Should I declare a java field 'final' when if it's not modified in code?

Question

My question is mainly about performance. The compiler knows better that, for example, some variable is NOT modified after object instantiation. So, why bother with final?

Answer 1

Using final for a field also has implications for thread safety. The Java Memory Model states that the values of final fields becomes visible to all threads that can access the object as soon as the constructor of that object finishes. That guarantee is similar to volatile fields, where any thread always sees the current value. There is no such guarantee for normal fields.

And, as others noted, the JIT can perform aggressive optimizations for final fields, which are not so easy for normal fields, where any newly loaded class could have write accesses to the field.