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ava多线程实现HTTP断点续传：原理、设计与代码实现

news 来源：原创 2025/6/10 7:14:13

一、引言

在当今互联网环境下，大文件下载需求日益增长。传统单线程下载方式效率低下，且一旦下载中断，需要重新开始。断点续传技术通过将文件分块并利用多线程并行下载，显著提升了下载效率，同时支持中断后继续下载。本文将详细介绍基于HTTP协议实现断点续传的原理、设计与Java代码实现。

二、HTTP断点续传原理

HTTP协议通过Range请求头支持断点续传，格式如下：

Range: bytes=start-end

其中：

start：起始字节位置（从0开始）
end：结束字节位置（可选，省略表示到文件末尾）

服务器响应状态码为206 Partial Content，并在响应头中包含Content-Range字段，指示实际返回的字节范围。

三、系统设计

1. 架构设计

├── DownloadManager (下载管理器)
├── DownloadTask (下载任务)
├── FileManager (文件管理器)
├── DownloadInfo (下载信息)
└── Main (主程序)

2. 核心模块

下载管理器：协调多个下载任务，管理线程池
下载任务：负责单个分块的下载
文件管理器：处理文件的分块写入和合并
下载信息：保存下载状态，支持持久化

四、代码实现

1. 下载信息类

// DownloadInfo.java
package com.httpdownloader.model;import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;public class DownloadInfo implements Serializable {private static final long serialVersionUID = 1L;private String url;private String savePath;private long fileSize;private int threadCount;private List<BlockInfo> blockInfos;private boolean completed;public DownloadInfo(String url, String savePath, int threadCount) {this.url = url;this.savePath = savePath;this.threadCount = threadCount;this.blockInfos = new ArrayList<>();this.completed = false;}// Getters and setterspublic String getUrl() { return url; }public String getSavePath() { return savePath; }public long getFileSize() { return fileSize; }public void setFileSize(long fileSize) { this.fileSize = fileSize; }public int getThreadCount() { return threadCount; }public List<BlockInfo> getBlockInfos() { return blockInfos; }public void addBlockInfo(BlockInfo info) { blockInfos.add(info); }public boolean isCompleted() { return completed; }public void setCompleted(boolean completed) { this.completed = completed; }@Overridepublic String toString() {return "DownloadInfo{" +"url='" + url + '\'' +", savePath='" + savePath + '\'' +", fileSize=" + fileSize +", threadCount=" + threadCount +", completed=" + completed +'}';}
}// BlockInfo.java
package com.httpdownloader.model;import java.io.Serializable;public class BlockInfo implements Serializable {private static final long serialVersionUID = 1L;private int blockId;private long startPos;private long endPos;private long currentPos;private boolean completed;public BlockInfo(int blockId, long startPos, long endPos) {this.blockId = blockId;this.startPos = startPos;this.endPos = endPos;this.currentPos = startPos;this.completed = false;}// Getters and setterspublic int getBlockId() { return blockId; }public long getStartPos() { return startPos; }public long getEndPos() { return endPos; }public long getCurrentPos() { return currentPos; }public void setCurrentPos(long currentPos) { this.currentPos = currentPos; }public boolean isCompleted() { return completed; }public void setCompleted(boolean completed) { this.completed = completed; }@Overridepublic String toString() {return "BlockInfo{" +"blockId=" + blockId +", startPos=" + startPos +", endPos=" + endPos +", currentPos=" + currentPos +", completed=" + completed +'}';}
}

2. 文件管理器

// FileManager.java
package com.httpdownloader.util;import com.httpdownloader.model.BlockInfo;
import com.httpdownloader.model.DownloadInfo;import java.io.*;
import java.nio.channels.FileChannel;
import java.util.List;public class FileManager {private static final String TEMP_DIR = "temp/";/*** 创建临时文件*/public static void createTempFile(DownloadInfo downloadInfo) throws IOException {File tempDir = new File(TEMP_DIR);if (!tempDir.exists()) {tempDir.mkdirs();}// 创建主文件File file = new File(downloadInfo.getSavePath());if (!file.getParentFile().exists()) {file.getParentFile().mkdirs();}if (!file.exists()) {file.createNewFile();}// 为每个分块创建临时文件for (BlockInfo block : downloadInfo.getBlockInfos()) {File tempFile = getTempFile(downloadInfo, block.getBlockId());if (!tempFile.exists()) {tempFile.createNewFile();}}}/*** 写入数据到临时文件*/public static synchronized void writeBlockData(DownloadInfo downloadInfo, int blockId, byte[] data, int length) throws IOException {File tempFile = getTempFile(downloadInfo, blockId);try (RandomAccessFile raf = new RandomAccessFile(tempFile, "rw")) {// 定位到当前写入位置raf.seek(getCurrentBlockPosition(downloadInfo, blockId));raf.write(data, 0, length);}}/*** 更新分块的当前位置*/public static void updateBlockPosition(DownloadInfo downloadInfo, int blockId, long newPosition) {for (BlockInfo block : downloadInfo.getBlockInfos()) {if (block.getBlockId() == blockId) {block.setCurrentPos(newPosition);break;}}}/*** 获取分块的当前位置*/public static long getCurrentBlockPosition(DownloadInfo downloadInfo, int blockId) {for (BlockInfo block : downloadInfo.getBlockInfos()) {if (block.getBlockId() == blockId) {return block.getCurrentPos();}}return 0;}/*** 合并所有临时文件到最终文件*/public static void mergeTempFiles(DownloadInfo downloadInfo) throws IOException {File finalFile = new File(downloadInfo.getSavePath());try (FileOutputStream fos = new FileOutputStream(finalFile);FileChannel outChannel = fos.getChannel()) {for (BlockInfo block : downloadInfo.getBlockInfos()) {File tempFile = getTempFile(downloadInfo, block.getBlockId());try (FileInputStream fis = new FileInputStream(tempFile);FileChannel inChannel = fis.getChannel()) {inChannel.transferTo(0, inChannel.size(), outChannel);}}}// 删除临时文件deleteTempFiles(downloadInfo);}/*** 删除临时文件*/private static void deleteTempFiles(DownloadInfo downloadInfo) {for (BlockInfo block : downloadInfo.getBlockInfos()) {File tempFile = getTempFile(downloadInfo, block.getBlockId());if (tempFile.exists()) {tempFile.delete();}}}/*** 获取分块的临时文件*/private static File getTempFile(DownloadInfo downloadInfo, int blockId) {String fileName = new File(downloadInfo.getSavePath()).getName();return new File(TEMP_DIR + fileName + ".part" + blockId);}/*** 保存下载信息*/public static void saveDownloadInfo(DownloadInfo downloadInfo) {String infoFile = getDownloadInfoFilePath(downloadInfo);try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(infoFile))) {oos.writeObject(downloadInfo);} catch (IOException e) {e.printStackTrace();}}/*** 加载下载信息*/public static DownloadInfo loadDownloadInfo(String url, String savePath) {String infoFile = getDownloadInfoFilePath(url, savePath);File file = new File(infoFile);if (file.exists()) {try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file))) {return (DownloadInfo) ois.readObject();} catch (IOException | ClassNotFoundException e) {e.printStackTrace();}}return null;}private static String getDownloadInfoFilePath(DownloadInfo downloadInfo) {return getDownloadInfoFilePath(downloadInfo.getUrl(), downloadInfo.getSavePath());}private static String getDownloadInfoFilePath(String url, String savePath) {String fileName = new File(savePath).getName();return TEMP_DIR + fileName + ".info";}
}

3. 下载任务

// DownloadTask.java
package com.httpdownloader.task;import com.httpdownloader.model.BlockInfo;
import com.httpdownloader.model.DownloadInfo;
import com.httpdownloader.util.FileManager;import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;public class DownloadTask implements Runnable {private static final int BUFFER_SIZE = 8192;private DownloadInfo downloadInfo;private BlockInfo blockInfo;public DownloadTask(DownloadInfo downloadInfo, BlockInfo blockInfo) {this.downloadInfo = downloadInfo;this.blockInfo = blockInfo;}@Overridepublic void run() {try {downloadBlock();} catch (IOException e) {e.printStackTrace();}}private void downloadBlock() throws IOException {URL url = new URL(downloadInfo.getUrl());HttpURLConnection conn = null;InputStream is = null;try {conn = (HttpURLConnection) url.openConnection();conn.setRequestMethod("GET");conn.setConnectTimeout(5000);conn.setReadTimeout(5000);// 设置Range请求头long startPos = blockInfo.getCurrentPos();long endPos = blockInfo.getEndPos();conn.setRequestProperty("Range", "bytes=" + startPos + "-" + endPos);// 检查响应码int responseCode = conn.getResponseCode();if (responseCode == HttpURLConnection.HTTP_PARTIAL) {is = conn.getInputStream();byte[] buffer = new byte[BUFFER_SIZE];int bytesRead;while ((bytesRead = is.read(buffer)) != -1) {// 写入数据到临时文件FileManager.writeBlockData(downloadInfo, blockInfo.getBlockId(), buffer, bytesRead);// 更新当前位置long newPos = blockInfo.getCurrentPos() + bytesRead;blockInfo.setCurrentPos(newPos);// 保存下载信息FileManager.saveDownloadInfo(downloadInfo);}// 标记分块完成blockInfo.setCompleted(true);System.out.println("分块 " + blockInfo.getBlockId() + " 下载完成");} else {System.err.println("服务器不支持断点续传，响应码: " + responseCode);}} finally {// 关闭资源if (is != null) {try {is.close();} catch (IOException e) {e.printStackTrace();}}if (conn != null) {conn.disconnect();}}}
}

4. 下载管理器

// DownloadManager.java
package com.httpdownloader.manager;import com.httpdownloader.model.BlockInfo;
import com.httpdownloader.model.DownloadInfo;
import com.httpdownloader.task.DownloadTask;
import com.httpdownloader.util.FileManager;import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class DownloadManager {private ExecutorService threadPool;private DownloadInfo downloadInfo;public DownloadManager(String url, String savePath, int threadCount) {this.downloadInfo = FileManager.loadDownloadInfo(url, savePath);if (this.downloadInfo == null) {this.downloadInfo = new DownloadInfo(url, savePath, threadCount);}this.threadPool = Executors.newFixedThreadPool(threadCount);}/*** 开始下载*/public void startDownload() throws IOException {if (downloadInfo.isCompleted()) {System.out.println("文件已下载完成");return;}// 如果是新下载，获取文件信息并初始化分块if (downloadInfo.getFileSize() == 0) {initDownloadInfo();}// 创建临时文件FileManager.createTempFile(downloadInfo);// 提交下载任务List<BlockInfo> blocks = downloadInfo.getBlockInfos();for (BlockInfo block : blocks) {if (!block.isCompleted()) {threadPool.submit(new DownloadTask(downloadInfo, block));}}// 监控下载进度monitorDownloadProgress();}/*** 初始化下载信息*/private void initDownloadInfo() throws IOException {URL url = new URL(downloadInfo.getUrl());HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setRequestMethod("HEAD");int responseCode = conn.getResponseCode();if (responseCode == HttpURLConnection.HTTP_OK) {// 获取文件大小long fileSize = conn.getContentLengthLong();downloadInfo.setFileSize(fileSize);// 计算分块int threadCount = downloadInfo.getThreadCount();long blockSize = fileSize / threadCount;List<BlockInfo> blocks = new ArrayList<>();for (int i = 0; i < threadCount; i++) {long start = i * blockSize;long end = (i == threadCount - 1) ? fileSize - 1 : start + blockSize - 1;blocks.add(new BlockInfo(i, start, end));}downloadInfo.getBlockInfos().addAll(blocks);// 保存下载信息FileManager.saveDownloadInfo(downloadInfo);} else {throw new IOException("无法获取文件信息，响应码: " + responseCode);}conn.disconnect();}/*** 监控下载进度*/private void monitorDownloadProgress() {Thread monitorThread = new Thread(() -> {while (true) {try {Thread.sleep(1000);// 计算已下载百分比long downloaded = 0;long total = downloadInfo.getFileSize();boolean allCompleted = true;for (BlockInfo block : downloadInfo.getBlockInfos()) {downloaded += (block.getCurrentPos() - block.getStartPos());if (!block.isCompleted()) {allCompleted = false;}}double percent = (double) downloaded / total * 100;System.out.printf("下载进度: %.2f%%\n", percent);// 检查是否全部完成if (allCompleted) {downloadInfo.setCompleted(true);FileManager.saveDownloadInfo(downloadInfo);// 合并临时文件try {FileManager.mergeTempFiles(downloadInfo);System.out.println("下载完成，文件已保存至: " + downloadInfo.getSavePath());} catch (IOException e) {e.printStackTrace();}// 关闭线程池threadPool.shutdown();break;}} catch (InterruptedException e) {e.printStackTrace();break;}}});monitorThread.start();}/*** 暂停下载*/public void pauseDownload() {threadPool.shutdownNow();System.out.println("下载已暂停");}
}

5. 主程序

// Main.java
package com.httpdownloader.main;import com.httpdownloader.manager.DownloadManager;import java.io.IOException;public class Main {public static void main(String[] args) {if (args.length < 2) {System.out.println("用法: java Main <下载URL> <保存路径> [线程数]");System.out.println("示例: java Main https://example.com/file.zip ./downloads/file.zip 4");return;}String url = args[0];String savePath = args[1];int threadCount = (args.length > 2) ? Integer.parseInt(args[2]) : 4;try {DownloadManager manager = new DownloadManager(url, savePath, threadCount);manager.startDownload();// 注册关闭钩子，确保程序意外退出时能保存下载状态Runtime.getRuntime().addShutdownHook(new Thread(() -> {System.out.println("程序关闭，保存下载状态...");}));} catch (IOException e) {e.printStackTrace();}}
}

五、线程安全与性能优化

1. 线程安全机制

同步方法：FileManager中的关键方法使用synchronized保证线程安全
原子操作：使用long类型的变量记录下载位置，避免多线程冲突
线程池管理：使用ExecutorService管理线程池，控制并发数量

2. 性能优化

缓冲读取：使用8KB缓冲区减少IO操作次数
并行下载：多线程并行下载不同分块，提高带宽利用率
断点续传：支持中断后继续下载，避免重复下载已完成部分

六、测试与验证

1. 测试用例

// DownloadManagerTest.java
package com.httpdownloader.test;import com.httpdownloader.manager.DownloadManager;
import org.junit.Test;import java.io.IOException;public class DownloadManagerTest {@Testpublic void testDownload() throws IOException {String url = "https://example.com/large-file.zip";String savePath = "./downloads/large-file.zip";int threadCount = 4;DownloadManager manager = new DownloadManager(url, savePath, threadCount);manager.startDownload();// 等待下载完成try {Thread.sleep(60000); // 等待1分钟} catch (InterruptedException e) {e.printStackTrace();}}@Testpublic void testResumeDownload() throws IOException {String url = "https://example.com/large-file.zip";String savePath = "./downloads/large-file.zip";int threadCount = 4;// 第一次下载（会被中断）DownloadManager manager1 = new DownloadManager(url, savePath, threadCount);manager1.startDownload();try {Thread.sleep(10000); // 下载10秒后中断} catch (InterruptedException e) {e.printStackTrace();}manager1.pauseDownload();// 恢复下载DownloadManager manager2 = new DownloadManager(url, savePath, threadCount);manager2.startDownload();// 等待下载完成try {Thread.sleep(60000);} catch (InterruptedException e) {e.printStackTrace();}}
}