Java实现断点下载功能的示例代码-Java开发

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。本文将用Java语言实现断点下载，需要的可以参考一下

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();
            }
        })

    }

    /**
     *  样式可以进行修改
     * @param {*} progressId   需要将进度条添加到那个元素下面
     */
    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();
    }

    /**
     * 分段下载文件
     * @param fileName  文件名称
     * @param begin  从文件什么位置开始读取
     * @param end  到什么位置结束
     * @param last  是否是最后一次读取
     * @param response
     */
    @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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