3.实现下载综合调度逻辑

本篇文章我们用Go实现一个简单的多线程下载器。

1.多线程下载原理

通过判断下载文件链接返回头信息中的 Accept-Ranges 字段，如果为 bytes 则表示支持断点续传。

然后在请求头中设置 Range 字段为 bytes=[start]-[end]，以请求下载文件的分段部分，然后将所有分段合并为一个完整文件。

2.构造一个下载器

type HttpDownloader struct { 
    url string
    filename string
    contentLength int    
    acceptRanges bool     // 是否支持断点续传
    numThreads int        // 同时下载线程数
} 

2.1 为下载器提供初始化方法

func New(url string, numThreads int) *HttpDownloader {
    var urlSplits []string = strings.Split(url, "/")
    var filename string = urlSplits[len(urlSplits)-1]

    res, err := http.Head(url)
    check(err)

    httpDownload := new(HttpDownloader)
    httpDownload.url = url
    httpDownload.contentLength = int(res.ContentLength)
    httpDownload.numThreads = numThreads
    httpDownload.filename = filename

    if len(res.Header["Accept-Ranges"]) != 0 && res.Header["Accept-Ranges"][0] == "bytes" {
        httpDownload.acceptRanges = true
    } else {
        httpDownload.acceptRanges = false
    }
    
    return httpDownload
}

3.实现下载综合调度逻辑

如果不支持多线程下载，就使用单线程下载。

func (h *HttpDownloader) Download() {
    f, err := os.Create(h.filename)
    check(err)
    defer f.Close()

    if h.acceptRanges == false {
        fmt.Println("该文件不支持多线程下载，单线程下载中：")
        resp, err := http.Get(h.url)
        check(err)
        save2file(h.filename, 0, resp)
    } else {
        var wg sync.WaitGroup
        for _, ranges := range h.Split() {
            fmt.Printf("多线程下载中:%d-%d\n", ranges[0], ranges[1])
            wg.Add(1)
            go func(start, end int) {
                defer wg.Done()
                h.download(start, end)
            }(ranges[0], ranges[1])
        }
        wg.Wait()
    }
}

3.1 下载文件分段

func (h *HttpDownloader) Split() [][]int {
    ranges := [][]int{}
    blockSize := h.contentLength / h.numThreads
    for i:=0; i<h.numThreads; i++ {
        var start int = i * blockSize
        var end int = (i + 1) * blockSize - 1
        if i == h.numThreads - 1 {
            end = h.contentLength - 1
        }
        ranges = append(ranges, []int{start, end})
    }
    return ranges
}

3.2 子线程下载函数

func (h *HttpDownloader) download(start, end int) {
    req, err := http.NewRequest("GET", h.url, nil)
    check(err)
    req.Header.Set("Range", fmt.Sprintf("bytes=%v-%v", start, end))
    req.Header.Set("User-Agent", userAgent)
    
    resp, err := http.DefaultClient.Do(req)
    check(err)
    defer resp.Body.Close() 

    save2file(h.filename, int64(start), resp)
}

4. 保存下载文件函数

func save2file(filename string, offset int64, resp *http.Response) {
    f, err := os.OpenFile(filename, os.O_WRONLY, 0660)
    check(err)
    f.Seek(offset, 0)
    defer f.Close()

    content, err := ioutil.ReadAll(resp.Body)
    check(err)  
    f.Write(content)
}

5.完整代码

package main

import (
    "fmt"
    "strings"
    "log"
    "os"
    "net/http"
    "sync"
    "io/ioutil"
)

const (
    userAgent = `Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36`
)

type HttpDownloader struct { 
    url string
    filename string
    contentLength int    
    acceptRanges bool     // 是否支持断点续传
    numThreads int        // 同时下载线程数
} 

func check(e error) {
    if e != nil {
        log.Println(e)
        panic(e)
    }
}           

func New(url string, numThreads int) *HttpDownloader {
    var urlSplits []string = strings.Split(url, "/")
    var filename string = urlSplits[len(urlSplits)-1]

    res, err := http.Head(url)
    check(err)

    httpDownload := new(HttpDownloader)
    httpDownload.url = url
    httpDownload.contentLength = int(res.ContentLength)
    httpDownload.numThreads = numThreads
    httpDownload.filename = filename

    if len(res.Header["Accept-Ranges"]) != 0 && res.Header["Accept-Ranges"][0] == "bytes" {
        httpDownload.acceptRanges = true
    } else {
        httpDownload.acceptRanges = false
    }
    
    return httpDownload
}

// 下载综合调度
func (h *HttpDownloader) Download() {
    f, err := os.Create(h.filename)
    check(err)
    defer f.Close()

    if h.acceptRanges == false {
        fmt.Println("该文件不支持多线程下载，单线程下载中：")
        resp, err := http.Get(h.url)
        check(err)
        save2file(h.filename, 0, resp)
    } else {
        var wg sync.WaitGroup
        for _, ranges := range h.Split() {
            fmt.Printf("多线程下载中:%d-%d\n", ranges[0], ranges[1])
            wg.Add(1)
            go func(start, end int) {
                defer wg.Done()
                h.download(start, end)
            }(ranges[0], ranges[1])
        }
        wg.Wait()
    }
}

// 下载文件分段
func (h *HttpDownloader) Split() [][]int {
    ranges := [][]int{}
    blockSize := h.contentLength / h.numThreads
    for i:=0; i<h.numThreads; i++ {
        var start int = i * blockSize
        var end int = (i + 1) * blockSize - 1
        if i == h.numThreads - 1 {
            end = h.contentLength - 1
        }
        ranges = append(ranges, []int{start, end})
    }
    return ranges
}

// 多线程下载
func (h *HttpDownloader) download(start, end int) {
    req, err := http.NewRequest("GET", h.url, nil)
    check(err)
    req.Header.Set("Range", fmt.Sprintf("bytes=%v-%v", start, end))
    req.Header.Set("User-Agent", userAgent)
    
    resp, err := http.DefaultClient.Do(req)
    check(err)
    defer resp.Body.Close() 

    save2file(h.filename, int64(start), resp)
}

// 保存文件
func save2file(filename string, offset int64, resp *http.Response) {
    f, err := os.OpenFile(filename, os.O_WRONLY, 0660)
    check(err)
    f.Seek(offset, 0)
    defer f.Close()

    content, err := ioutil.ReadAll(resp.Body)
    check(err)  
    f.Write(content)
}


func main() {
    var url string = "https://dl.softmgr.qq.com/original/im/QQ9.5.0.27852.exe"
    
    httpDownload := New(url, 4)
    fmt.Printf("Bool:%v\nContent:%d\n", httpDownload.acceptRanges, httpDownload.contentLength)

    httpDownload.Download()
}