Re: [Swig-user] Controlling getter/setter generation for structs

[William S F. <ws...@fu...>]

Gibbs, Solomon wrote:
> Hello,
>  
> I hope this isn't an obvious question, but I have looked through the
> docs and I can't figure it out.
>  
> I'm exporting an existing C library to Java. I have a C struct along the
> lines of:
> typedef 
>    struct {
>       char* next_in;
>       unsigned int avail_in;
>  
>       char* next_out;
>       unsigned int avail_out;
>  
>    } bz_stream;
>  
> bz_stream gets wrapped with a Java proxy class, and I can specify a
> C-method for backing the constructor with the %extend directive. Since
> the library functions as a stream filter, I allocate heap space in Java
> using two direct Java NIO Buffers and override %typemap(in) char* and
> %typemap(memberin) char* so that the setter method on the bz_stream
> proxy class sets the bz_stream->next_in and next_out as the address of
> the direct Buffer.
>  
> There are two problems with this: Because the proxy class only retains a
> reference to the underlying array and not the actual Java Buffer object,
> there is the potential for the Buffer to be garbage collected
> prematurely. In addition, the getter generated for the bz_stream proxy
> makes no sense, because it cannot return the Buffer object.
>  
> 
> How do I modify the proxy code for the setters to retain a reference to
> the Buffer object and the code for the getters to return that reference?
> 
Use the same techniques as outlined here:
http://www.swig.org/Doc1.3/Java.html#java_memory_management_member_variables


>  
> As a matter of style and productivity, is it better to spend time fixing
> up the proxy classes to present a clean interface, or should I just plan
> on having a somewhat messy boundary layer between Java and C and try to
> smooth it out by adding a layer of abstraction in Java?
>  
Unless you have many similar types of structs, I would write an 
abstraction layer with a pure Java wrapper class which manages the 
memory references. It would wrap the SWIG generated proxy. You could 
squeeze this functionality into the proxy class, but it requires a lot 
of messing around with typemaps and features and will take someone not 
familiar with SWIG somewhat longer to do.

William