Monday, 6 June 2011

jjmpeg - why jni?

I started writing a reply to Michael's comment on the last entry but because I tend to ramble, it ended up so long I thought I'd promote it to a post.
have you considered using JNA? ...
... otherwise gluegen if you want to stay on the JNI road.
I looked at JNA previously some time ago, and found some problems with using it. I can't remember what they were at the moment but I was so displeased with it I know it ruled me out ever bothering with it again. It looks really good on paper but as I chose to write JNI directly at the time (for cross platform code too) there must have been a good reason. FWIW I didn't look into SWIG or any other option either.

For ffmpeg specifically, you need to access random fields of big structures and it would be impractical (or impossible) to map them using jna - many of the fields are private and the public ones are spread out through the structure. So i'd be forced to write a library to define accessors anyway, and then the jna objects to call those, so in the end i'd have to write something twice whereas now i don't even need to write it once (just a simple config file entry, assuming i didn't write a generator for jna - but then there would be no reason to use it).

I tried gluegen because it looks pretty nice and i've had nothing but positive experiences with jocl and jogl, but it's preprocessor and parser just weren't up to the task - the ffmpeg headers are mostly internal headers which have become public, they are not a cleaned-up public/standard api. They contain a ton of cruft that isn't public as well as public stuff that is behind conditional compilation (using expressions) and the like. I tried pre-processing it using cpp -dD (iirc) which preserves the #defines but then the inline code or other stuff threw it and i couldn't even work out which bit of code was the problem from the terse error messages. After giving up on it i found some other tools that might generate a simple/clean enough file to process (e.g. cproto can dump cleaned up types as well as clean prototypes) - but by then i'd moved on.

The perl script is a bit of a mess but most of the binding is automatic. At minimum i only have to write a constructor method for the public class. Accessors and most methods are automatic (once defined in the config file). I only need 2 classes for every wrapped 'object', one auto-generated.

There are a few special cases, but I find JNI pretty easy to use for those - given what it does it's about as simple as could be expected. And having attempted or worked with interfaces for similar purposes in the past I think JNI is actually quite nice. For example .NET's native binding looks really nice on paper too (it's more like a 'built-in' JNA) but there are actually more gotchas because it's trying to automate more - it's good most of the time but can be a real pain when things get complicated.

I'm already spending more time trying to work out how to use the libraries, the binding itself is mostly looking after itself, even if it is still incomplete.

Having said all that .... I realise that I may have made a mistake and there will be outstanding issues yet to resolve. But at least i'm fighting with my own mistakes and not finding the hidden limitations of tools i know little about - which simply makes it a lot more fun for a spare-time project.

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