Question

I am currently working with JNI (Java Native Interface) to send data between Java and C++. After implementing a little of code I realized the code for each method was always similar. An example could be:

JNIEXPORT void JNICALL Java_com_trial_jni_Receiver_setData__II(JNIEnv * env, jobject thiz, jint nativeObject, jint value)  
{  
    reinterpret_cast<Receiver *>(nativeObject)->setData(value);  
}  

JNIEXPORT void JNICALL Java_com_trial_jni_Receiver_setData__ILjava_lang_String_2(JNIEnv *env, jobject thiz, jint nativeObject, jstring value)  
{  
    reinterpret_cast<Receiver *>(nativeObject)->setData(value);  
}

Due to all the code has a similar structure, I decided generate a set of macros to generate automatically all this code. Thanks to Gregory Pakosz’ answer in this link Is it possible to iterate over arguments in variadic macros? I am now able to verify using the preprocessor how many parameters I introduced in a macro and process every single param.

But from the previous example there is a thing I am unable to achive that I would like to. Suppose I have this method inside a macro called JNI_METHOD. I would like to do something like this:

#define JNI_METHOD(package,clazz,method,...) \  
    JNIEXPORT void JNICALL Java_ ##package## _ ##clazz## _ ##method##__II(JNIEnv * env, jobject thiz, jint nativeObject, SET_DECLARATION_PARAMS(__VA_ARGS__ )) \  
    { \  
        reinterpret_cast<clazz *>(nativeObject)->method(SET_DECLARED_PARAMS(__VA_ARGS__)); \  
    }  

JNI_METHOD(com_trial_jni,Receiver,setData,jint);  
JNI_METHOD(com_trial_jni,Receiver,setData,jstring);  

In order to avoid having this question too long I didn’t paste the declaration of SET_DECLARATION_PARAMS and SET_DECLARED_PARAMS but the first one will result in something like ‘jint arg1’ and the second in ‘arg1’ without the type.

The question is: is there any way to generate a macro returning ‘I’ for ‘jint’ or ‘Ljava_lang_String_2’ for ‘jstring’. Note that the stringification cannot be used and this is needed to have ‘ILjava_lang_String_2’ instead of ‘II’ in the second generated method name.

Thanks!

Answer 1

Well, the link you provided pretty much gives you the solution you need. Consider this:

#define CONCATENATE(arg1, arg2)   CONCATENATE1(arg1, arg2)
#define CONCATENATE1(arg1, arg2)  CONCATENATE2(arg1, arg2)
#define CONCATENATE2(arg1, arg2)  arg1##arg2

#define JNI_TRANSLATE_TYPE_jint I
#define JNI_TRANSLATE_TYPE_jstring Ljava_lang_String_2

#define JNI_TRANSLATE_TYPE(T) CONCATENATE(JNI_TRANSLATE_TYPE_, T)

Test, on VS2010:

#define STRINGIZE(arg)  STRINGIZE1(arg)
#define STRINGIZE1(arg) STRINGIZE2(arg)
#define STRINGIZE2(arg) #arg

#pragma message("jint: " STRINGIZE(JNI_TRANSLATE_TYPE(jint)))
#pragma message("jstring: " STRINGIZE(JNI_TRANSLATE_TYPE(jstring)))

Output:

1>  jint: I
1>  jstring: Ljava_lang_String_2