A class in Symfony2 can be loaded by four different mechanisms:
* bootstrap.php: This file contains classes that are always required and
needed very early in the request handling;
* classes.php: This file contains classes that are always required and
managed by extensions via addClassesToCompile();
* MapFileClassLoader: This autoloader uses a map of class/file to load
classes (classes are managed by extensions via addClassesToAutoloadMap(),
and should contain often used classes);
* UniversalAutolaoder: This autoloader loads all other classes (it's the
slowest one).
