## defer 推遲執行 官方文檔：“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**函數返回**、**函數結束**或者對應的**goroutine發生panic**的時候defer就會執行。