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
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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
}
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2.1 为下载器提供初始化方法
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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
}
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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。
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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()
}
}
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3.1 下载文件分段
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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
}
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3.2 子线程下载函数
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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)
}
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4. 保存下载文件函数
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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)
}
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5.完整代码
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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()
}
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原文链接：https://www.cnblogs.com/qxcheng/p/15378472.html