final instance variables

[agrah upadhyay]

Why final instance variables of a servlet are considered to be thread safe?

[Vijay Venkat]

1. final --> implies it is immutable.
It holds good in any java environment.
Beware: If you are using object, if the properties inside the objects are not final then they are mutable and not thread safe.

Eg private final int _FIVE = 5
private final List _SHAPES = new ArrayList();

Here you cannot assign new values to _FIVE and _SHAPES cannot be made to refer to a different list.

This is invalid : Somewhere in your code you cannot do
_FIVE = 10
_SHAPES = new ArrayList()
On doing above you will get compilation errors.

But the objects with in _SHAPES can be mutated, depending up the objects which have been added to _SHAPES.

Vijay Venkataraman