使用Go并發下載圖片資源

對爬蟲已爬到的數據進行下載，響應時間與請求量成正比。

package img

import (
    "database/sql"
    "fmt"
    _ "github.com/go-sql-driver/mysql"
    "io"
    "net/http"
    "os"
    "runtime"
    "strconv"
    "sync"
    "time"
)
const (
    DataRoot     = "./tmp/" // 存放封面圖的根目錄
    TimeoutLimit = 10       // 設置超時時間
)
// 表示章節ID和封面URL的對應關系
type VolumeCover struct {
    VolumeID int
    Url      string
    Lock     sync.Mutex
    Msg      chan string
    Flag     bool    "timeout_flag"
}
// 將圖片下載并保存到本地
func SaveImage(vc *VolumeCover) {
    res, err := http.Get(vc.Url)
    defer res.Body.Close()
    if err != nil {
        vc.Msg <- (strconv.Itoa(vc.VolumeID) + " HTTP_ERROR")
    }
    // 創建文件
    dst, err := os.Create(DataRoot + strconv.Itoa(vc.VolumeID) + ".jpg")
    if err != nil {
        vc.Msg <- (strconv.Itoa(vc.VolumeID) + " OS_ERROR")
    }
    // 生成文件
    io.Copy(dst, res.Body)
    // goroutine通信
    vc.Lock.Lock()
    vc.Msg <- "in"
    vc.Flag = true
    vc.Lock.Unlock()
}
func Start(name string, password string, limit int) error {
    runtime.GOMAXPROCS(4)
    sl, err := sql.Open("mysql", name+":"+password+"@/slnovel?charset=utf8")
    defer sl.Close()
    if err != nil {
        return err
    }
    // 構造SELECT語句并檢索
    queryStr := "SELECT VolumeID, ImageUrl FROM volume "
    if limit > 0 {
        queryStr += "limit " + strconv.Itoa(limit)
    }
    rows, err := sl.Query(queryStr)
    defer rows.Close()
    if err != nil {
        return err
    }

// 構建列表
i := 0
vclist := make([]*VolumeCover, limit)
for rows.Next() {
    vc := &VolumeCover{}
    if err := rows.Scan(&vc.VolumeID, &vc.Url); err != nil {
        return err
    }
    vc.Msg = make(chan string, 1)
    //vc.To = make(chan bool, 1)
    vc.Lock = *new(sync.Mutex)
    vclist[i] = vc
    i++
}

// start goroutines
for i := 0; i < len(vclist); i++ {
    go SaveImage(vclist[i])
    go func(i int) {
        time.Sleep(TimeoutLimit * time.Second)
        if vclist[i].Flag == false {
            vclist[i].Msg <- "out"
        }
    }(i)
}

// 阻塞地獲取結果
for i := 0; i < len(vclist); i++ {
    func(c *VolumeCover) {
        select {
        case m := <-c.Msg:
            fmt.Println(m)
        }

    }(vclist[i])
}
return nil

}</pre></span>