How to extract textures and sprites from a game on the libGDX engine?

[gameripper]

How to extract textures and sprites from a game on the libGDX engine?

That's not an engine but a framework (meaning each game using it has its own way of storing resources, no common format exists).

The entire game is packed into a single exe file, there are no resource files.

LibGDX is a Java library, but exe files contain machine code (so not Java bytecode). Therefore either this game is not using libGDX, or it also uses some magic to encapsulate the Java Runtime Environment too (in which case odds are it is using some even more exotic resource management method).

Regardless to the engine, you might be able to extract assets from an exe. Maybe try Microsoft's PEdump, but it's not easy to use, so I'd rather suggest ripPE or petools (or any other PE analysis and dumper tool) instead. If you're lucky you can find the assets in one of the sections, most likely in the one named '.rsrc' or '.rdata', but with a custom engine, anything is possible.

EDIT: give a try to GAME RIPPER, easy to use: just start it and drag'n'drop your the exe file on its window and that's it. I've implemented asset scanning in EXE and JAR files, so should work with a libGDX game too.