java - Why defining class as final improves JVM performance?

Question

Quoting from http://sites.google.com/site/gson/gson-design-document:

Why are most classes in Gson marked as final?

While Gson provides a fairly extensible architecture by providing pluggable serializers and deserializers, Gson classes were not specifically designed to be extensible. Providing non-final classes would have allowed a user to legitimately extend Gson classes, and then expect that behavior to work in all subsequent revisions. We chose to limit such use-cases by marking classes as final, and waiting until a good use-case emerges to allow extensibility. Marking a class final also has a minor benefit of providing additional optimization opportunities to Java compiler and virtual machine.

Why is this the case? [If I would guess: of JVM knows class is final it does not maintain method override tables? Are there any other reasons?]

What is the benefit in performance?

Does this applies to classes that are frequency instantiated (POJO?) or perhaps to class that are holders static methods (Utility classes) ?

Are methods defined as final also can theoretically improve performance?

Are there any implications?

Thank you, Maxim.

Answer 1

Virtual (overridden) methods generally are implemented via some sort of table (vtable) that is ultimately a function pointer. Each method call has the overhead of having to go through that pointer. When classes are marked final then all of the methods cannot be overridden and the use of a table is not needed anymore - this it is faster.

Some VMs (like HotSpot) may do things more intelligently and know when methods are/are not overridden and generate faster code as appropriate.

Here is some more specific info on HotSpot. And some general info too.