sync.Map,不需要用make初始化
单纯的用map开启goroutine是不安全的，如：

package main

import (
	"fmt"
	"strconv"
	"sync"
)

var m = make(map[string]int)
var lock sync.Mutex


func get(key string)int{
	return m[key]
}

func set(key string, value int){
	m[key] = value
}

func main() {
	wg := sync.WaitGroup{}
	for i := 0; i < 20; i++ {
		wg.Add(1)
		go func(n int) {
			key := strconv.Itoa(n)
			//lock.Lock()
			set(key,n)
			//lock.Unlock()
			fmt.Printf("k=:%v,v:=%v\n", key, get(key))
			wg.Done()
		}(i)
	}
	wg.Wait()
}

会发生致命错误

fatal error: concurrent map writes

所以需要用到sync.Map

var m2 = sync.Map{}
func main() {
	wg := sync.WaitGroup{}
 	for i := 0; i < 20; i++ {
 		wg.Add(1)
		go func(n int) {
			key := strconv.Itoa(n)
 			m2.Store(key,n) //必须使用sync.Map内置的Store方法设置键值对
			value,_ := m2.Load(key)//必须使用sync.Map内置的Load方法根据key值取值
			fmt.Printf("k=:%v,v:=%v\n", key, value)
 			wg.Done()
		}(i)
 	}
 	wg.Wait()
}