Android NDK java.lang.UnsatisfiedLinkError on sample hello-jni

Question

I need to run some native C code on android and am trying to setup the NDK. I am using Android studio.

I've imported the hello-jni sample, but it simply doesn't work. I always get a java.lang.UnsatisfiedLinkError error.

ndk-build:

$ ndk-build
[arm64-v8a] Gdbserver      : [aarch64-linux-android-4.9] libs/arm64-v8a/gdbserver
[arm64-v8a] Gdbsetup       : libs/arm64-v8a/gdb.setup
[x86_64] Gdbserver      : [x86_64-4.9] libs/x86_64/gdbserver
[x86_64] Gdbsetup       : libs/x86_64/gdb.setup
[mips64] Gdbserver      : [mips64el-linux-android-4.9] libs/mips64/gdbserver
[mips64] Gdbsetup       : libs/mips64/gdb.setup
[armeabi-v7a] Gdbserver      : [arm-linux-androideabi-4.8] libs/armeabi-v7a/gdbserver
[armeabi-v7a] Gdbsetup       : libs/armeabi-v7a/gdb.setup
[armeabi] Gdbserver      : [arm-linux-androideabi-4.8] libs/armeabi/gdbserver
[armeabi] Gdbsetup       : libs/armeabi/gdb.setup
[x86] Gdbserver      : [x86-4.8] libs/x86/gdbserver
[x86] Gdbsetup       : libs/x86/gdb.setup
[mips] Gdbserver      : [mipsel-linux-android-4.8] libs/mips/gdbserver
[mips] Gdbsetup       : libs/mips/gdb.setup
[arm64-v8a] Compile        : hello-jni <= hello-jni.c
[arm64-v8a] SharedLibrary  : libhello-jni.so
[arm64-v8a] Install        : libhello-jni.so => libs/arm64-v8a/libhello-jni.so
[x86_64] Compile        : hello-jni <= hello-jni.c
[x86_64] SharedLibrary  : libhello-jni.so
[x86_64] Install        : libhello-jni.so => libs/x86_64/libhello-jni.so
[mips64] Compile        : hello-jni <= hello-jni.c
[mips64] SharedLibrary  : libhello-jni.so
[mips64] Install        : libhello-jni.so => libs/mips64/libhello-jni.so
[armeabi-v7a] Compile thumb  : hello-jni <= hello-jni.c
[armeabi-v7a] SharedLibrary  : libhello-jni.so
[armeabi-v7a] Install        : libhello-jni.so => libs/armeabi-v7a/libhello-jni.so
[armeabi] Compile thumb  : hello-jni <= hello-jni.c
[armeabi] SharedLibrary  : libhello-jni.so
[armeabi] Install        : libhello-jni.so => libs/armeabi/libhello-jni.so
[x86] Compile        : hello-jni <= hello-jni.c
[x86] SharedLibrary  : libhello-jni.so
[x86] Install        : libhello-jni.so => libs/x86/libhello-jni.so
[mips] Compile        : hello-jni <= hello-jni.c
[mips] SharedLibrary  : libhello-jni.so
[mips] Install        : libhello-jni.so => libs/mips/libhello-jni.so

jni/Android.mk:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello-jni
LOCAL_SRC_FILES := hello-jni.c

include $(BUILD_SHARED_LIBRARY)

jni/hello-jni.c:

jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{
#if defined(__arm__)
  #if defined(__ARM_ARCH_7A__)
    #if defined(__ARM_NEON__)
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a/NEON (hard-float)"
      #else
        #define ABI "armeabi-v7a/NEON"
      #endif
    #else
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a (hard-float)"
      #else
        #define ABI "armeabi-v7a"
      #endif
    #endif
  #else
   #define ABI "armeabi"
  #endif
#elif defined(__i386__)
   #define ABI "x86"
#elif defined(__x86_64__)
   #define ABI "x86_64"
#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
   #define ABI "mips64"
#elif defined(__mips__)
   #define ABI "mips"
#elif defined(__aarch64__)
   #define ABI "arm64-v8a"
#else
   #define ABI "unknown"
#endif

    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");
}

com.example.hellojni.HelloJni:

public class HelloJni extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        /* Create a TextView and set its content.
         * the text is retrieved by calling a native
         * function.
         */
        TextView  tv = new TextView(this);
        tv.setText( stringFromJNI() );
        setContentView(tv);
    }

    /* A native method that is implemented by the
     * 'hello-jni' native library, which is packaged
     * with this application.
     */
    public native String  stringFromJNI();

    /* This is another native method declaration that is *not*
     * implemented by 'hello-jni'. This is simply to show that
     * you can declare as many native methods in your Java code
     * as you want, their implementation is searched in the
     * currently loaded native libraries only the first time
     * you call them.
     *
     * Trying to call this function will result in a
     * java.lang.UnsatisfiedLinkError exception !
     */
    public native String  unimplementedStringFromJNI();

    /* this is used to load the 'hello-jni' library on application
     * startup. The library has already been unpacked into
     * /data/data/com.example.hellojni/lib/libhello-jni.so at
     * installation time by the package manager.
     */
    static {
        System.loadLibrary("hello-jni");
    }
}

Error:

11-13 22:38:40.227  19386-19386/com.example.hellojni E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.hellojni, PID: 19386
    java.lang.UnsatisfiedLinkError: Couldn't load hello-jni from loader dalvik.system.PathClassLoader[DexPathList[[zip file "/data/app/com.example.hellojni-7.apk"],nativeLibraryDirectories=[/data/app-lib/com.example.hellojni-7, /vendor/lib, /system/lib]]]: findLibrary returned null
            at java.lang.Runtime.loadLibrary(Runtime.java:358)
            at java.lang.System.loadLibrary(System.java:526)
            at com.example.hellojni.HelloJni.<clinit>(HelloJni.java:64)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1208)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1067)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2399)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2599)
            at android.app.ActivityThread.access$900(ActivityThread.java:174)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5748)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)

Please, if you have any idea what is causing this, help me. I've wasted a whole work day on this already and am about to jump out the window (taking the computer with me).

Answer 1

This is because Android Studio ignore your Android.mk and generate own one on-the-fly, taking settings from gradle script. So you should either specify correct settings in gradle script (which works only in simplest cases due to incomplete NDK support in Android Studio), or completely disable Android Studio's limited NDK support and call ndk-build directly from gradle.

To specify NDK-related settings in gradle script, add the following section:

defaultConfig {
    ...
    ndk {
        moduleName "my-module-name"
        cFlags "-std=c++11 -fexceptions"
        ldLibs "log"
        stl "gnustl_shared"
        abiFilter "armeabi-v7a"
    }
}

And set your project specific values there. Android Studio's gradle plugin then will take those settings into account and auto-generate Android.mk on-the-fly with these settings.

Note, however, that the only available NDK options are those listed above. If you have more complicated NDK setup (for example, several NDK modules, depending each on other, or you want use prebuilt libraries, or whatever else), you should completely disable Android Studio's limited NDK support and call ndk-build directly from gradle. To do that, first disable auto-generation of Android.mk:

android {
    .....
    // disable automatic ndk-build call, which ignore our Android.mk
    sourceSets.main.jni.srcDirs = []
}

Then add the following lines:

android {
    .....
    sourceSets.main.jniLibs.srcDir 'src/main/libs'

    // call regular ndk-build(.cmd) script from app directory
    task ndkBuild(type: Exec) {
        workingDir file('src/main')
        commandLine getNdkBuildCmd()
    }

    tasks.withType(JavaCompile) {
        compileTask -> compileTask.dependsOn ndkBuild
    }

    task cleanNative(type: Exec) {
        workingDir file('src/main')
        commandLine getNdkBuildCmd(), 'clean'
    }

    clean.dependsOn cleanNative
}

Finally, add helper functions to the end of gradle script:

def getNdkDir() {
    if (System.env.ANDROID_NDK_ROOT != null)
        return System.env.ANDROID_NDK_ROOT

    Properties properties = new Properties()
    properties.load(project.rootProject.file('local.properties').newDataInputStream())
    def ndkdir = properties.getProperty('ndk.dir', null)
    if (ndkdir == null)
        throw new GradleException("NDK location not found. Define location with ndk.dir in the local.properties file or with an ANDROID_NDK_ROOT environment variable.")

    return ndkdir
}

def getNdkBuildCmd() {
    def ndkbuild = getNdkDir() + "/ndk-build"
    if (Os.isFamily(Os.FAMILY_WINDOWS))
        ndkbuild += ".cmd"

    return ndkbuild
}

Ah, and don't forget to add "import" to the beginning of the gradle script:

import org.apache.tools.ant.taskdefs.condition.Os

Here I've described it more detailed.