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]
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[ 4.7 ms ] story [ 17.4 ms ] threadThe list of languages they support is impressive: http://www.compileonline.com/
Trickier ways of doing this include Emscripten [1]
1: http://www.rfk.id.au/blog/entry/pypy-js-first-steps/
Skulpt is a Javascript implementation of the Python programming language.