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From: Steven Simpson <ss@domain.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: JNI and GetFieldID
Date: Fri, 06 Sep 2013 22:34:32 +0300
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On 06/09/13 21:44, Borneq wrote:
> public interface ICallback {
>        void OnPercent(int percent);
>        void OnPercentEx(CmplxStruct cs);
> }

While in Java, you should use Java conventions, e.g. onPercent for a 
method name.  Can't say I'm a fan of prefixing "Callback" with "I" to 
show it's an interface either.

> public class CmplxStruct {
>        int fieldI;
>        String str;
> }

CmplxStruct?  Ky, 'll tk tht s jst n ex.

> How place number and string?

> jmethodID mid;
> mid = env->GetMethodID(cls, "OnPercentEx", "(LCmplxStruct;)V");
> env->CallVoidMethod(param1, mid, SOMETHING);

> how fill SOMETHING?

Never learned to do that - looks too error-prone.  Instead of mucking 
around in JNI, creating objects, accessing fields, etc, I prefer to do 
as much as I can in Java, then declare the native methods as taking the 
already dismantled fields as separate arguments.

You could do similar for a callback.  Write a private static method to 
do the fiddly stuff:

   private static void callBack(ICallback dest, int i, String s) {
       CmplxStruct obj = new CmplxStruct();
       obj.fieldI = i;
       obj.str = s;
       dest.OnPercentEx(obj);
   }

Now, in JNI, you use something like:

   jobject dest = ...;
   jint i = ...;
   jstring s = ...;

   mid = env->GetStaticMethodID(cls, "callBack", "(LICallback;ILjava/lang/String;)V");
   env->CallStaticVoidMethod(dest, i, s);


-- 
ss at comp dot lancs dot ac dot uk