Spring Boot实现大文件分片下载

关于文件的上传和下载前面已经讲了2节课，今天我们主要讲一下如何分片下载，历史文章详解下面链接

1.分片下载的好处

使用分片下载：将大文件分割成多个小块进行下载，可以降低内存占用和网络传输中断的风险。这样可以避免一次性下载整个大文件造成的性能问题。
断点续传：实现断点续传功能，即在下载中途中断后，可以从已下载的部分继续下载，而不需要重新下载整个文件。
进度条显示：在页面上展示下载进度，让用户清晰地看到文件下载的进度。如果一次全部下载可以从process中直接拿到参数计算得出（很精细），如果是分片下载，也是计算已下载的和总大小，只不过已下载的会成片成片的增加（不是很精细）。

分片下载原理

其实和大文件的分片上传原理一样，就是将一个大的文件对象进行切片，然后并发下载分片，最后再进行组装。只是下载文件需要服务端进行文件的分片，客户端（浏览器）进行文件的组装合并。

HTTP 范围请求

分片下载文件需要用到一个 HTTP 范围请求：

HTTP 协议范围请求允许服务器只发送 HTTP 消息的一部分到客户端。

范围请求在传送大的媒体文件，或者与文件下载的断点续传功能搭配使用时非常有用。如果在响应中存在 Accept-Ranges 首部（并且它的值不为 "none"），那么表示该服务器支持范围请求。在一个 Range 首部中，可以一次性请求多个部分，服务器会以 multipart 文件的形式将其返回。如果服务器返回的是范围响应，需要使用 206 Partial Content 状态码。假如所请求的范围不合法，那么服务器会返回 416 Range Not Satisfiable 状态码，表示客户端错误。服务器允许忽略 Range 首部，从而返回整个文件，状态码用 200 。

Range 请求语法

Range: <unit>=<range-start>-
Range: <unit>=<range-start>-<range-end>
Range: <unit>=<range-start>-<range-end>, <range-start>-<range-end>
Range: <unit>=<range-start>-<range-end>, <range-start>-<range-end>, <range-start>-<range-end>

unit：范围请求所采用的单位，通常是字节（bytes）。
<range-start>：一个整数，表示在特定单位下，范围的起始值。
<range-end>：一个整数，表示在特定单位下，范围的结束值。这个值是可选的，如果不存在，表示此范围一直延伸到文档结束。

2.代码工程

实验目的

利用http range实现分片下载

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>springboot-demo</artifactId>
        <groupId>com.et</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>file</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpmime</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-core</artifactId>
            <version>5.8.15</version>
        </dependency>
        <dependency>
            <groupId>commons-fileupload</groupId>
            <artifactId>commons-fileupload</artifactId>
            <version>1.3.1</version>
        </dependency>
        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>
    </dependencies>
</project>

分片下载

package com.et.controller;

import com.et.bean.DownLoadFileInfo;
import org.apache.commons.io.FileUtils;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.HttpClients;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLDecoder;
import java.net.URLEncoder;
import java.util.concurrent.*;


@RestController
public class DownloadController {
    private static final Logger LOGGER = LoggerFactory.getLogger(DownloadController.class);
    private final static long PER_PAGE = 1024L * 1024L * 50L;
    private final static String DOWN_PATH = "D:\\tmp";
    private static final String UTF8 = "UTF-8";
    ExecutorService taskExecutor = Executors.newFixedThreadPool(10);
    @RequestMapping("/download")
    public void downLoadFile(HttpServletRequest request, HttpServletResponse response) throws IOException {
        File file = new File("D:\\SoftWare\\oss-browser-win32-ia32.zip");
        response.setCharacterEncoding(UTF8);
        InputStream is = null;
        OutputStream os = null;
        try {
            // chunk download Range  bytes=100-1000  100-
            long fSize = file.length();
            response.setContentType("application/x-download");
            String fileName = URLEncoder.encode(file.getName(), UTF8);
            response.addHeader("Content-Disposition", "attachment;filename=" + fileName);
            // Accept-Range
            response.setHeader("Accept-Range", "bytes");
            response.setHeader("fSize", String.valueOf(fSize));
            response.setHeader("fName", fileName);

            long pos = 0, last = fSize - 1, sum = 0;
            if (null != request.getHeader("Range")) {
                response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);
                String numberRange = request.getHeader("Range").replaceAll("bytes=", "");
                String[] strRange = numberRange.split("-");
                if (strRange.length == 2) {
                    pos = Long.parseLong(strRange[0].trim());
                    last = Long.parseLong(strRange[1].trim());
                    if (last > fSize-1) {
                        last = fSize - 1;
                    }
                } else {
                    pos = Long.parseLong(numberRange.replaceAll("-", "").trim());
                }
            }
            long rangeLength = last - pos + 1;
            String contentRange = new StringBuffer("bytes").append(pos).append("-").append(last).append("/").append(fSize).toString();
            response.setHeader("Content-Range", contentRange);
            response.setHeader("Content-Length", String.valueOf(rangeLength));

            os = new BufferedOutputStream(response.getOutputStream());
            is = new BufferedInputStream(new FileInputStream(file));
            is.skip(pos);
            byte[] buffer = new byte[1024];
            int length = 0;
            while (sum < rangeLength) {
                int readLength = (int) (rangeLength - sum);
                length = is.read(buffer, 0, (rangeLength - sum) <= buffer.length ? readLength : buffer.length);
                sum += length;
                os.write(buffer,0, length);
            }
            System.out.println("download finish");
        }finally {
            if (is != null){
                is.close();
            }
            if (os != null){
                os.close();
            }
        }
    }

    @RequestMapping("/downloadFile")
    public String downloadFile() {

        DownLoadFileInfo fileInfo = download(0, 10, -1, null);
        if (fileInfo != null) {
            long pages =  fileInfo.fSize / PER_PAGE;
            for (long i = 0; i <= pages; i++) {
                Future<DownLoadFileInfo> future = taskExecutor.submit(new DownloadThread(i * PER_PAGE, (i + 1) * PER_PAGE - 1, i, fileInfo.fName));
                if (!future.isCancelled()) {
                    try {
                        fileInfo = future.get();
                    } catch (InterruptedException | ExecutionException e) {
                        e.printStackTrace();
                    }
                }
            }
            return System.getProperty("user.home") + "\\Downloads\\" + fileInfo.fName;
        }
        return null;
    }


    private DownLoadFileInfo download(long start, long end, long page, String fName) {
        File dir = new File(DOWN_PATH);
        if (!dir.exists()) {
            dir.mkdirs();
        }

        File file = new File(DOWN_PATH, page + "-" + fName);
        if (file.exists() && page != -1 && file.length() == PER_PAGE) {
            return null;
        }
        try {
            HttpClient client = HttpClients.createDefault();
            HttpGet httpGet = new HttpGet("http://127.0.0.1:8080/download");
            httpGet.setHeader("Range", "bytes=" + start + "-" + end);
            HttpResponse response = client.execute(httpGet);
            String fSize = response.getFirstHeader("fSize").getValue();
            fName = URLDecoder.decode(response.getFirstHeader("fName").getValue(), "UTF-8");
            HttpEntity entity = response.getEntity();
            InputStream is = entity.getContent();
            FileOutputStream fos = new FileOutputStream(file);
            byte[] buffer = new byte[1024];
            int ch;
            while ((ch = is.read(buffer)) != -1) {
                fos.write(buffer, 0, ch);
            }
            is.close();
            fos.flush();
            fos.close();
            // last part
            if (end - Long.parseLong(fSize) > 0) {
                // merge file
                mergeFile(fName, page);
            }

            return new DownLoadFileInfo(Long.parseLong(fSize), fName);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    private void mergeFile(String fName, long page) {
        File file = new File(DOWN_PATH, fName);
        try {
            BufferedOutputStream os = new BufferedOutputStream(new FileOutputStream(file));
            for (long i = 0; i <= page; i++) {
                File tempFile = new File(DOWN_PATH, i + "-" + fName);
                while (!file.exists() || (i != page && tempFile.length() < PER_PAGE)) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                byte[] bytes = FileUtils.readFileToByteArray(tempFile);
                os.write(bytes);
                os.flush();
                tempFile.delete();
            }
            File testFile = new File(DOWN_PATH, -1 + "-null");
            testFile.delete();
            os.flush();
            os.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * get file size
     */
    private long getRemoteFileSize(String remoteFileUrl) throws IOException {
        long fileSize = 0;
        HttpURLConnection httpConnection = (HttpURLConnection) new URL(remoteFileUrl).openConnection();
        //使用HEAD方法
        httpConnection.setRequestMethod("HEAD");
        int responseCode = httpConnection.getResponseCode();
        if (responseCode >= 400) {
            LOGGER.debug("Web responsible fail!");
            return 0;
        }
        String sHeader;
        for (int i = 1;; i++) {
            sHeader = httpConnection.getHeaderFieldKey(i);
            if (sHeader != null && sHeader.equals("Content-Length")) {
                LOGGER.debug("file size ContentLength:" + httpConnection.getContentLength());
                fileSize = Long.parseLong(httpConnection.getHeaderField(sHeader));
                break;
            }
        }
        return fileSize;
    }

    class DownloadThread implements Callable<DownLoadFileInfo> {
        long start;
        long end;
        long page;
        String fName;

        public DownloadThread(long start, long end, long page, String fName) {
            this.start = start;
            this.end = end;
            this.page = page;
            this.fName = fName;
        }

        @Override
        public DownLoadFileInfo call() {
            return download(start, end, page, fName);
        }
    }
}

以上只是一些关键代码，所有代码请参见下面代码仓库

代码仓库

GitHub - Harries/springboot-demo: a simple springboot demo with some components for example: redis,solr,rockmq and so on.（File）

3.测试

启动Spring Boot应用
访问http://127.0.0.1:8080/downloadFile
查看日志，发现分为3块下载

download finish
download finish
download finish