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