java自带四种线程池：
- FixedThreadPool
- SingleThreadExecutor
- CachedThreadPool
- ScheduledThreadPool
- submit方法与execute方法的区别 submit方法调用execute方法，submit方法有返回值，execute方法没有返回值
- offer和add方法区别 add方法比offer方法多一个异常，add方法不会返回false
abortPolicy
callerRunsPolicy
discardPolicy
discardOldestPolicy