What does final guarantee, and what breaks that guarantee?

There is a freeze at the end of the constructor: any thread that obtains a reference to the object through a normal read is guaranteed to see the final fields with their constructed values, with no synchronisation of any kind. This is what makes String and the immutable collections safe to share freely.

The guarantee has one condition and one limit. The condition: this must not escape during construction. Registering a listener, starting a thread or passing this to a callback from a constructor hands out a reference before the freeze, and the guarantee is void. The limit: final freezes the reference, not the object behind it. A final List field whose list is mutated is not thread safe in the slightest.