

LOCAL_SRC_FILES  := DetectionBasedTracker_jni.cpp
LOCAL_C_INCLUDES += $(LOCAL_PATH)
LOCAL_LDLIBS     += -llog -ldl

LOCAL_MODULE     := detection_based_tracker
include $(BUILD_SHARED_LIBRARY)

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := hello
LOCAL_SRC_FILE := org_opencv_samples_facedetect_Hello_jni.cpp
LOCAL_LDLIBS     += -llog 

include $(BUILD_SHARED_LIBRARY)

from my main activity, i call Hello.sayHello() (static). This triggers the error. Tried: - Removing/adding 'Extern "C"' to sayHello() in the cpp file. - Change function from native to not-native

The library seems to load fine, this is the whole error:

I/JNI(21440): Hello.sayHello() called
I/JNI(21440): Loading hello
D/dalvikvm(21440): Trying to load lib /data/data/org.opencv.samples.facedetect/lib/libhello.so 0x4248c960
D/dalvikvm(21440): Added shared lib /data/data/org.opencv.samples.facedetect/lib/libhello.so 0x4248c960
D/dalvikvm(21440): No JNI_OnLoad found in /data/data/org.opencv.samples.facedetect/lib/libhello.so 0x4248c960, skipping init
W/dalvikvm(21440): No implementation found for native Lorg/opencv/samples/facedetect/Hello;.sayHello:()V
D/AndroidRuntime(21440): Shutting down VM
W/dalvikvm(21440): threadid=1: thread exiting with uncaught exception (group=0x41a4e2a0)
E/AndroidRuntime(21440): FATAL EXCEPTION: main
E/AndroidRuntime(21440): java.lang.UnsatisfiedLinkError: Native method not found: org.opencv.samples.facedetect.Hello.sayHello:()V
E/AndroidRuntime(21440):    at     org.opencv.samples.facedetect.Hello.sayHello(Native Method)

If anybody can help me out, you would be a hero! I just can't get it to work...

Thanks in advance :)

Jelmer

Answer 1

Your dalvik runtime tells you exactly what is wrong:

W/dalvikvm(21440): No implementation found for native Lorg/opencv/samples/facedetect/Hello;.sayHello:()VD/AndroidRuntime(21440): Shutting down VM

Which means that you haven't properly implemented your native method and java runtime can't find it. Such error occurs in runtime only, the lib compiles ok, which gives me a hunch, that either method name (in .h and .cpp file) is wrong and doesn't correspond to the jni naming format, or that there is an error in .cpp file.

Answer 2

It's working now, the problem was the Android.mk file. Thanks Vorren for giving the solution ;) The working make file is:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

#OPENCV_CAMERA_MODULES:=off
#OPENCV_INSTALL_MODULES:=off
#OPENCV_LIB_TYPE:=SHARED
include ../../sdk/native/jni/OpenCV.mk

LOCAL_SRC_FILES  := DetectionBasedTracker_jni.cpp
LOCAL_C_INCLUDES += $(LOCAL_PATH)
LOCAL_LDLIBS     += -llog -ldl

LOCAL_MODULE     := detection_based_tracker

include $(BUILD_SHARED_LIBRARY)

include $(CLEAR_VARS)

LOCAL_SRC_FILES := Hello_jni.cpp
LOCAL_C_INCLUDES := $(LOCAL_PATH)
LOCAL_LDLIBS     += -llog 

LOCAL_MODULE := hello

include $(BUILD_SHARED_LIBRARY)

there were two errors; syntax error LOCAL_SRC_FILE (missing the S), and i called 'my-dir' 2 times in the make file, which is prohibited (found this here: https://groups.google.com/forum/#!topic/android-ndk/Qmr_WQH-uKk )