cron是什么

cron的意思就是：计划任务，说白了就是定时任务。我和系统约个时间，你在几点几分几秒或者每隔几分钟跑一个任务(job)，就那么简单。

cron表达式　　

cron表达式是一个好东西，这个东西不仅Java的quartZ能用到，Go语言中也可以用到。我没有用过Linux的cron，但网上说Linux也是可以用crontab -e 命令来配置定时任务。Go语言和Java中都是可以精确到秒的，但是Linux中不行。

cron表达式代表一个时间的集合，使用6个空格分隔的字段表示：

字段名 是否必须 允许的值　 允许的特定字符 秒(Seconds) 是 0-59 * / , - 分(Minute) 是 0-59 * / , - 时(Hours) 是 0-23 * / , - 日(Day of month) 是 1-31 * / , - "htmlcode">

 # ┌───────────── min (0 - 59)
 # │ ┌────────────── hour (0 - 23)
 # │ │ ┌─────────────── day of month (1 - 31)
 # │ │ │ ┌──────────────── month (1 - 12)
 # │ │ │ │ ┌───────────────── day of week (0 - 6) (0 to 6 are Sunday to
 # │ │ │ │ │         Saturday, or use names; 7 is also Sunday)
 # │ │ │ │ │
 # │ │ │ │ │
 # * * * * * command to execute

cron特定字符说明

1）星号(*)

表示 cron 表达式能匹配该字段的所有值。如在第5个字段使用星号(month)，表示每个月

2）斜线(/)

表示增长间隔，如第1个字段(minutes) 值是 3-59/15，表示每小时的第3分钟开始执行一次，之后每隔 15 分钟执行一次（即 3、18、33、48 这些时间点执行），这里也可以表示为：3/15

3）逗号(,)

用于枚举值，如第6个字段值是 MON,WED,FRI，表示 星期一、三、五 执行

4）连字号(-)

表示一个范围，如第3个字段的值为 9-17 表示 9am 到 5pm 直接每个小时（包括9和17）

5）问号("htmlcode">

constantdelay.go   #一个最简单的秒级别定时系统。与cron无关
constantdelay_test.go #测试
cron.go        #Cron系统。管理一系列的cron定时任务（Schedule Job）
cron_test.go     #测试
doc.go        #说明文档
LICENSE        #授权书 
parser.go       #解析器，解析cron格式字符串城一个具体的定时器（Schedule）
parser_test.go    #测试
README.md       #README
spec.go        #单个定时器（Schedule）结构体。如何计算自己的下一次触发时间
spec_test.go     #测试

cron.go

结构体：

// Cron keeps track of any number of entries, invoking the associated func as
// specified by the schedule. It may be started, stopped, and the entries may
// be inspected while running. 
// Cron保持任意数量的条目的轨道，调用相关的func时间表指定。它可以被启动，停止和条目，可运行的同时进行检查。
type Cron struct {
  entries []*Entry  　　　　// 任务
  stop   chan struct{}   // 叫停止的途径
  add   chan *Entry    // 添加新任务的方式
  snapshot chan []*Entry   // 请求获取任务快照的方式
  running bool        // 是否在运行
  ErrorLog *log.Logger    // 出错日志(新增属性)
  location *time.Location   // 所在地区(新增属性)    
}

// Entry consists of a schedule and the func to execute on that schedule.
// 入口包括时间表和可在时间表上执行的func
type Entry struct {
    // 计时器
  Schedule Schedule
  // 下次执行时间
  Next time.Time
  // 上次执行时间
  Prev time.Time
  // 任务
  Job Job
}

关键方法：

// 开始任务
// Start the cron scheduler in its own go-routine, or no-op if already started.
func (c *Cron) Start() {
  if c.running {
    return
  }
  c.running = true
  go c.run()
}
// 结束任务
// Stop stops the cron scheduler if it is running; otherwise it does nothing.
func (c *Cron) Stop() {
  if !c.running {
    return
  }
  c.stop <- struct{}{}
  c.running = false
}

// 执行定时任务
// Run the scheduler.. this is private just due to the need to synchronize
// access to the 'running' state variable.
func (c *Cron) run() {
  // Figure out the next activation times for each entry.
  now := time.Now().In(c.location)
  for _, entry := range c.entries {
    entry.Next = entry.Schedule.Next(now)
  }
    // 无限循环
  for {
      //通过对下一个执行时间进行排序，判断那些任务是下一次被执行的，防在队列的前面.sort是用来做排序的
    sort.Sort(byTime(c.entries))

    var effective time.Time
    if len(c.entries) == 0 || c.entries[0].Next.IsZero() {
      // If there are no entries yet, just sleep - it still handles new entries
      // and stop requests.
      effective = now.AddDate(10, 0, 0)
    } else {
      effective = c.entries[0].Next
    }

    timer := time.NewTimer(effective.Sub(now))
    select {
    case now = <-timer.C: // 执行当前任务
      now = now.In(c.location)
      // Run every entry whose next time was this effective time.
      for _, e := range c.entries {
        if e.Next != effective {
          break
        }
        go c.runWithRecovery(e.Job)
        e.Prev = e.Next
        e.Next = e.Schedule.Next(now)
      }
      continue

    case newEntry := <-c.add: // 添加新的任务
      c.entries = append(c.entries, newEntry)
      newEntry.Next = newEntry.Schedule.Next(time.Now().In(c.location))

    case <-c.snapshot: // 获取快照
      c.snapshot <- c.entrySnapshot()

    case <-c.stop:  // 停止任务
      timer.Stop()
      return
    }

    // 'now' should be updated after newEntry and snapshot cases.
    now = time.Now().In(c.location)
    timer.Stop()
  }
}

spec.go

结构体及关键方法：

// SpecSchedule specifies a duty cycle (to the second granularity), based on a
// traditional crontab specification. It is computed initially and stored as bit sets.
type SpecSchedule struct {
  // 表达式中锁表明的，秒，分，时，日，月，周，每个都是uint64
  // Dom:Day of Month,Dow:Day of week
  Second, Minute, Hour, Dom, Month, Dow uint64
}

// bounds provides a range of acceptable values (plus a map of name to value).
// 定义了表达式的结构体
type bounds struct {
  min, max uint
  names  map[string]uint
}


// The bounds for each field.
// 这样就能看出各个表达式的范围
var (
    seconds = bounds{0, 59, nil}
    minutes = bounds{0, 59, nil}
    hours  = bounds{0, 23, nil}
    dom   = bounds{1, 31, nil}
    months = bounds{1, 12, map[string]uint{
       "jan": 1,
       "feb": 2,
       "mar": 3,
       "apr": 4,
       "may": 5,
       "jun": 6,
       "jul": 7,
       "aug": 8,
       "sep": 9,
       "oct": 10,
       "nov": 11,
       "dec": 12,
    }}
    dow = bounds{0, 6, map[string]uint{
       "sun": 0,
       "mon": 1,
       "tue": 2,
       "wed": 3,
       "thu": 4,
       "fri": 5,
       "sat": 6,
    }}
)

const (
    // Set the top bit if a star was included in the expression.
    starBit = 1 << 63
)

看了上面的东西肯定有人疑惑为什么秒分时这些都是定义了unit64,以及定义了一个常量starBit = 1 << 63这种写法，这是逻辑运算符。表示二进制1向左移动63位。原因如下：

cron表达式是用来表示一系列时间的，而时间是无法逃脱自己的区间的 ， 分，秒 0 - 59 ， 时 0 - 23 ， 天/月 0 - 31 ， 天/周 0 - 6 ， 月0 - 11 。 这些本质上都是一个点集合，或者说是一个整数区间。 那么对于任意的整数区间 ， 可以描述cron的如下部分规则。

1. * | "htmlcode">

   /* 
     ------------------------------------------------------------
     第64位标记任意 ， 用于 日/周 ， 日 / 月 的相互干扰。
     63 - 0 为 表示区间 [63 , 0] 的 每一个点。
     ------------------------------------------------------------
   
     假设区间是 0 - 63 ， 则有如下的例子 ：
   
     比如 0/3 的表示如下 ： (表示每隔两位为1)
     * / "htmlcode">
   
   package cron
   
   import (
     "fmt"
     "math"
     "strconv"
     "strings"
     "time"
   )
   
   // Configuration options for creating a parser. Most options specify which
   // fields should be included, while others enable features. If a field is not
   // included the parser will assume a default value. These options do not change
   // the order fields are parse in.
   type ParseOption int
   
   const (
     Second   ParseOption = 1 << iota // Seconds field, default 0
     Minute               // Minutes field, default 0
     Hour                // Hours field, default 0
     Dom                 // Day of month field, default *
     Month                // Month field, default *
     Dow                 // Day of week field, default *
     DowOptional             // Optional day of week field, default *
     Descriptor             // Allow descriptors such as @monthly, @weekly, etc.
   )
   
   var places = []ParseOption{
     Second,
     Minute,
     Hour,
     Dom,
     Month,
     Dow,
   }
   
   var defaults = []string{
     "0",
     "0",
     "0",
     "*",
     "*",
     "*",
   }
   
   // A custom Parser that can be configured.
   type Parser struct {
     options  ParseOption
     optionals int
   }
   
   // Creates a custom Parser with custom options.
   //
   // // Standard parser without descriptors
   // specParser := NewParser(Minute | Hour | Dom | Month | Dow)
   // sched, err := specParser.Parse("0 0 15 */3 *")
   //
   // // Same as above, just excludes time fields
   // subsParser := NewParser(Dom | Month | Dow)
   // sched, err := specParser.Parse("15 */3 *")
   //
   // // Same as above, just makes Dow optional
   // subsParser := NewParser(Dom | Month | DowOptional)
   // sched, err := specParser.Parse("15 */3")
   //
   func NewParser(options ParseOption) Parser {
     optionals := 0
     if options&DowOptional > 0 {
       options |= Dow
       optionals++
     }
     return Parser{options, optionals}
   }
   
   // Parse returns a new crontab schedule representing the given spec.
   // It returns a descriptive error if the spec is not valid.
   // It accepts crontab specs and features configured by NewParser.
   // 将字符串解析成为SpecSchedule 。 SpecSchedule符合Schedule接口
   
   func (p Parser) Parse(spec string) (Schedule, error) {
   　　// 直接处理特殊的特殊的字符串
     if spec[0] == '@' && p.options&Descriptor > 0 {
       return parseDescriptor(spec)
     }
   
     // Figure out how many fields we need
     max := 0
     for _, place := range places {
       if p.options&place > 0 {
         max++
       }
     }
     min := max - p.optionals
   
     // cron利用空白拆解出独立的items。
     fields := strings.Fields(spec)
   
     // 验证表达式取值范围
     if count := len(fields); count < min || count > max {
       if min == max {
         return nil, fmt.Errorf("Expected exactly %d fields, found %d: %s", min, count, spec)
       }
       return nil, fmt.Errorf("Expected %d to %d fields, found %d: %s", min, max, count, spec)
     }
   
     // Fill in missing fields
     fields = expandFields(fields, p.options)
   
     var err error
     field := func(field string, r bounds) uint64 {
       if err != nil {
         return 0
       }
       var bits uint64
       bits, err = getField(field, r)
       return bits
     }
   
     var (
       second   = field(fields[0], seconds)
       minute   = field(fields[1], minutes)
       hour    = field(fields[2], hours)
       dayofmonth = field(fields[3], dom)
       month   = field(fields[4], months)
       dayofweek = field(fields[5], dow)
     )
     if err != nil {
       return nil, err
     }
     // 返回所需要的SpecSchedule
     return &SpecSchedule{
       Second: second,
       Minute: minute,
       Hour:  hour,
       Dom:  dayofmonth,
       Month: month,
       Dow:  dayofweek,
     }, nil
   }
   
   func expandFields(fields []string, options ParseOption) []string {
     n := 0
     count := len(fields)
     expFields := make([]string, len(places))
     copy(expFields, defaults)
     for i, place := range places {
       if options&place > 0 {
         expFields[i] = fields[n]
         n++
       }
       if n == count {
         break
       }
     }
     return expFields
   }
   
   var standardParser = NewParser(
     Minute | Hour | Dom | Month | Dow | Descriptor,
   )
   
   // ParseStandard returns a new crontab schedule representing the given standardSpec
   // (https://en.wikipedia.org/wiki/Cron). It differs from Parse requiring to always
   // pass 5 entries representing: minute, hour, day of month, month and day of week,
   // in that order. It returns a descriptive error if the spec is not valid.
   //
   // It accepts
   //  - Standard crontab specs, e.g. "* * * * "
   //  - Descriptors, e.g. "@midnight", "@every 1h30m"
   // 这里表示不仅可以使用cron表达式，也可以使用@midnight @every等方法
   
   func ParseStandard(standardSpec string) (Schedule, error) {
     return standardParser.Parse(standardSpec)
   }
   
   var defaultParser = NewParser(
     Second | Minute | Hour | Dom | Month | DowOptional | Descriptor,
   )
   
   // Parse returns a new crontab schedule representing the given spec.
   // It returns a descriptive error if the spec is not valid.
   //
   // It accepts
   //  - Full crontab specs, e.g. "* * * * * "
   //  - Descriptors, e.g. "@midnight", "@every 1h30m"
   func Parse(spec string) (Schedule, error) {
     return defaultParser.Parse(spec)
   }
   
   // getField returns an Int with the bits set representing all of the times that
   // the field represents or error parsing field value. A "field" is a comma-separated
   // list of "ranges".
   func getField(field string, r bounds) (uint64, error) {
     var bits uint64
     ranges := strings.FieldsFunc(field, func(r rune) bool { return r == ',' })
     for _, expr := range ranges {
       bit, err := getRange(expr, r)
       if err != nil {
         return bits, err
       }
       bits |= bit
     }
     return bits, nil
   }
   
   // getRange returns the bits indicated by the given expression:
   //  number | number "-" number [ "/" number ]
   // or error parsing range.
   func getRange(expr string, r bounds) (uint64, error) {
     var (
       start, end, step uint
       rangeAndStep   = strings.Split(expr, "/")
       lowAndHigh    = strings.Split(rangeAndStep[0], "-")
       singleDigit   = len(lowAndHigh) == 1
       err       error
     )
   
     var extra uint64
     if lowAndHigh[0] == "*" || lowAndHigh[0] == "" {
       start = r.min
       end = r.max
       extra = starBit
     } else {
       start, err = parseIntOrName(lowAndHigh[0], r.names)
       if err != nil {
         return 0, err
       }
       switch len(lowAndHigh) {
       case 1:
         end = start
       case 2:
         end, err = parseIntOrName(lowAndHigh[1], r.names)
         if err != nil {
           return 0, err
         }
       default:
         return 0, fmt.Errorf("Too many hyphens: %s", expr)
       }
     }
   
     switch len(rangeAndStep) {
     case 1:
       step = 1
     case 2:
       step, err = mustParseInt(rangeAndStep[1])
       if err != nil {
         return 0, err
       }
   
       // Special handling: "N/step" means "N-max/step".
       if singleDigit {
         end = r.max
       }
     default:
       return 0, fmt.Errorf("Too many slashes: %s", expr)
     }
   
     if start < r.min {
       return 0, fmt.Errorf("Beginning of range (%d) below minimum (%d): %s", start, r.min, expr)
     }
     if end > r.max {
       return 0, fmt.Errorf("End of range (%d) above maximum (%d): %s", end, r.max, expr)
     }
     if start > end {
       return 0, fmt.Errorf("Beginning of range (%d) beyond end of range (%d): %s", start, end, expr)
     }
     if step == 0 {
       return 0, fmt.Errorf("Step of range should be a positive number: %s", expr)
     }
   
     return getBits(start, end, step) | extra, nil
   }
   
   // parseIntOrName returns the (possibly-named) integer contained in expr.
   func parseIntOrName(expr string, names map[string]uint) (uint, error) {
     if names != nil {
       if namedInt, ok := names[strings.ToLower(expr)]; ok {
         return namedInt, nil
       }
     }
     return mustParseInt(expr)
   }
   
   // mustParseInt parses the given expression as an int or returns an error.
   func mustParseInt(expr string) (uint, error) {
     num, err := strconv.Atoi(expr)
     if err != nil {
       return 0, fmt.Errorf("Failed to parse int from %s: %s", expr, err)
     }
     if num < 0 {
       return 0, fmt.Errorf("Negative number (%d) not allowed: %s", num, expr)
     }
   
     return uint(num), nil
   }
   
   // getBits sets all bits in the range [min, max], modulo the given step size.
   func getBits(min, max, step uint) uint64 {
     var bits uint64
   
     // If step is 1, use shifts.
     if step == 1 {
       return ^(math.MaxUint64 << (max + 1)) & (math.MaxUint64 << min)
     }
   
     // Else, use a simple loop.
     for i := min; i <= max; i += step {
       bits |= 1 << i
     }
     return bits
   }
   
   // all returns all bits within the given bounds. (plus the star bit)
   func all(r bounds) uint64 {
     return getBits(r.min, r.max, 1) | starBit
   }
   
   // parseDescriptor returns a predefined schedule for the expression, or error if none matches.
   func parseDescriptor(descriptor string) (Schedule, error) {
     switch descriptor {
     case "@yearly", "@annually":
       return &SpecSchedule{
         Second: 1 << seconds.min,
         Minute: 1 << minutes.min,
         Hour:  1 << hours.min,
         Dom:  1 << dom.min,
         Month: 1 << months.min,
         Dow:  all(dow),
       }, nil
   
     case "@monthly":
       return &SpecSchedule{
         Second: 1 << seconds.min,
         Minute: 1 << minutes.min,
         Hour:  1 << hours.min,
         Dom:  1 << dom.min,
         Month: all(months),
         Dow:  all(dow),
       }, nil
   
     case "@weekly":
       return &SpecSchedule{
         Second: 1 << seconds.min,
         Minute: 1 << minutes.min,
         Hour:  1 << hours.min,
         Dom:  all(dom),
         Month: all(months),
         Dow:  1 << dow.min,
       }, nil
   
     case "@daily", "@midnight":
       return &SpecSchedule{
         Second: 1 << seconds.min,
         Minute: 1 << minutes.min,
         Hour:  1 << hours.min,
         Dom:  all(dom),
         Month: all(months),
         Dow:  all(dow),
       }, nil
   
     case "@hourly":
       return &SpecSchedule{
         Second: 1 << seconds.min,
         Minute: 1 << minutes.min,
         Hour:  all(hours),
         Dom:  all(dom),
         Month: all(months),
         Dow:  all(dow),
       }, nil
     }
   
     const every = "@every "
     if strings.HasPrefix(descriptor, every) {
       duration, err := time.ParseDuration(descriptor[len(every):])
       if err != nil {
         return nil, fmt.Errorf("Failed to parse duration %s: %s", descriptor, err)
       }
       return Every(duration), nil
     }
   
     return nil, fmt.Errorf("Unrecognized descriptor: %s", descriptor)
   }
   
   
   
   
   项目中应用
   
   
   package main
   import (
     "github.com/robfig/cron"
     "log"
   )
   
   func main() {
     i := 0
     c := cron.New()
     spec := "*/5 * * * * "
     c.AddFunc(spec, func() {
       i++
       log.Println("cron running:", i)
     })
     c.AddFunc("@every 1h1m", func() {
       i++
       log.Println("cron running:", i)
     })
     c.Start()
   }
   
   
   
   注： @every 用法比较特殊，这是Go里面比较特色的用法。同样的还有 @yearly @annually @monthly @weekly @daily @midnight @hourly 这里面就不一一赘述了。希望大家能够自己探索。
   以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。