JUC-关于CopyOnWriteArrayList的疑问

        transient final ReentrantLock lock = new ReentrantLock();           .....................           .....................                      private static final Unsafe unsafe = Unsafe.getUnsafe();        private static final long lockOffset;        static {          try {               lockOffset = unsafe.objectFieldOffset                  (CopyOnWriteArrayList.class.getDeclaredField("lock"));               } catch (Exception ex) { throw new Error(ex); }        }        private void resetLock() {            unsafe.putObjectVolatile(this, lockOffset, new ReentrantLock());        }