[concurrency-interest] ThreadPoolExecutor implement question!

[yangjs]

hi,all

when I read ThreadPoolExecutor code ,I found it's declare the mainLock use "final" ,

  private final ReentrantLock mainLock = new ReentrantLock();

many method use the lock  guarding state, use case as follow:

     final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // some code 
        } finally {
            mainLock.unlock();
        }

My question is the instance field already use "final" to mainLock, 
why every method need copy mainLock reference to final  local var.

this puzzle me,who can tell me why ? thanks.

Best regards,
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