JNIEnv

jstring toUpperString
(
    JNIEnv        *env,
    jobject        thiz,
    jstring        str
)
{
    //create String instance
    //find the string class
    jclass stringClass = env->FindClass("java/lang/String");
    if(stringClass == nullptr) {
        std::cout << "Failed to find the String class" << std::endl;
        return nullptr;
    }

    //find the contructor id
    jmethodID constructorID = env->GetMethodID(stringClass, "<init>", "(Ljava/lang/String;)V");
    if(constructorID == nullptr) {
        std::cout << "Failed to find the constructor of String class" << std::endl;
        return nullptr;
    }

    jstring stringObject = static_cast<jstring>( env->NewObject(stringClass, constructorID, str) );

    //find the toUpperCase Method
    jmethodID toUpperCaseID = env->GetMethodID(stringClass, "toUpperCase", "()Ljava/lang/String;");
    if(toUpperCaseID == nullptr) {
        std::cout << "Failed to find the toUpperCase method" << std::endl;
        return nullptr;
    }

    //call method
    return static_cast<jstring>(env->CallObjectMethod(stringObject, toUpperCaseID));
}

jstring toLowerString
(
    JNIEnv        *env,
    jobject        thiz,
    jstring        str
)
{
    //find the string class
    jclass stringClass = env->FindClass("java/lang/String");
    if(stringClass == nullptr) {
        std::cout << "Failed to find the String class" << std::endl;
        return nullptr;
    }

    //find the toLowerCase Method
    jmethodID toLowerCaseID = env->GetMethodID(stringClass, "toLowerCase", "()Ljava/lang/String;");
    if(toLowerCaseID == nullptr) {
        std::cout << "Failed to find the toLowerCase method" << std::endl;
        return nullptr;
    }

    //call method
    return static_cast<jstring>(env->CallObjectMethod(str, toLowerCaseID));
}


void doJobAndThenCallback
(
    JNIEnv        *env,
    jobject        thiz,
    jint            value
)
{
    //find the class of thiz object
    jclass thizClass = env->GetObjectClass(thiz);

    //find the onStart Method
    jmethodID onStartID = env->GetMethodID(thizClass, "onStart", "()V");
    if(onStartID == nullptr) {
        std::cout << "Failed to find the onStart method" << std::endl;
        return;
    }

    //call the onStart callback Method
    env->CallVoidMethod(thiz, onStartID);

    //do some jobs
    for(auto i=0; i<value; ++i) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    //find the onFinish Method
    jmethodID onFinishID = env->GetMethodID(thizClass, "onFinish", "()V");
    if(onFinishID == nullptr) {
        std::cout << "Failed to find the onFinish method" << std::endl;
        return ;
    }

    //call the onFinish callback Method
    env->CallVoidMethod(thiz, onFinishID);
}

JNIEXPORT jint JNICALL JNI_OnLoad
(
    JavaVM      *vm,
    void        *reserved
)
{
    JNIEnv      *env;
    if(vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6)) {
        return -1;
    }

    JNINativeMethod nm[3] ={

        {
            const_cast<char*>("toUpperString"),
            const_cast<char*>("(Ljava/lang/String;)Ljava/lang/String;"),
            reinterpret_cast<void*>(toUpperString)
        }
        ,
        {
            const_cast<char*>("toLowerString"),
            const_cast<char*>("(Ljava/lang/String;)Ljava/lang/String;"),
            reinterpret_cast<void*>(toLowerString)
        }
        ,
        {
            const_cast<char*>("doJobAndThenCallback"),
            const_cast<char*>("(I)V"),
            reinterpret_cast<void*>(doJobAndThenCallback)
        }

    };

    jclass cls = env->FindClass("Client");
    env->RegisterNatives(cls, nm, 3);
    return JNI_VERSION_1_6;
}