It uses NIO direct ByteBuffers to allow some of the binding to be done in managed code, and the rest is done with custom callbacks.
I only need it for reading video, so that's all i've implemented; and even then only a tiny subset thereof. I don't know if it will get any further than that.
I did look into using gluegen to do the binding but I couldn't get it to work - for starters the ffmpeg headers are too complex for it's limited parser. Even stripping out the conditionals and the inline code didn't help (using cpp -dD and some editing). So I just manually select which fields to bind and I have some C which works out the offsets (and auto-generates some classes) from some embedded tables (why write a config parser when the c compiler has one?), and hand-code the function calls.
I use 3 classes per 'class' to allow me to get away with binding generated code with hand-written code, as well as supporting 32 or 64 bit (perhaps, eventually - only 64 bit done so far). Base abstract class is auto-generated and sets up the interface for the auto-generated field accessors. The middle abstract class is the public api and includes the hand-written bindings and factory methods (where appropriate). The final concrete class implements the accessors for each processor size.
Up until now i've been using Xuggler ... which I probably will keep using for some software, particularly cross platform stuff. But I don't really like the callback api it uses and for the most part the ffmpeg library api's are fairly simple on their own (despite a bit of bitrot).