Swig typemap java object

Question

I am trying to generate java bindings through swig for these two c++ functions

C++ functions

void SetUserData(void* data);
void* GetUserData() const;
         


        

Answer 1

You probably want to do:

%apply jobject { void* };

because %apply copies the typemaps defined on one C++ type to another C++ type. Object is a Java type, not a C++ one so doesn't have any typemaps to be copied. jobject on the other hand is the JNI equivalent.

Additionally assuming you want to have 'normal' GC semantics (i.e. not be required to retain whatever data you pass in) you'll need to do some more work, for example:

%module test

%{
#include "test.hh"
%}

%apply jobject { void * };

%typemap(in) void * {
  void *old = getData();
  if (old) JCALL1(DeleteGlobalRef, jenv, (jobject)old);
  $1 = JCALL1(NewGlobalRef, jenv, $input);
}

%typemap(out) void * {
  $result = (jobject)($1);
}

%include "test.hh"

This makes a new Global Reference for your data, which could be the only thing keeping it from getting freed by the GC.

Given a header file:

void setData(void *);
void *getData();

and an implementation:

#include "test.hh"

namespace {
  void *d = nullptr;
}

void setData(void *d) {
  ::d = d;
}

void *getData() {
  return d;
}

That is sufficient to allow:

public class run {
  public static void main(String[] argv) {
    System.loadLibrary("test");
    Object example = "HELLO";
    test.setData(example);
    System.out.println(test.getData());
  }
}

to work correctly.

As written these typemaps are pretty ugly - they'll impact all usage of void *. So if you really used this you would want to use %apply or %clear a second time to limit their impact. You could also name the arguments in the header file and use that to limit where the typemap gets applied.