Golang官方log包详解

以下全是代码, 详解在注释中, 请从头到尾看

// 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.
// 官方包注释, 这个包让你自己不需要手动实现日志包, 但是还是我建议还是使用第三方包!
package golog

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

// 稍加注释!
// These flags define which text to prefix to each log entry generated by the Logger.
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
	// 二进制或标志!
	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
	// 默认不带文件名
	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.
// 这个可同时被多个协程使用
type Logger struct {
	// 加锁
	mu     sync.Mutex // ensures atomic writes; protects the following fields
	// 打日志的前缀
	prefix string     // prefix to write at beginning of each line
	// 日志格式的标志
	flag   int        // properties
	// 写入的目标
	out    io.Writer  // destination for output
	// 日志缓存, 待写入
	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.
// 新一个日志记录, 这个对象(结构对象)承接了日志输出的责任.
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.
// 设置日志输出的地方
func (l *Logger) SetOutput(w io.Writer) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.out = w
}

// 默认记录对象, 输出为标准输出, 日志没前缀, 格式是标准: 2009/01/23 01:23:23 message
var std = New(os.Stderr, "", LstdFlags)

// Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.
// 数字填充, 方便时间对齐
func itoa(buf *[]byte, i int, wid int) {
	// Assemble decimal in reverse order.
	// 填充后最长长度20!
	var b [20]byte
	bp := len(b) - 1

	// 类似辗转相除法取最大公约数, 此处是每次取数字的后一位, 等到数字取不到位了就按照wid开始填充
	// 比如 i=106 wid=5:
	//		wid q byte i
	//		4 10 6 106
	//		3 1 0 10
	//		2 0 1 1
	//		1 0 0 0  ===> 不符合i>=0 || wid>1, 跳出
	//		00106
	for i >= 10 || wid > 1 {
		wid--
		// 商, 如果第一次i是106, 那么就变成10
		// 填到q为0时
		q := i / 10
		// 余数, 第一次i-q*10就是6 ==> '0' + 6 ==> '6' 原封不动
		// 填到q为0时, i也为0了, 这时就按wid开始不断填'0'
		b[bp] = byte('0' + i - q*10)
		bp--
		//fmt.Println(wid,q,string(byte('0' + i - q*10)),i)
		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).
// 日志输出时日志头部的格式化
func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
	// 前缀增加, 如果前缀为空, 啥都没发生!
	*buf = append(*buf, l.prefix...)
	// 二进制或的魅力来了
	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
		// 以上表示有使用了时间格式
		// 如果有, 转换为UTC时间
		if l.flag&LUTC != 0 {
			t = t.UTC()
		}

		// 日期转换
		if l.flag&Ldate != 0 {
			year, month, day := t.Date()
			// 年份填充, 长度4位
			itoa(buf, year, 4)
			*buf = append(*buf, '/')
			itoa(buf, int(month), 2)
			*buf = append(*buf, '/')
			itoa(buf, day, 2)

			// 空一点后面接文件名或者日志消息
			*buf = append(*buf, ' ')
		}

		// 时间或微妙格式
		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)

			// 正常时间完, 是否有微秒?
			if l.flag&Lmicroseconds != 0 {
				*buf = append(*buf, '.')
				itoa(buf, t.Nanosecond()/1e3, 6)  // 纳秒转微秒, 长度6位
			}
			// 空一点后面接文件名或者日志消
			*buf = append(*buf, ' ')
		}
	}

	// 长文件名, 短文件名输出
	if l.flag&(Lshortfile|Llongfile) != 0 {
		// 短文件名那么截取最后一个, 如a/b/c ==> c
		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
		}

		// 写入缓冲
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')

		// 行数, 代码打日志所在的地方, wid为-1表示不填冲
		itoa(buf, line, -1)

		// 后面开始接消息, :再空一点!
		*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.
// 日志输出最重要的地方来了!
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()

	// 使用文件标志格式, 可能会产生bug!
	// 此篇文章: https://www.cnblogs.com/zhangym/p/6709282.html
	// 重复加锁时协程会阻塞, 直到锁被解开, 只要不让主协程阻塞, 就不会死锁! 要及时解开锁!
	if l.flag&(Lshortfile|Llongfile) != 0 {
		// Release lock while getting caller info - it's expensive.
		// 先将锁释放, 方便其他协程能更快获取到调用的文件路径. 文件调用获取路径是一个昂贵的过程
		// 在这个解锁期间, 其他协程中的一个又会进来Output进行加锁, 加锁的这一个瞬间, 如果获取路径过程结束了, 之前的协程会再次加锁, 两次加锁会报错!
		// may BE BUG
		l.mu.Unlock()
		var ok bool
		// 0 表示获取自己的路径
		// 1 表示获取上一层调用的路径
		// 越往上那么逐次加一

		// 在此, 直接调用Output, 那么calldepth为1时可以知道是谁调用了Output, 但是Output被Print等又封装了一层, 所以是2!
		_, file, line, ok = runtime.Caller(calldepth)  // 昂贵!
		if !ok {
			file = "???"
			line = 0
		}

		// 路径获取完后, 在加锁之后, 其他协程可能也进入开头加锁! Maybe bug
		l.mu.Lock()
		// 下面这个测试, 可以Lock两次? 只要保证主进程不阻塞即可?
		//go func(){
		//	time.Sleep(time.Duration(2)*time.Second)
		//	l.mu.Unlock()   // 最终还是会解开锁, Golang的智能检测会发现死锁? 如果主协程在一定时间内还没有响应!
		//}()
		//l.mu.Lock()  // 加锁加不成功难度会阻塞?
	}

	// 以下这一段完全隔离的
	l.buf = l.buf[:0]  // 清空上次缓冲!

	// 格式化
	l.formatHeader(&l.buf, now, file, line)
	l.buf = append(l.buf, s...)

	// 加换行符号
	if len(s) == 0 || s[len(s)-1] != '\n' {
		l.buf = append(l.buf, '\n')
	}
	_, err := l.out.Write(l.buf)
	return err
}



// 下面的都是采用Output包装而来
// 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)
}

// 以下都是原子更新
// 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
}

// 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)
}

// 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.

// 默认的让你再包装一层的函数...
func Output(calldepth int, s string) error {
	return std.Output(calldepth+1, s) // +1 for this frame.
}