my jni helpers

jni_helper.cpp

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

#define _MACRO_CONCAT(a,b) a ## b
#define _MACRO_2_STR(a) #a
#define _TOSTR(a) _MACRO_2_STR(a)

#define JMETHOD_VAR(n) _MACRO_CONCAT(jmethodId, n)
#define JMETHOD_DECLARE(cn,n,s) static jmethodID JMETHOD_VAR(n);
#define JMETHOD_GET(cn, n,s)  JMETHOD_VAR(n) = jEnv->GetMethodID(_MACRO_CONCAT(jclass, cn), #n, s);

#define JCLASS_VAR_STATMENT(n) _MACRO_CONCAT(jclass, n)
#define JCLASS_VAR JCLASS_VAR_STATMENT(JCLASS_SIMPLENAME)
#define JCLASS_GET JCLASS_VAR = jEnv->FindClass(_TOSTR(JCLASS_FULLNAME));
#define JCLASS_DECLARE static jclass JCLASS_VAR;

/* example */

#define JCLASS_FULLNAME cn.iflyos.sdk.MediaPlayer
#define JCLASS_SIMPLENAME MediaPlayer
#define JMETHODS \
JMETHOD(JCLASS_SIMPLENAME, setStringSource, "(J)Ljava.lang.String;") \
JMETHOD(JCLASS_SIMPLENAME, setStreamSource, "(J)Lcn.iflyos.sdk.NativeStream;")

JCLASS_DECLARE

#define JMETHOD JMETHOD_DECLARE
JMETHODS
#undef JMETHOD

extern "C" JNIEXPORT jstring JNICALL
Java_cn_iflyos_open_myapplication_MainActivity_stringFromJNI(
        JNIEnv *jEnv,
jobject /* this */) {
    std::string hello = "Hello from C++";
    JCLASS_GET
    #define JMETHOD JMETHOD_GET
    JMETHODS
    #undef JMETHOD

    jEnv->CallLongMethod(JCLASS_VAR, JMETHOD_VAR(setStringSource), nullptr);

    return jEnv->NewStringUTF(hello.c_str());
}

#undef JMETHODS
#undef JCLASS_FULLNAME
#undef JCLASS_SIMPLENAME