该类定义了一个内部接口EventExecutorChooser,该接口有一个方法: next, EventExecutorChooserFactory类有个默认实现类: DefaultEventExecutorChooserFactory,该类有个newChooser方法用来返回EventExecutorChooser 可能会返回两种类型的EventExecutorChooser: PowerOfTwoEventExecutorChooser和GenericEventExecutorChooser