Java实现断点下载功能的示例代码
介绍
当下载一个很大的文件时，如果下载到一半暂停，如果继续下载呢？断点下载就是解决这个问题的。
具体原理:
利用indexedDb,将下载的数据存储到用户的本地中,这样用户就算是关电脑那么下次下载还是从上次的位置开始的
- 先去看看本地缓存中是否存在这个文件的分片数据,如果存在那么就接着上一个分片继续下载(起始位置)
- 下载前先去后端拿文件的大小,然后计算分多少次下载(n/(1024*1024*10)) (结束位置)
- 每次下载的数据放入一个Blob中,然后存储到本地indexedDB
- 当全部下载完毕后,将所有本地缓存的分片全部合并,然后给用户
有很多人说必须使用content-length、Accept-Ranges、Content-Range还有Range。 但是这只是一个前后端的约定而已,所有没必须非要遵守,只要你和后端约定好怎么拿取数据就行
难点都在前端:
- 怎么存储
- 怎么计算下载多少次
- 怎么获取最后下载的分片是什么
- 怎么判断下载完成了
- 怎么保证下载的分片都是完整的
- 下载后怎么合并然后给用户
效果
前端代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta Http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<h1>HTML5大文件断点下载传</h1>
<div id="progressBar"></div>
<Button id="but">下载</Button>
<Button id="stop">暂停</Button>
<script type="module">
import FileSliceDownload from '/src/file/FileSliceDownload.js'
let downloadUrl = "http://localhost:7003/fileslice/dwnloadsFIleSlice"
let fileSizeUrl = "http://localhost:7003/fileslice/fIleSliceDownloadSize"
let fileName = "Downloads.zip"
let but = document.querySelector("#but")
let stop = document.querySelector("#stop")
let fileSliceDownload = new FileSliceDownload(downloadUrl, fileSizeUrl);
fileSliceDownload.addProgress("#progressBar")
but.addEventListener("click", function () {
fileSliceDownload.startDownload(fileName)
})
stop.addEventListener("click", function () {
fileSliceDownload.stop()
})
</script>
</body>
</html>
class BlobUtls{
// blob转文件并下载
static downloadFileByBlob(blob, fileName = "file") {
let blobUrl = window.URL.createObjectURL(blob)
let link = document.createElement('a')
link.download = fileName || 'defaultName'
link.style.display = 'none'
link.href = blobUrl
// 触发点击
document.body.appendChild(link)
link.click()
// 移除
document.body.removeChild(link)
}
}
export default BlobUtls;
//导包要从项目全路径开始,也就是最顶部
import BlobUtls from '/WEB-js/src/blob/BlobUtls.js'
//导包
class FileSliceDownload{
#m1=1024*1024*10 //1mb 每次下载多少
#db //indexedDB库对象
#downloadUrl // 下载文件的地址
#fileSizeUrl // 获取文件大小的url
#fileSiez=0 //下载的文件大小
#fileName // 下载的文件名称
#databaseName="dbDownload"; //默认库名称
#tableDadaName="tableDada" //用于存储数据的表
#tableInfoName="tableInfo" //用于存储信息的表
#fIleReadCount=0 //文件读取次数
#fIleStartReadCount=0//文件起始的位置
#barId = "bar"; //进度条id
#progressId = "progress";//进度数值ID
#percent=0 //百分比
#checkDownloadInterval=null; //检测下载是否完成定时器
#mergeInterval=null;//检测是否满足合并分片要求
#stop=false; //是否结束
//下载地址
constructor(downloadUrl,fileSizeUrl) {
this.check()
this.#downloadUrl=downloadUrl;
this.#fileSizeUrl=fileSizeUrl;
}
check(){
let indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB ;
if(!indexedDB){
alert('不支持');
}
}
//初始化
#init(fileName){
return new Promise((resolve,reject)=>{
this.#fileName=fileName;
this.#percent=0;
this.#stop=false;
const request = window.indexedDB.open(this.#databaseName, 1)
request.onupgradeneeded = (e) => {
const db = e.target.result
if (!db.objectStoreNames.contains(this.#tableDadaName)) {
db.createObjectStore(this.#tableDadaName, { keyPath: 'serial',autoIncrement:false })
db.createObjectStore(this.#tableInfoName, { keyPath: 'primary',autoIncrement:false })
}
}
request.onsuccess = e => {
this.#db = e.target.result
resolve()
}
})
}
#getFileSize(){
return new Promise((resolve,reject)=>{
let ref=this;
var xhr = new XMLHttpRequest();
//同步
xhr.open("GET", this.#fileSizeUrl+"/"+this.#fileName,false)
xhr.send()
if (xhr.readyState === 4 && xhr.status === 200) {
let ret = JSON.parse(xhr.response)
if (ret.code === 20000) {
ref.#fileSiez=ret.data
}
resolve()
}
})
}
#getTransactionDadaStore(){
let transaction = this.#db.transaction([this.#tableDadaName], 'readwrite')
let store = transaction.objectStore(this.#tableDadaName)
return store;
}
#getTransactionInfoStore(){
let transaction = this.#db.transaction([this.#tableInfoName], 'readwrite')
let store = transaction.objectStore(this.#tableInfoName)
return store;
}
#setBlob(begin,end,i,last){
return new Promise((resolve,reject)=>{
var xhr = new XMLHttpRequest();
xhr.open("GET", this.#downloadUrl+"/"+this.#fileName+"/"+begin+"/"+end+"/"+last)
xhr.responseType="blob" // 只支持异步,默认使用 text 作为默认值。
xhr.send()
xhr.onload = ()=> {
if (xhr.status === 200) {
let store= this.#getTransactionDadaStore()
let obj={serial:i,blob:xhr.response}
//添加分片到用户本地的库中
store.add(obj)
let store2= this.#getTransactionInfoStore()
//记录下载了多少个分片了
store2.put({primary:"count",count:i})
//调整进度条
let percent1= Math.ceil( (i/this.#fIleReadCount)*100)
if(this.#percent<percent1){
this.#percent=percent1;
}
this.#dynamicProgress()
resolve()
}
}
})
}
#mergeCallback(){
// 读取全部字节到blob里,处理合并
let arrayBlobs = [];
let store1 = this.#getTransactionDadaStore()
//按顺序找到全部的分片
for (let i = 0; i <this.#fIleReadCount; i++) {
let result= store1.get(IDBKeyRange.only(i))
result.onsuccess=(data)=>{
arrayBlobs.push(data.target.result.blob)
}
}
//分片合并下载
this.#mergeInterval= setInterval(()=> {
if(arrayBlobs.length===this.#fIleReadCount){
clearInterval(this.#mergeInterval);
//多个Blob进行合并
let fileBlob = new Blob(arrayBlobs);//合并后的数组转成⼀个Blob对象。
BlobUtls.downloadFileByBlob(fileBlob,this.#fileName)
//下载完毕后清除数据
this. #clear()
}
},200)
}
#clear(){
let store2 = this.#getTransactionDadaStore()
let store3 = this.#getTransactionInfoStore()
store2.clear() //清除本地全下载的数据
store3.delete("count")//记录清除
this.#fIleStartReadCount=0 //起始位置
this.#db=null;
this.#fileName=null;
this.#fileSiez=0;
this.#fIleReadCount=0 //文件读取次数
this.#fIleStartReadCount=0//文件起始的位置
}
//检测是否有分片在本地
#checkSliceDoesIsExist(){
return new Promise((resolve,reject)=>{
let store1 = this.#getTransactionInfoStore()
let result= store1.get(IDBKeyRange.only("count"))
result.onsuccess=(data)=>{
let count= data.target.result?.count
if(count){
//防止因为网络的原因导致分片损坏,所以不要最后一个分片
this.#fIleStartReadCount=count-1;
}
resolve();
}
})
}
addProgress (progressSelect) {
let bar = document.createElement("div")
bar.setAttribute("id", this.#barId);
let num = document.createElement("div")
num.setAttribute("id", this.#progressId);
num.innerText = "0%"
bar.appendChild(num);
document.querySelector(progressSelect).appendChild(bar)
}
#dynamicProgress(){
//调整进度
let bar = document.getElementById(this.#barId)
let progressEl = document.getElementById(this.#progressId)
bar.style.width = this.#percent + '%';
bar.style.backgroundColor = 'red';
progressEl.innerHTML = this.#percent + '%'
}
stop(){
this.#stop=true;
}
startDownload(fileName){
//同步代码块
;(async ()=>{
//初始化
await this.#init(fileName)
//自动调整分片,如果本地以下载了那么从上一次继续下载
await this.#checkSliceDoesIsExist()
//拿到文件的大小
await this.#getFileSize()
let begin=0; //开始读取的字节
let end=this.#m1; // 结束读取的字节
let last=false; //是否是最后一次读取
this.#fIleReadCount= Math.ceil( this.#fileSiez/this.#m1)
for (let i = this.#fIleStartReadCount; i < this.#fIleReadCount; i++) {
if(this.#stop){
return
}
begin=i*this.#m1;
end=begin+this.#m1
if(i===this.#fIleReadCount-1){
last=true;
}
//添加分片
await this.#setBlob(begin,end,i,last)
}
//定时检测存下载的分片数量是否够了
this.#checkDownloadInterval= setInterval(()=> {
let store = this.#getTransactionDadaStore()
let result = store.count()
result.onsuccess = (data) => {
if (data.target.result === this.#fIleReadCount) {
clearInterval(this.#checkDownloadInterval);
//如果分片够了那么进行合并下载
this.#mergeCallback()
}
}
},200)
})()
}
}
export default FileSliceDownload;
后端代码
package com.controller.commontools.fileDownload;
import com.application.Result;
import com.container.ArrayByteUtil;
import com.file.FileWebDownLoad;
import com.file.ReadWriteFileUtils;
import com.path.ResourceFileUtil;
import org.springframework.web.bind.annotation.*;
import javax.servlet.http.httpservletResponse;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.OutputStream;
import java.net.URLEncoder;
@RestController
@RequestMapping("/fileslice")
public class FIleSliceDownloadController {
private final String uploaddir="uploads"+ File.separator+"real"+File.separator;//实际文件目录
// 获取文件的大小
@GetMapping("/fIleSliceDownloadSize/{fileName}")
public Result getFIleSliceDownloadSize(@PathVariable String fileName){
String absoluteFilePath = ResourceFileUtil.getAbsoluteFilePathAndCreate(uploaddir)+File.separator+fileName;
File file= new File(absoluteFilePath);
if(file.exists()&&file.isFile()){
return Result.Ok(file.length(),Long.class);
}
return Result.Error();
}
@GetMapping("/dwnloadsFIleSlice/{fileName}/{begin}/{end}/{last}")
public void dwnloadsFIleSlice(@PathVariable String fileName, @PathVariable long begin, @PathVariable long end, @PathVariable boolean last, HttpServletResponse response){
String absoluteFilePath = ResourceFileUtil.getAbsoluteFilePathAndCreate(uploaddir)+File.separator+fileName;
File file= new File(absoluteFilePath);
try(OutputStream toClient = new BufferedOutputStream(response.getOutputStream())) {
long readSize = end - begin;
//读取文件的指定字节
byte[] bytes = new byte[(int)readSize];
ReadWriteFileUtils.randoMaccessFileRead(file.getAbsolutePath(),(int)begin,bytes);
if(readSize<=file.length()||last){
bytes=ArrayByteUtil.getActualBytes(bytes); //去掉多余的
}
response.setContentType("application/octet-stream");
response.addHeader("Content-Length", String.valueOf(bytes.length));
response.setHeader("Content-Disposition", "attachment;filename*=UTF-8''" + URLEncoder.encode(fileName, "UTF-8"));
toClient.write(bytes);
} catch (Exception e) {
e.printStackTrace();
}
}
}
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