Java多线程文件下载器开发教程 – 高效并发下载解决方案 | Java编程指南

一、项目概述

多线程下载器通过将大文件分割成多个部分，使用多个线程同时下载不同部分，最后合并成一个完整文件，可以显著提高下载速度。本教程将实现一个功能完整的Java多线程下载器。

二、项目结构设计

/src
  /main
    /java
      /com
        /downloader
          MultiThreadDownloader.java   # 主下载器类
          DownloadTask.java            # 下载任务类
          ProgressBar.java             # 进度条显示类
          FileUtils.java               # 文件工具类
    /resources
  /test
    /java
      /com
        /downloader
          DownloadTest.java            # 测试类
        

三、核心组件实现

1. 主下载器类 (MultiThreadDownloader.java)

package com.downloader;

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class MultiThreadDownloader {
    private String fileUrl;
    private String savePath;
    private int threadCount;
    private long fileSize;
    private ProgressBar progressBar;
    
    public MultiThreadDownloader(String fileUrl, String savePath, int threadCount) {
        this.fileUrl = fileUrl;
        this.savePath = savePath;
        this.threadCount = threadCount;
        this.progressBar = new ProgressBar();
    }
    
    public void download() throws Exception {
        // 获取文件大小
        fileSize = getFileSize();
        System.out.println("文件大小: " + FileUtils.formatFileSize(fileSize));
        
        // 创建空文件
        RandomAccessFile file = new RandomAccessFile(savePath, "rw");
        file.setLength(fileSize);
        file.close();
        
        // 计算每个线程下载的字节范围
        long chunkSize = fileSize / threadCount;
        
        // 创建线程池
        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
        
        // 提交下载任务
        for (int i = 0; i < threadCount; i++) {
            long startByte = i * chunkSize;
            long endByte = (i == threadCount - 1) ? fileSize - 1 : (i + 1) * chunkSize - 1;
            
            executor.execute(new DownloadTask(fileUrl, savePath, i, startByte, endByte, progressBar));
        }
        
        // 显示进度条
        progressBar.start(fileSize);
        
        // 关闭线程池
        executor.shutdown();
        
        // 等待所有任务完成
        try {
            executor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            throw new RuntimeException("下载被中断", e);
        }
        
        // 停止进度条
        progressBar.stop();
        
        System.out.println("n下载完成! 文件保存至: " + savePath);
    }
    
    private long getFileSize() throws IOException {
        URL url = new URL(fileUrl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("HEAD");
        
        int responseCode = conn.getResponseCode();
        if (responseCode != HttpURLConnection.HTTP_OK) {
            throw new IOException("服务器返回错误响应: " + responseCode);
        }
        
        String contentLength = conn.getHeaderField("Content-Length");
        if (contentLength == null) {
            throw new IOException("无法获取文件大小");
        }
        
        return Long.parseLong(contentLength);
    }
}
        

2. 下载任务类 (DownloadTask.java)

package com.downloader;

import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadTask implements Runnable {
    private String fileUrl;
    private String savePath;
    private int threadId;
    private long startByte;
    private long endByte;
    private ProgressBar progressBar;
    
    public DownloadTask(String fileUrl, String savePath, int threadId, 
                       long startByte, long endByte, ProgressBar progressBar) {
        this.fileUrl = fileUrl;
        this.savePath = savePath;
        this.threadId = threadId;
        this.startByte = startByte;
        this.endByte = endByte;
        this.progressBar = progressBar;
    }
    
    @Override
    public void run() {
        InputStream inputStream = null;
        RandomAccessFile file = null;
        
        try {
            // 创建HTTP连接
            URL url = new URL(fileUrl);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Range", "bytes=" + startByte + "-" + endByte);
            
            int responseCode = conn.getResponseCode();
            if (responseCode != HttpURLConnection.HTTP_PARTIAL) {
                throw new IOException("服务器不支持范围请求，响应码: " + responseCode);
            }
            
            // 获取输入流
            inputStream = conn.getInputStream();
            
            // 随机访问文件写入数据
            file = new RandomAccessFile(savePath, "rw");
            file.seek(startByte);
            
            byte[] buffer = new byte[4096];
            int bytesRead;
            long totalRead = 0;
            
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                file.write(buffer, 0, bytesRead);
                totalRead += bytesRead;
                
                // 更新进度
                progressBar.update(bytesRead);
            }
            
            System.out.println("线程 " + threadId + " 下载完成: " + (endByte - startByte + 1) + " 字节");
            
        } catch (IOException e) {
            System.err.println("线程 " + threadId + " 下载出错: " + e.getMessage());
        } finally {
            try {
                if (inputStream != null) inputStream.close();
                if (file != null) file.close();
            } catch (IOException e) {
                System.err.println("关闭资源出错: " + e.getMessage());
            }
        }
    }
}
        

3. 进度条显示类 (ProgressBar.java)

package com.downloader;

public class ProgressBar {
    private volatile long totalDownloaded = 0;
    private long totalSize;
    private boolean running = false;
    private Thread displayThread;
    
    public void start(long totalSize) {
        this.totalSize = totalSize;
        this.running = true;
        
        displayThread = new Thread(() -> {
            while (running) {
                printProgress();
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }
            printProgress(); // 最终打印一次
        });
        
        displayThread.start();
    }
    
    public void stop() {
        running = false;
        try {
            displayThread.join();
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
    
    public synchronized void update(long bytes) {
        totalDownloaded += bytes;
    }
    
    private synchronized void printProgress() {
        double percent = (double) totalDownloaded / totalSize * 100;
        int filledLength = (int) (50 * totalDownloaded / totalSize);
        
        StringBuilder bar = new StringBuilder("[");
        for (int i = 0; i < 50; i++) {
            if (i ");
            } else {
                bar.append(" ");
            }
        }
        bar.append("]");
        
        System.out.printf("r%s %.2f%% (%s/%s)", 
                bar.toString(), 
                percent,
                FileUtils.formatFileSize(totalDownloaded),
                FileUtils.formatFileSize(totalSize));
    }
}
        

4. 文件工具类 (FileUtils.java)

package com.downloader;

public class FileUtils {
    public static String formatFileSize(long size) {
        if (size < 1024) {
            return size + " B";
        } else if (size < 1024 * 1024) {
            return String.format("%.2f KB", size / 1024.0);
        } else if (size < 1024 * 1024 * 1024) {
            return String.format("%.2f MB", size / (1024.0 * 1024));
        } else {
            return String.format("%.2f GB", size / (1024.0 * 1024 * 1024));
        }
    }
}
        

四、使用示例

1. 创建测试类 (DownloadTest.java)

package com.downloader;

import java.util.Scanner;

public class DownloadTest {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("=== Java多线程下载器 ===");
        System.out.print("请输入下载URL: ");
        String url = scanner.nextLine();
        
        System.out.print("请输入保存路径: ");
        String savePath = scanner.nextLine();
        
        System.out.print("请输入线程数(建议1-8): ");
        int threadCount = scanner.nextInt();
        
        try {
            MultiThreadDownloader downloader = new MultiThreadDownloader(url, savePath, threadCount);
            downloader.download();
        } catch (Exception e) {
            System.err.println("下载失败: " + e.getMessage());
            e.printStackTrace();
        } finally {
            scanner.close();
        }
    }
}
        

2. 运行示例

=== Java多线程下载器 ===
请输入下载URL: https://example.com/largefile.zip
请输入保存路径: C:/Downloads/largefile.zip
请输入线程数(建议1-8): 4

文件大小: 256.78 MB
[=================>                  ] 45.32% (116.42 MB/256.78 MB)
        

五、高级功能扩展

1. 断点续传实现

通过保存每个线程的下载进度，可以在程序重启后继续未完成的下载：

// 在DownloadTask中添加断点续传支持
private long currentByte;

public void run() {
    // 读取保存的进度
    long startPoint = readDownloadProgress();
    
    // 修改Range请求
    conn.setRequestProperty("Range", "bytes=" + startPoint + "-" + endByte);
    
    // 下载过程中保存进度
    saveDownloadProgress(startPoint + bytesRead);
}
        

2. 下载速度限制

添加下载速度控制功能，避免占用过多带宽：

public class SpeedLimiter {
    private final long maxBytesPerSecond;
    private long bytesThisSecond;
    private long startTime;
    
    public SpeedLimiter(long maxBytesPerSecond) {
        this.maxBytesPerSecond = maxBytesPerSecond;
        this.bytesThisSecond = 0;
        this.startTime = System.currentTimeMillis();
    }
    
    public void acquire(int bytes) throws InterruptedException {
        bytesThisSecond += bytes;
        
        long elapsed = System.currentTimeMillis() - startTime;
        if (elapsed >= 1000) {
            resetCounter();
            return;
        }
        
        long expectedTime = (long) (bytesThisSecond / (maxBytesPerSecond / 1000.0));
        long sleepTime = expectedTime - elapsed;
        
        if (sleepTime > 0) {
            Thread.sleep(sleepTime);
        }
    }
    
    private void resetCounter() {
        bytesThisSecond = 0;
        startTime = System.currentTimeMillis();
    }
}
        

3. 代理服务器支持

添加代理服务器支持，方便在企业网络环境下使用：

public class ProxySupport {
    public static HttpURLConnection createConnection(String url, Proxy proxy) throws IOException {
        if (proxy != null) {
            return (HttpURLConnection) new URL(url).openConnection(proxy);
        } else {
            return (HttpURLConnection) new URL(url).openConnection();
        }
    }
}
        

六、异常处理与优化

1. 网络异常重试机制

public class RetryMechanism {
    public static <T> T executeWithRetry(Callable<T> task, int maxRetries) 
            throws Exception {
        int retries = 0;
        while (true) {
            try {
                return task.call();
            } catch (Exception e) {
                if (retries >= maxRetries) {
                    throw e;
                }
                retries++;
                System.out.println("操作失败，第" + retries + "次重试...");
                Thread.sleep(1000 * retries); // 指数退避
            }
        }
    }
}
        

2. 资源清理确保

// 使用try-with-resources确保资源正确释放
try (InputStream inputStream = conn.getInputStream();
     RandomAccessFile file = new RandomAccessFile(savePath, "rw")) {
    
    // 下载逻辑
    
} catch (IOException e) {
    // 异常处理
}
        

七、项目构建与运行

1. Maven依赖配置

<dependencies>
    <!-- 本项目无需额外依赖，使用标准Java库 -->
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>3.2.0</version>
            <configuration>
                <archive>
                    <manifest>
                        <mainClass>com.downloader.DownloadTest</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>
        

2. 编译与打包

# 使用Maven打包
mvn clean package

# 运行程序
java -jar downloader.jar
        

八、总结

本教程详细介绍了如何使用Java实现一个功能完整的多线程文件下载器。通过这个项目，你学习了：

1. Java多线程编程和线程池管理
2. HTTP范围请求的处理方法
3. 文件分块下载与合并技术
4. 进度显示和用户交互实现
5. 异常处理和资源管理

这个下载器不仅具有实际使用价值，也是学习Java并发和网络编程的绝佳项目。你可以在此基础上继续扩展功能，如添加GUI界面、支持下载队列管理、集成到更大系统中等。