final methods are inlined?

Question

Are Java final methods automatically inlined?

Many books says yes many books says no!!!

Answer 1

Inlining of methods is performed by the JIT compiler, not javac.

Modern JIT compilers (including Hotspot) can often inline even non-final methods, "undoing" the optimisation appropriately if necessary. They're basically scarily clever.

In short: it entirely depends on the VM. In my opinion, you should make your methods final or not based on what produces the cleanest code rather than performance. I'm personally a fan of "design for inheritance or prohibit it" but that's a different discussion :)

Answer 2

Interesting question, prompted me to look into it further. 2 interesting remarks I found -

• 1 comment that automatic inlining is a bug:

Contrary to the implication of many tips, methods declared as final cannot be safely inlined by the compiler, because the method could have a non-final declaration at runtime.

To see why, suppose the compiler looks at class A and subclass B, and sub-subclass C and sees a final method in A which it inlines into C. But then at runtime the versions loaded for A and B are different and the method is not final in A, and overridden in B. Then C uses the incorrectly inlined version. T

And, a bit more authoritatively, from a sun whitepaper, writing that methods can be left virtual,

Because the Java HotSpot VM can automatically inline the vast majority of virtual method invocations, this performance penalty is dramatically reduced, and in many cases, eliminated altogether.

Here's a more direct reference on the mechanism.