log
Logger
通过New(out io.Writer, prefix string, flag int)可以创建一个Logger，其中：
out：日志输出目标prefix：日志数据前缀flag：标志位，控制日志的输出格式
其中flag可取值为：
const (
// Bits or'ed together to control what's printed.
// There is no control over the order they appear (the order listed
// here) or the format they present (as described in the comments).
// The prefix is followed by a colon only when Llongfile or Lshortfile
// is specified.
// For example, flags Ldate | Ltime (or LstdFlags) produce,
// 2009/01/23 01:23:23 message
// while flags Ldate | Ltime | Lmicroseconds | Llongfile produce,
// 2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message
Ldate = 1 << iota // the date in the local time zone: 2009/01/23
Ltime // the time in the local time zone: 01:23:23
Lmicroseconds // microsecond resolution: 01:23:23.123123. assumes Ltime.
Llongfile // full file name and line number: /a/b/c/d.go:23
Lshortfile // final file name element and line number: d.go:23. overrides Llongfile
LUTC // if Ldate or Ltime is set, use UTC rather than the local time zone
LstdFlags = Ldate | Ltime // initial values for the standard logger
)
下面是一个例子，首先是输出日志到标准输出，然后更改设置，输出到文件：
logger := log.New(os.Stdout, "my logger: ", log.LstdFlags)
fmt.Println(logger.Prefix()) // "my logger: "
fmt.Println(logger.Flags()) // 3
logger.Println("hello world") // my logger: 2017/05/19 15:22:17 hello world
file, err := os.OpenFile("data.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
if err != nil {
panic("open log file error")
}
defer file.Close()
logger.SetOutput(file)
logger.SetPrefix("hello: ")
logger.SetFlags(log.Lmicroseconds | log.Lshortfile)
logger.Println("hello world") // hello: 15:22:17.050889 main.go:27: hello world
Logger的输出方式有三种:
- Print: Print, Printf, Println
- Fatal: Fatal, Fatalf, Fatalln
- Panic: Panic, Panicf, Panicln
其中Fatal相当于是Print之后跟了一个os.Exit(1)操作，Panic相当于是Print后跟了一个panic()。
这三种输出方式，本质上都是调用了Output方法，一般我们不会主动调用该方法。
全局Logger
logger模块中有一个非导出的全局默认Logger：
var std = New(os.Stderr, "", LstdFlags)
logger模块的全局方法，实际上就是调用了std的相应方法。