Ask HN: How did they make an online compiler?

At a (semi-educated) guess, they've got a box (or several) running Javac 1.7.0_09 (like it says in the title) and a script that pushes stdin and stdout around and back to the web page with the results of the compilation.

Trickier ways of doing this include Emscripten [1]

1: http://www.rfk.id.au/blog/entry/pypy-js-first-steps/

I assume it's a standard Java compiler. The hard part is preventing security breaches when the program is run.

The list of languages they support is impressive: http://www.compileonline.com/