Online compiler running

How are these things are made ? How can java be written on the web and compiled on web or C++ or other languag开发者_JS百科es, I saw a lot of sites, that have this option. How do they make them, and is there some example code, some source or project ?

The TryRuby project is hosted on Github, just have a look at how they do it (There shouldn't be that big of a difference between using a compiler and interpreter).