Convert JNI types to Native types

Question

While there is documentation regarding turning a jstring to a native string (string nativeString = env->GetStringUTFChars(jStringVariable, NULL);

Answer 1

Same issue–fixed. In my case I'm using openFrameworks so I don't know if this applies to non-openFrameworks projects (haven't tested). However, it appears that the first two arguments in an external function are always "env" and "thiz" and these need to be defined explicitly for each new extern function.

extern "C"{

// casts the variable properly
void Java_com_package_JavaClass_someFunction( JNIEnv*  env, jobject  thiz, jboolean yourBool ){
    myTestApp->someFunction( (bool) yourBool );
}

// "yourBool" will always be "1" because its taking the spot of "thiz" which is not null
void Java_com_package_JavaClass_someFunction( JNIEnv*  env, jboolean yourBool ){
    myTestApp->someFunction( (bool) yourBool );
}

// "yourBool" will always be "1" because its taking the spot of "env" which is not null
void Java_com_package_JavaClass_someFunction( jboolean yourBool ){
    myTestApp->someFunction( (bool) yourBool );
}


}

Answer 2

You just need to cast jintto int using C style casts. Same for jboolean to bool (if you're using C99 bool type) or to uint8_t (if you're using std int types) or to unsigned char.

Open $NDK_ROOT/platforms/android-8/arch-arm/usr/include/jni.h and you'll see jint, jboolean etc are just typedefs.

Answer 3

This has worked for me:

bool isValue = bool(jbooleanValue);

Answer 4

The odd one out is jchar. It's defined as unsigned short, and depending on your compilation settings, that may or may not be equivalent to wchar_t. Depending on your underlying platform, you may be better off working with UTF8 strings. At least those are bitwise equivalent to ASCII for the ASCII subset of characters.

On Windows and Mac OS/Cocoa, however, the native wide string representation is exactly unsigned short. Java strings fit naturally into that.

Answer 5

If you just want to use it in if(..) statement, this is working for me without conversion:

if (jboolean == true) {
  return true;
} else {
  return false;
}

Answer 6

To cast a jboolean (which may only contain the values JNI_FALSE or JNI_TRUE) to a native bool I would use something like this :

(bool)(jboolean == JNI_TRUE)

If perhaps the jboolean isn't coming from the JVM, then testing for jboolean != JNI_FALSE might be considered safer.