Threads and Locks - The Java Language Specification - page 585

17.5. final Field Semantics
Fields declared final are initialized once, but never changed under normal circumstances.
The detailed semantics of final fields are somewhat different from those of normal fields.
In particular, compilers have a great deal of freedom to move reads of final fields across
synchronization barriers and calls to arbitrary or unknown methods. Correspondingly, com-
pilers are allowed to keep the value of a final field cached in a register and not reload it from
memory in situations where a non- final field would have to be reloaded.
final fields also allow programmers to implement thread-safe immutable objects without
synchronization. A thread-safe immutable object is seen as immutable by all threads, even
if a data race is used to pass references to the immutable object between threads. This can
provide safety guarantees against misuse of an immutable class by incorrect or malicious
code. final fields must be used correctly to provide a guarantee of immutability.
An object is considered to be completely initialized when its constructor finishes. A thread
that can only see a reference to an object after that object has been completely initialized is
guaranteed to see the correctly initialized values for that object's final fields.
The usage model for final fields is a simple one: Set the final fields for an object in that ob-
ject's constructor; and do not write a reference to the object being constructed in a place
where another thread can see it before the object's constructor is finished. If this is fol-
lowed, then when the object is seen by another thread, that thread will always see the cor-
rectly constructed version of that object's final fields. It will also see versions of any object
or array referenced by those final fields that are at least as up-to-date as the final fields are.
Example 17.5-1. final Fields In The Java Memory Model
The program below illustrates how final fields compare to normal fields.
Click here to view code image
class FinalFieldExample {
final int x;
int y;
static FinalFieldExample f;
public FinalFieldExample() {
x = 3;
y = 4;
}
static void writer() {
f = new FinalFieldExample();
}