## defer 推迟执行 官方文档：“A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.” 意思就是**函数返回**、**函数结束**或者对应的**goroutine发生panic**的时候defer就会执行。