Go语言log日志包详解及使用

前言

在计算机世界里，日志文件记录了发生在操作系统或其他软件运行时的事件或状态。技术人员可以通过日志记录进而判断系统的运行状态，寻找导致系统出错、崩溃的成因等。这是我们分析程序问题常用的手段。在研究log日志文件之前，先来看看日志是什么。

Centos 7系统里/var/log

/var目录是所有服务的登录的文件或错误信息文件（LOG FILES)都在/var/log下，此外，一些数据库如MySQL则在/var/lib下，还有，用户未读的邮件的默认存放地点为/var/spool/mail

系统日志一般都存在/var/log下，常用的系统日志如下:

核心启动日志：/var/log/dmesg
系统报错日志：/var/log/messages
邮件系统日志：/var/log/maillog
FTP系统日志：/var/log/xferlog
安全信息和系统登录与网络连接的信息：/var/log/secure
登录记录：/var/log/wtmp 记录登录者讯录，二进制文件，须用last来读取内容 who -u /var/log/wtmp 查看信息
News日志：/var/log/spooler
引导日志：/var/log/boot.log 记录开机启动讯息，dmesg | more
cron(定制任务日志)日志：/var/log/cron

那么Golang打印的日志长什么样？下面开始进入本篇的正题。

Golang的log日志包

Golang源码的日志目录结构是这样的：
其中包含了两个包，一个是syslog系统日志包，另一个是log包。这里本篇主要以log包讲解。说到log包，直接看源码：

log.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package log implements a simple logging package. It defines a type, Logger,
// with methods for formatting output. It also has a predefined 'standard'
// Logger accessible through helper functions Print[f|ln], Fatal[f|ln], and
// Panic[f|ln], which are easier to use than creating a Logger manually.
// That logger writes to standard error and prints the date and time
// of each logged message.
// Every log message is output on a separate line: if the message being
// printed does not end in a newline, the logger will add one.
// The Fatal functions call os.Exit(1) after writing the log message.
// The Panic functions call panic after writing the log message.
package log

import (
	"fmt"
	"io"
	"os"
	"runtime"
	"sync"
	"time"
)

// These flags define which text to prefix to each log entry generated by the Logger.
// Bits are 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
const (
	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
)

// A Logger represents an active logging object that generates lines of
// output to an io.Writer. Each logging operation makes a single call to
// the Writer's Write method. A Logger can be used simultaneously from
// multiple goroutines; it guarantees to serialize access to the Writer.
type Logger struct {
	mu     sync.Mutex // ensures atomic writes; protects the following fields
	prefix string     // prefix to write at beginning of each line
	flag   int        // properties
	out    io.Writer  // destination for output
	buf    []byte     // for accumulating text to write
}

// New creates a new Logger. The out variable sets the
// destination to which log data will be written.
// The prefix appears at the beginning of each generated log line.
// The flag argument defines the logging properties.
func New(out io.Writer, prefix string, flag int) *Logger {
	return &Logger{out: out, prefix: prefix, flag: flag}
}

// SetOutput sets the output destination for the logger.
func (l *Logger) SetOutput(w io.Writer) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.out = w
}

var std = New(os.Stderr, "", LstdFlags)

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
func itoa(buf *[]byte, i int, wid int) {
	// Assemble decimal in reverse order.
	var b [20]byte
	bp := len(b) - 1
	for i >= 10 || wid > 1 {
		wid--
		q := i / 10
		b[bp] = byte('0' + i - q*10)
		bp--
		i = q
	}
	// i < 10
	b[bp] = byte('0' + i)
	*buf = append(*buf, b[bp:]...)
}

// formatHeader writes log header to buf in following order:
//   * l.prefix (if it's not blank),
//   * date and/or time (if corresponding flags are provided),
//   * file and line number (if corresponding flags are provided).
func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
	*buf = append(*buf, l.prefix...)
	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
		if l.flag&LUTC != 0 {
			t = t.UTC()
		}
		if l.flag&Ldate != 0 {
			year, month, day := t.Date()
			itoa(buf, year, 4)
			*buf = append(*buf, '/')
			itoa(buf, int(month), 2)
			*buf = append(*buf, '/')
			itoa(buf, day, 2)
			*buf = append(*buf, ' ')
		}
		if l.flag&(Ltime|Lmicroseconds) != 0 {
			hour, min, sec := t.Clock()
			itoa(buf, hour, 2)
			*buf = append(*buf, ':')
			itoa(buf, min, 2)
			*buf = append(*buf, ':')
			itoa(buf, sec, 2)
			if l.flag&Lmicroseconds != 0 {
				*buf = append(*buf, '.')
				itoa(buf, t.Nanosecond()/1e3, 6)
			}
			*buf = append(*buf, ' ')
		}
	}
	if l.flag&(Lshortfile|Llongfile) != 0 {
		if l.flag&Lshortfile != 0 {
			short := file
			for i := len(file) - 1; i > 0; i-- {
				if file[i] == '/' {
					short = file[i+1:]
					break
				}
			}
			file = short
		}
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')
		itoa(buf, line, -1)
		*buf = append(*buf, ": "...)
	}
}

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is used to recover the PC and is
// provided for generality, although at the moment on all pre-defined
// paths it will be 2.
func (l *Logger) Output(calldepth int, s string) error {
	now := time.Now() // get this early.
	var file string
	var line int
	l.mu.Lock()
	defer l.mu.Unlock()
	if l.flag&(Lshortfile|Llongfile) != 0 {
		// Release lock while getting caller info - it's expensive.
		l.mu.Unlock()
		var ok bool
		_, file, line, ok = runtime.Caller(calldepth)
		if !ok {
			file = "???"
			line = 0
		}
		l.mu.Lock()
	}
	l.buf = l.buf[:0]
	l.formatHeader(&l.buf, now, file, line)
	l.buf = append(l.buf, s...)
	if len(s) == 0 || s[len(s)-1] != '\n' {
		l.buf = append(l.buf, '\n')
	}
	_, err := l.out.Write(l.buf)
	return err
}

// Printf calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Printf.
func (l *Logger) Printf(format string, v ...interface{}) {
	l.Output(2, fmt.Sprintf(format, v...))
}

// Print calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Print.
func (l *Logger) Print(v ...interface{}) { l.Output(2, fmt.Sprint(v...)) }

// Println calls l.Output to print to the logger.
// Arguments are handled in the manner of fmt.Println.
func (l *Logger) Println(v ...interface{}) { l.Output(2, fmt.Sprintln(v...)) }

// Fatal is equivalent to l.Print() followed by a call to os.Exit(1).
func (l *Logger) Fatal(v ...interface{}) {
	l.Output(2, fmt.Sprint(v...))
	os.Exit(1)
}

// Fatalf is equivalent to l.Printf() followed by a call to os.Exit(1).
func (l *Logger) Fatalf(format string, v ...interface{}) {
	l.Output(2, fmt.Sprintf(format, v...))
	os.Exit(1)
}

// Fatalln is equivalent to l.Println() followed by a call to os.Exit(1).
func (l *Logger) Fatalln(v ...interface{}) {
	l.Output(2, fmt.Sprintln(v...))
	os.Exit(1)
}

// Panic is equivalent to l.Print() followed by a call to panic().
func (l *Logger) Panic(v ...interface{}) {
	s := fmt.Sprint(v...)
	l.Output(2, s)
	panic(s)
}

// Panicf is equivalent to l.Printf() followed by a call to panic().
func (l *Logger) Panicf(format string, v ...interface{}) {
	s := fmt.Sprintf(format, v...)
	l.Output(2, s)
	panic(s)
}

// Panicln is equivalent to l.Println() followed by a call to panic().
func (l *Logger) Panicln(v ...interface{}) {
	s := fmt.Sprintln(v...)
	l.Output(2, s)
	panic(s)
}

// Flags returns the output flags for the logger.
func (l *Logger) Flags() int {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.flag
}

// SetFlags sets the output flags for the logger.
func (l *Logger) SetFlags(flag int) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.flag = flag
}

// Prefix returns the output prefix for the logger.
func (l *Logger) Prefix() string {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.prefix
}

// SetPrefix sets the output prefix for the logger.
func (l *Logger) SetPrefix(prefix string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.prefix = prefix
}

// Writer returns the output destination for the logger.
func (l *Logger) Writer() io.Writer {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.out
}

// SetOutput sets the output destination for the standard logger.
func SetOutput(w io.Writer) {
	std.mu.Lock()
	defer std.mu.Unlock()
	std.out = w
}

// Flags returns the output flags for the standard logger.
func Flags() int {
	return std.Flags()
}

// SetFlags sets the output flags for the standard logger.
func SetFlags(flag int) {
	std.SetFlags(flag)
}

// Prefix returns the output prefix for the standard logger.
func Prefix() string {
	return std.Prefix()
}

// SetPrefix sets the output prefix for the standard logger.
func SetPrefix(prefix string) {
	std.SetPrefix(prefix)
}

// Writer returns the output destination for the standard logger.
func Writer() io.Writer {
	return std.Writer()
}

// These functions write to the standard logger.

// Print calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Print.
func Print(v ...interface{}) {
	std.Output(2, fmt.Sprint(v...))
}

// Printf calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Printf.
func Printf(format string, v ...interface{}) {
	std.Output(2, fmt.Sprintf(format, v...))
}

// Println calls Output to print to the standard logger.
// Arguments are handled in the manner of fmt.Println.
func Println(v ...interface{}) {
	std.Output(2, fmt.Sprintln(v...))
}

// Fatal is equivalent to Print() followed by a call to os.Exit(1).
func Fatal(v ...interface{}) {
	std.Output(2, fmt.Sprint(v...))
	os.Exit(1)
}

// Fatalf is equivalent to Printf() followed by a call to os.Exit(1).
func Fatalf(format string, v ...interface{}) {
	std.Output(2, fmt.Sprintf(format, v...))
	os.Exit(1)
}

// Fatalln is equivalent to Println() followed by a call to os.Exit(1).
func Fatalln(v ...interface{}) {
	std.Output(2, fmt.Sprintln(v...))
	os.Exit(1)
}

// Panic is equivalent to Print() followed by a call to panic().
func Panic(v ...interface{}) {
	s := fmt.Sprint(v...)
	std.Output(2, s)
	panic(s)
}

// Panicf is equivalent to Printf() followed by a call to panic().
func Panicf(format string, v ...interface{}) {
	s := fmt.Sprintf(format, v...)
	std.Output(2, s)
	panic(s)
}

// Panicln is equivalent to Println() followed by a call to panic().
func Panicln(v ...interface{}) {
	s := fmt.Sprintln(v...)
	std.Output(2, s)
	panic(s)
}

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is the count of the number of
// frames to skip when computing the file name and line number
// if Llongfile or Lshortfile is set; a value of 1 will print the details
// for the caller of Output.
func Output(calldepth int, s string) error {
	return std.Output(calldepth+1, s) // +1 for this frame.
}

log包是go语言提供的一个简单的日志记录功能，其中定义了一个结构体类型 Logger，是整个包的基础部分，包中的其他方法都是围绕这整个结构体创建的．

Logger结构

// A Logger represents an active logging object that generates lines of
// output to an io.Writer. Each logging operation makes a single call to
// the Writer's Write method. A Logger can be used simultaneously from
// multiple goroutines; it guarantees to serialize access to the Writer.
type Logger struct {
	mu     sync.Mutex // ensures atomic writes; protects the following fields
	prefix string     // prefix to write at beginning of each line
	flag   int        // properties
	out    io.Writer  // destination for output
	buf    []byte     // for accumulating text to write
}

它表示一个活动的日志对象，给io.Writer生成多行输出。每次记录都简单地调用io.Writer的write方法。一个Logger可以被多个goroutines同步执行。

对各个成员含义解析：

mu ：是sync.Mutex，它是一个同步互斥锁，用于保证日志记录的原子性．
prefix ：是输入的日志每一行的前缀
flag ：是一个标志，用于设置日志的打印格式
out ：日志的输出目标，需要是一个实现了 io.Writer接口的对象，如: os.Stdout, os.Stderr, os.File等等
buf ：用于缓存数据

flag可选值

在log包里首先定义了一些常量，它们是日志输出前缀的标识:

// These flags define which text to prefix to each log entry generated by the Logger.
// Bits are 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
const (
	Ldate         = 1 << iota     // the date in the local time zone: 2009/01/23 //1 << 0 当地时区的日期: 2009/01/23
	Ltime                         // the time in the local time zone: 01:23:23 //1 << 1 当地时区的时间: 01:23:23
	Lmicroseconds                 // microsecond resolution: 01:23:23.123123.  assumes Ltime. //1 << 2 显示精度到微秒: 01:23:23.123123 (应该和Ltime一起使用)
	Llongfile                     // full file name and line number: /a/b/c/d.go:23   // 1 << 3 显示完整文件路径和行号: /a/b/c/d.go:23
	Lshortfile                    // final file name element and line number: d.go:23. overrides Llongfile  // 1 << 4 显示当前文件名和行号: d.go:23 (如果与Llongfile一起出现，此项优先) 
	LUTC                          // if Ldate or Ltime is set, use UTC rather than the local time zone  // 1 << 5如果设置了Ldata或者Ltime, 最好使用 UTC 时间而不是当地时区
	LstdFlags     = Ldate | Ltime // initial values for the standard logger    // 标准日志器的初始值
)

这是log包定义的一些抬头信息，有日期、时间、毫秒时间、绝对路径和行号、文件名和行号等。

主要函数讲解

辅助函数

在 log 包中，定义了下面几组方法:

func (l *Logger) Printf(format string, v ...interface{})
func (l *Logger) Print(v ...interface{})
func (l *Logger) Println(v ...interface{}) 

func (l *Logger) Fatal(v ...interface{})
func (l *Logger) Fatalf(format string, v ...interface{})
func (l *Logger) Fatalln(v ...interface{})

func (l *Logger) Panic(v ...interface{})
func (l *Logger) Panicf(format string, v ...interface{})
func (l *Logger) Panicln(v ...interface{})

即 Print*, Fatal*, Painc*, 这里方法结尾的 f 或者 ln 就跟 fmt.Print 的含义是相同的，因此上面这九个方法的使用方式其实与 fmt.Print/f/ln 是一样的。
fmt.Print/f/ln 示例：

package main

import (
	"log"
)
func main() {
	log.Print("欢迎关注\n")
	log.Printf("微信公众号\n")
	log.Println("程序猿编码")
}

编译输出：

log.Fatal/f/ln 示例：

package main

import (
	"fmt"
	"log"
)

func test() {
	fmt.Println("first defer")
	fmt.Println("second defer")
}

func main() {

	defer test()
	log.Fatal("this is log fatal test\n")
	log.Fatalf("this is log fatalf test\n")
	log.Fatalln("this is log fatalln test\n")

}

编译输出：
Panic 示例：

package main

import (
	"fmt"
	"log"
)

func test(){
	fmt.Println("first defer")
	if err := recover(); err != nil {
		fmt.Println(err)
	}

}

func main() {

	defer test()

	log.Panic("this is log panic")
	defer func() {
		fmt.Println("second defer")
	}()
}

编译输出：

我们直接以没有 f 或 ln 的方法为例来看看三组方法的代码：

func (l *Logger) Print(v ...interface{}) { 
    l.Output(2, fmt.Sprint(v...))  
}

func (l *Logger) Fatal(v ...interface{}) {
    l.Output(2, fmt.Sprint(v...))
    os.Exit(1)    
}

func (l *Logger) Panic(v ...interface{}) {
    s := fmt.Sprint(v...)
    l.Output(2, s)
    panic(s)
}

可以看到其实三个方法 都调用了接收者(也就是Logger类型的实例或指针)的 Output 方法。

New

// New creates a new Logger. The out variable sets the
// destination to which log data will be written.
// The prefix appears at the beginning of each generated log line.
// The flag argument defines the logging properties.
func New(out io.Writer, prefix string, flag int) *Logger {
	return &Logger{out: out, prefix: prefix, flag: flag}
}

New创建一个Logger。参数out设置日志信息写入的目的地。
参数prefix会添加到生成的每一条日志前面。参数flag定义日志的属性（时间、文件等等）。

SetOutput

// SetOutput sets the output destination for the logger.
func (l *Logger) SetOutput(w io.Writer) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.out = w
}

它用来将Logger的out域赋值为w。io.Writer是个接口类型，任何实现了方法Write(p []byte)(n int, err error)的类型都可以在这里使用。

itoa

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
func itoa(buf *[]byte, i int, wid int) {
	// Assemble decimal in reverse order.
	var b [20]byte
	bp := len(b) - 1
	for i >= 10 || wid > 1 {
		wid--
		q := i / 10
		b[bp] = byte('0' + i - q*10)
		bp--
		i = q
	}
	// i < 10
	b[bp] = byte('0' + i)
	*buf = append(*buf, b[bp:]...)
}

有辅助函数，将整型转换为定长十进制ASCII码。赋予负数宽度来防止左侧补0(zero-padding)

Output

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is used to recover the PC and is
// provided for generality, although at the moment on all pre-defined
// paths it will be 2.
func (l *Logger) Output(calldepth int, s string) error {
	now := time.Now() // get this early.
	var file string
	//加锁，保证多goroutine下的安全
	var line int
	l.mu.Lock()
	defer l.mu.Unlock()
	//如果配置了获取文件和行号的话
	if l.flag&(Lshortfile|Llongfile) != 0 {
		// Release lock while getting caller info - it's expensive.
		l.mu.Unlock()
		var ok bool
		_, file, line, ok = runtime.Caller(calldepth)
		if !ok {
			file = "???"
			line = 0
		}
		//获取到行号等信息后，再加锁，保证安全
		l.mu.Lock()
	}
	//把我们的日志信息和设置的日志抬头进行拼接
	l.buf = l.buf[:0]
	l.formatHeader(&l.buf, now, file, line)
	l.buf = append(l.buf, s...)
	if len(s) == 0 || s[len(s)-1] != '\n' {
		l.buf = append(l.buf, '\n')
	}
	//输出拼接好的缓冲buf里的日志信息到目的地
	_, err := l.out.Write(l.buf)
	return err
}

在 Output 方法中，做了下面这些事情：

1、 获取当前事件
2、对 Logger实例进行加锁操作
3、 判断Logger的标志位是否包含 Lshortfile 或 Llongfile,　如果包含进入步骤4, 如果不包含进入步骤5
4、获取当前函数调用所在的文件和行号信息
5、格式化数据，并将数据写入到 l.out 中，完成输出
6、解锁操作

最后，来一个生成日志（log）文件例子，对上述的函数做一个总结：

package main

import (
	"fmt"
	"log"
	"os"
)

func Debug(logName string) {
	logFile, err := os.OpenFile(logName, os.O_CREATE|os.O_APPEND|os.O_RDWR, 0666)
	if err != nil {
		fmt.Printf("create ./test.log err : %v\n", err)
	}
	if logFile != nil {
		defer func(file *os.File) { file.Close() }(logFile)
	}

	debugLog := log.New(logFile, "[Debug]", log.Ldate)

	debugLog.SetPrefix("[Debug]")
	debugLog.SetFlags(log.Lshortfile)
	debugLog.Println("this is Debug log")
}
func Waring(logName string) {
	logFile, err := os.OpenFile(logName, os.O_CREATE|os.O_APPEND|os.O_RDWR, 0666)
	if err != nil {
		fmt.Printf("create ./test.log err : %v\n", err)
	}
	if logFile != nil {
		defer func(file *os.File) { file.Close() }(logFile)
	}

	debugLog := log.New(logFile, "[Waring]", log.Ldate)

	debugLog.SetPrefix("[Waring]")
	debugLog.SetFlags(log.Lshortfile)
	debugLog.Println("this is Waring log")
}

func main() {
	logName := "./test.log"
	Debug(logName)
	Waring(logName)
}

编译输出：

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参考:Go1.13.6 源代码