Here are the new simplified rules:
* Required cache warmers are *always* executed when the Kernel boots for the first time;
* Optional cache warmers are *only* executed from the CLI via cache:warmup
These new rules means that all the configuration settings for the cache
warmers have been removed. So, if you want the best performance, remember to
warmup the cache when going to production.
This also fixed quite a few bugs.
This feature added complexity to the framework but wasn't used in the core anyway.
You can still use the Map class loader in your application though. But most of the time, using the APC
autoloader is just better.
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).