Unsatisfiedlinkerror: Unable to load custom NDK JNI library in Nfc system app

Question

I have written a small C program using JNI which I would like to call from inside the NFC system app, specifically the NfcDispatcher.java class. I have done the following so far:

Created a /jni directory inside the /AOSP/packages/apps/Nfc/ where I have written the following JNI code. Nfc/jni/ dir has 2 files, viz mycustomlib.c and Android.mk which are as follows

mycustomlib.c at /AOSP/packages/apps/Nfc/jni/mycustomlib.c

#include <string.h>
#include <jni.h>

jstring Java_com_android_nfc_NfcDispatcher_gettagkey( JNIEnv* env, jobject thiz, jstring tagKey )
{
// do something
    return tagKey;
}

Android.mk at /AOSP/packages/apps/Nfc/jni/Android.mk

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

I am calling the native method "gettagkey" from inside the NfcDispatcher.java file as follows.

NfcDispatcher.java at /AOSP/packages/apps/Nfc/src/com/android/nfc/NfcDispatcher.java

public class NfcDispatcher {
    ..
    ..
    public static native String gettagkey(String tagKey);
    ..
    static class DispatchInfo {
        ..
    }

    public boolean dispatchTag(Tag tag) {
    ..
    ..
        Log.d(TAG, "NFC Key Tag from C code : " + gettagkey(sb.toString()));
    ..
    }
    ..
    static {
        System.loadLibrary("mycustomlib");
    }

}

Then we compile the C code using ndk-build -C path_to_c_code and then we did a "make -j32" to compile the code changed in the Nfc system app and then we flash the new system.img to the Nexus 7 device. The OS boot properly but we get the following error:

W/dalvikvm(  767): Exception Ljava/lang/UnsatisfiedLinkError; thrown while initializing Lcom/android/nfc/NfcDispatcher;
E/AndroidRuntime(  767): at com.android.nfc.NfcDispatcher.<clinit>(NfcDispatcher.java:571)
E/AndroidRuntime(  767): at com.android.nfc.NfcService.<init>(NfcService.java:390)
E/AndroidRuntime(  767): at com.android.nfc.NfcApplication.onCreate(NfcApplication.java:43)

I have read all related question but I am still not sure what's happening. Does anyone have any clue? Thanks!

Answer 1

I have the feeling that the native library is not being loaded before the native call to the native method.

I would try to replace the gettagkey body, with the library load call, and then call the native method. Something like:

public static String gettagkey(String tagKey){

         System.loadLibrary("mycustomlib");
         gettagkeyNative(tagKey);
}

public static native String gettagkeyNative(String tagKey);

Don't forget to modify the native part:

#include <string.h>
#include <jni.h>

jstring Java_com_android_nfc_NfcDispatcher_gettagkeyNative( JNIEnv* env, jobject thiz, jstring tagKey)
{
   // do something
   return tagKey;
}

This solution would work, unless the system call to the native method requires that the gettagkey method is native.

Hope it helps!