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

• Spring Boot实现文件上传和下载
• Spring Boot实现大文件分块上传

1.分片下载的好处

1. 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。
2. 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。
3. 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-Rangeresponse.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 partif (end - Long.parseLong(fSize) > 0) {// merge filemergeFile(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;}@Overridepublic 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

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

download finish
download finish
download finish

4.引用

• HTTP Range Request explained
• Spring Boot实现大文件分片下载 | Harries Blog™