Question

My application makes calls to a native library, and then within the native library, makes calls back to the Java code. I have the calls TO the native library working correctly ( I’m pretty sure ), but not its telling me that it can’t find the Java functions when trying to call them from the C file.

The application doesn’t even crash, there is no pop up telling me it closed unexpectedly, it just flashes a black screen and nothing happens.

The code I am trying to reproduce was originally made in C for Palm Pilot, and is being transferred to an Android device. This is the original call in C I am trying to duplicate…

InitRelay(Changeit,getit,putit,flushit,delayit);

The parameters in the call above are function names, the functions are here below:

void Changeit(WORD baud){
    ChangeRate(baud);
}

extern Boolean ChangeRate(UInt32 baud)
{...}

Int16 getit(UInt16 t) {...}

void putit( BYTE p ) {...}

void flushit(void){...}

extern void delayit( UInt16 wait ) {...}

This is InitRelay shown in my .c file:

BYTE __stdcall InitRelay(fp_setbaud _setbaud, fp_get _get, fp_put _put, fp_flush _flush, fp_delay _delay){

    RelayAPI_SetBaud=_setbaud;
    RelayAPI_get=_get;
    RelayAPI_put=_put;
    RelayAPI_flush=_flush;
    RelayAPI_delay=_delay;
    ....
}

The parameters shown in above are typedefed in my .h file:

typedef void (__stdcall *fp_setbaud)(WORD);
typedef short (__stdcall *fp_get)(WORD);
typedef void (__stdcall *fp_put)(BYTE);
typedef void (__stdcall *fp_flush)(void);
typedef void (__stdcall *fp_delay)(WORD);

The WORD/BYTE/DWORD types are defined in a separate .h file shown here:

typedef unsigned char BYTE;
typedef unsigned short WORD;
typedef unsigned long DWORD;

Now, in my Android code, I call a function I made called InitRelayJava(), all of my data for the application is stored in a separate class called RelayAPIModel. I created a nested class within that to store all of my native functions. I did this so that I can access these functions the same way no matter what Activity the application is currently in.

public class RelayAPIModel {
    ....
    public static class NativeCalls {
        static {
            System.loadLibrary( "RelayAPI" );
        }

        public native static byte InitRelayJava();

    public native static void FreeRelayJava();


        public static void changeItJavaWrapper( short l ) {
            mModelService.changeitJava( l );
        }

        public static void flushItJavaWrapper() {
            mModelService.flushitJava();
        }

        public static void putItJavaWrapper( byte[] p ) {
            mModelService.putitJava( p );
        }

        public static void delayItJavaWrapper( short wait ) {
            mModelService.delayitJava( wait );
        }

        public static short getItJavaWrapper( short s ) {           
            return mModelService.getitJava( s );
        }
    }
 }

The calls made inside the *Wrapper functions go to a separate class that I have to handle all of the applications bluetooth capabilities. I do not think that those are needed for this problem.

This is what InitRelayJava looks like in my C code…

BYTE Java_my_eti_commander_RelayAPIModel_00024NativeCalls_InitRelayJava( JNIEnv *env, jobject obj  ) {

    myEnv = (env);
    bluetoothClass = (*env)->GetObjectClass( env, obj );
    myObject = obj;

    changeID = (*myEnv)->GetMethodID( myEnv, myObject, "changeitJavaWrapper", "(S)V"  );
    getID    = (*myEnv)->GetMethodID( myEnv, myObject, "getitJavaWrapper"   , "(S)S"   );
    putID    = (*myEnv)->GetMethodID( myEnv, myObject, "putitJavaWrapper"   , "(B)V" );
    flushID  = (*myEnv)->GetMethodID( myEnv, myObject, "flushitJavaWrapper" , "()V"   );
    delayID  = (*myEnv)->GetMethodID( myEnv, myObject, "delayitJavaWrapper" , "(S)V"  );
...
}

This is the LogCat I am receiving…

08-02 10:27:32.406: D/dalvikvm(28376): Trying to load lib /data/data/my.eti.commander/lib/libRelayAPI.so 0x40515430
08-02 10:27:32.406: D/dalvikvm(28376): Added shared lib /data/data/my.eti.commander/lib/libRelayAPI.so 0x40515430
08-02 10:27:32.406: D/dalvikvm(28376): No JNI_OnLoad found in /data/data/my.eti.commander/lib/libRelayAPI.so 0x40515430, skipping init
08-02 10:27:32.406: D/dalvikvm(28376): GetMethodID: method not found: Lmy/eti/commander/RelayAPIModel$NativeCalls;.changeitJavaWrapper:(S)V
08-02 10:27:32.413: D/dalvikvm(28376): GetMethodID: method not found: Lmy/eti/commander/RelayAPIModel$NativeCalls;.getitJavaWrapper:(S)S
08-02 10:27:32.413: E/dalvikvm(28376): Class lookup Ljava/lang/NoSuchMethodError; attempted while exception Ljava/lang/NoSuchMethodError; pending

Answer 1

InitRelayJava is a static method – that means your second parameter (obj) is a class object pointer, not a this pointer. So get rid of the following line:

bluetoothClass = (*env)->GetObjectClass( env, obj );

and instead pass obj to GetMethodID(), like this:

changeID = (*myEnv)->GetMethodID( myEnv, obj, "changeitJavaWrapper", "(I)Z"  ); 

EDIT: also, your parameter/return type signatures are wrong. Short is not the same as int, so the signature for changeitJavaWrapper is “(S)Z”, for getitJavaWrapper is “()I” as it takes no parameters and returns an int. Be more careful please; it does not take an advanced knowledge of C to get those right, just some self-checking. This project of yours is inching into What have you tried? territory.

Cheat sheet on JNI type codes here.

Let me try and anticipate your next question – you cannot call those methods. Of course you cannot, it’s not possible to call a nonstatic method from a static one. Your Java callbacks are all nonstatic, while the native method is static. Either make them static, or make the native method nonstatic and insert the GetObjectClass back.

EDIT2: so you changed your Java methods to static without telling. Now instead of (*env)->GetMethodID() you need to call (*env)->GetStaticMethodID() to get the method ID. Same parameters.