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

in arrayblockingqueue, methods require lock copy local final variable before calling lock().

public boolean offer(e e) {     if (e == null) throw new nullpointerexception();     final reentrantlock lock = this.lock;     lock.lock();     try {         if (count == items.length)             return false;         else {             insert(e);             return true;         }     } {         lock.unlock();     } } 

is there reason copy this.lock local variable lock when field this.lock final?

additionally, uses local copy of e[] before acting on it:

private e extract() {     final e[] items = this.items;     e x = items[takeindex];     items[takeindex] = null;     takeindex = inc(takeindex);     --count;     notfull.signal();     return x; } 

is there reason copying final field local final variable?

it's extreme optimization doug lea, author of class, likes use. here's post on a recent thread on core-libs-dev mailing list exact subject answers question pretty well.

from post:

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