In ArrayBlockingQueue, why copy final member field into local final variable?

Question

In ArrayBlockingQueue, all the methods that require the lock copy it to a local final variable before calling lock().

Answer 1

This thread gives some answers. In substance:

• the compiler can't easily prove that a final field does not change within a method (due to reflection / serialization etc.)
• most current compilers actually don't try and would therefore have to reload the final field everytime it is used which could lead to a cache miss or a page fault
• storing it in a local variable forces the JVM to perform only one load

Answer 2

It's an extreme optimization Doug Lea, the author of the class, likes to use. Here's a post on a recent thread on the core-libs-dev mailing list about this exact subject which answers your question pretty well.

from the post:

...copying to locals produces the smallest bytecode, and for low-level code it's nice to write code that's a little closer to the machine