Question

I perform some decoding process in the native code and call a Java method from it to write samples into the AudioTrack instance. Decoding process runs fine, the callback from native code to Java:

private void writeToAudioTrack(final byte[] buffer)

is called , but once the Runnable object starts to write samples into the AudioTrack I get a NullPointerException immediately. I am quite sure it is caused by wrong threading, but cannot figure out what is wrong there. I am attaching the complete Java code:

public class Player
{
    private AudioTrack track;
    private boolean isInitialized = false;
    private static Handler handler = new Handler();

    public void init(String mediaSource)
    {
        // Call native function initEngine
        isInitialized = initEngine(mediaSource);

        if (isInitialized)
        {
            int bufSize = AudioTrack.getMinBufferSize(44100, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT);
            track = new AudioTrack(AudioManager.STREAM_MUSIC, 44100, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT, bufSize, AudioTrack.MODE_STREAM);
            track.play();
        }
    }

    public void play()
    {
        // Call native rendering function in a separate thread
        if (isInitialized)
        {
            new Thread(new Runnable()
            {
                @Override
                public void run()
                {
                    renderAudio();
                }
            }).start();
        }
    }

    public void release()
    {
        isInitialized = false;
        releaseEngine();
    }

    // This is callback from native code
    private void writeToAudioTrack(final byte[] buffer)
    {
        handler.post(new Runnable()
        {
            @Override
            public void run()
            {
                try
                {
                    track.write(buffer, 0, buffer.length);
                }
                catch (Exception e)
                {
                    e.printStackTrace();
                }
            }
        });
    }

    // 
    static
    {
        // Load native library
        System.loadLibrary("decoder");
    }

    // Private native methods
    private static native boolean initEngine(String mediaSource);
    private static native void releaseEngine();
    private static native void renderAudio();

Even if I create a buffer in place of try-catch block:

byte [] buffer = new byte[256];
track.write(buffer, 0, buffer.length);

I get the same result – NullPointerException.

Stack Trace:

java.lang.NullPointerException
at com.mautilus.audioplayer.Player$2.run(Player.java:99)
at android.os.Handler.handleCallback(Handler.java:587)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4627)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:876)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:634)
at dalvik.system.NativeStart.main(Native Method)

Answer 1

The error was in the type of one of the native methods – initEngine, it must not be declared as static – in this case.

According to the documentation:

http://docs.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html#wp16660

The GetMethodID() causes an uninitialized class to be initialized. So if the library loading is not tied with any class instance (that’s normal):

static
{
    // Load native library
    System.loadLibrary("decoder");
}

then the native code:

jmethodID writeToAudioTrackMethodID = NULL;
jclass cls = (*env)->FindClass(env, "com.mautilus.audioplayer.Player");

if (!cls)
writeToAudioTrackMethodID = (*env)->GetMethodID(env, cls, "writeToAudioTrack", "([B)V");

called from a static method – initEngine in my case, can’t obtain any existing instance of the class (even if such instance exists) and creates new one (with fields initialized to default values) – that’s it…