Java多线程下载实例详解

本文将介绍Java多线程下载的实现方法和步骤，并提供两个示例说明。

实现步骤

Java多线程下载的实现步骤如下：

1. 获取需要下载的文件的URL地址。
2. 创建多个线程，每个线程负责下载文件的不同部分。
3. 启动多个线程，通过HTTP请求下载各自负责的文件部分。
4. 合并下载完成的文件部分。
5. 完成文件下载。

示例一：Java多线程文件下载

以下示例是一个基于Java多线程的文件下载器，该程序支持并发下载多个文件，每个文件都会分成多个线程进行下载。

import java.io.File;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MultiThreadFileDownloader {

    private String url;  // 下载链接
    private String fileName;  // 本地保存文件名
    private String fileSavePath; // 文件保存目录
    private int threadNum;  // 线程数量

    public MultiThreadFileDownloader(String url, String fileSavePath, String fileName, int threadNum) {
        this.url = url;
        this.fileSavePath = fileSavePath;
        this.fileName = fileName;
        this.threadNum = threadNum;
    }

    public void download() throws Exception {
        URL fileUrl = new URL(url);
        HttpURLConnection connection = (HttpURLConnection) fileUrl.openConnection();

        // 获取下载文件长度
        int fileSize = connection.getContentLength();
        connection.disconnect();

        // 保存文件夹不存在则创建
        File saveDir = new File(fileSavePath);
        if (!saveDir.exists()) {
            saveDir.mkdirs();
        }

        // 计算每个线程要下载的字节数
        int blockSize = fileSize / threadNum + 1;

        // 创建线程池
        ExecutorService executorService = Executors.newCachedThreadPool();

        // 创建下载任务
        DownloadTask[] downloadTasks = new DownloadTask[threadNum];
        for (int i = 0; i < threadNum; i++) {
            int startIndex = i * blockSize;  // 计算下载起始位置
            int endIndex = (i + 1) * blockSize - 1;  // 计算下载结束位置

            // 最后一个线程下载到文件末尾
            if (i == threadNum - 1) {
                endIndex = fileSize;
            }

            // 创建下载任务
            DownloadTask downloadTask = new DownloadTask(url, fileSavePath, fileName, startIndex, endIndex);
            downloadTasks[i] = downloadTask;

            // 将下载任务交给线程池执行
            executorService.execute(downloadTask);
        }

        // 等待所有线程下载完成
        for (DownloadTask downloadTask : downloadTasks) {
            downloadTask.waitFinish();
        }

        // 合并下载文件
        mergeDownloadFile();

        // 关闭线程池
        executorService.shutdown();
    }

    // 合并下载文件
    private void mergeDownloadFile() throws Exception {
        // 创建下载文件
        File downloadFile = new File(fileSavePath + fileName);
        RandomAccessFile randomAccessFile = new RandomAccessFile(downloadFile, "rw");

        // 遍历所有下载文件块，并将文件块内容依次写入下载文件
        for (int i = 0; i < threadNum; i++) {
            File downloadTempFile = new File(fileSavePath + fileName + ".temp" + i);
            RandomAccessFile tempFile = new RandomAccessFile(downloadTempFile, "r");

            byte[] buffer = new byte[1024];
            int length;
            while ((length = tempFile.read(buffer)) != -1) {
                randomAccessFile.write(buffer, 0, length);
            }

            tempFile.close();

            // 删除临时文件
            downloadTempFile.delete();
        }

        randomAccessFile.close();
    }

    // 下载任务
    private class DownloadTask implements Runnable {
        private String url;  // 下载链接
        private String fileSavePath; // 本地保存路径
        private String fileName; // 保存文件名
        private int startIndex;  // 起始下载位置
        private int endIndex;  // 结束下载位置
        private boolean isFinish = false; // 是否下载完成

        public DownloadTask(String url, String fileSavePath, String fileName, int startIndex, int endIndex) {
            this.url = url;
            this.fileSavePath = fileSavePath;
            this.fileName = fileName;
            this.startIndex = startIndex;
            this.endIndex = endIndex;
        }

        @Override
        public void run() {
            try {
                URL fileUrl = new URL(this.url);
                HttpURLConnection connection = (HttpURLConnection) fileUrl.openConnection();

                // 设置下载文件的范围
                connection.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);

                byte[] buffer = new byte[1024];
                int length;
                int totalLength = 0;

                // 创建临时文件
                File file = new File(fileSavePath + fileName + ".temp" + Thread.currentThread().getId());
                RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");

                // 获取输入流并将数据写入临时文件
                while ((length = connection.getInputStream().read(buffer)) != -1) {
                    randomAccessFile.write(buffer, 0, length);
                    totalLength += length;
                }

                randomAccessFile.close();
                connection.disconnect();

                // 标记下载任务已经完成
                isFinish = true;

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        // 等待下载任务完成
        public void waitFinish() throws InterruptedException {
            while (!isFinish) {
                Thread.sleep(100);
            }
        }
    }
}

示例二：Java多线程图片下载

以下示例展示了如何使用Java多线程下载图片，并将其保存到本地磁盘中。

import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultiThreadImageDownloader {

    public static void downloadImage(String imageUrl, String savePath) {
        try {
            // 获取连接
            URL url = new URL(imageUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            // 获取文件长度
            int fileSize = connection.getContentLength();
            System.out.println("文件大小为：" + fileSize);

            // 分配线程下载
            int threadNum = 4;
            int blockSize = fileSize / threadNum + 1;
            FileDownloadThread[] threads = new FileDownloadThread[threadNum];

            // 创建文件夹
            File dir = new File(savePath);
            if (!dir.exists()) {
                dir.mkdirs();
            }

            // 分别开启多个线程下载不同的文件块
            for (int i = 0; i < threadNum; i++) {
                int start = i * blockSize;
                int end = (i + 1) * blockSize - 1;
                if (i == threadNum - 1) {
                    end = fileSize - 1;
                }
                System.out.println("线程 " + i + " 下载：" + start + " ~ " + end + " 字节");

                FileDownloadThread thread = new FileDownloadThread(url, start, end, new File(savePath + "/" + getFileName(imageUrl)), savePath);
                thread.start();
                threads[i] = thread;
            }

            // 等待所有的线程下载完成
            for (int i = 0; i < threadNum; i++) {
                threads[i].join();
            }

            System.out.println("文件下载完成");

        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("下载文件出现异常");
        }
    }

    // 根据图片URL获取图片名称
    public static String getFileName(String imageUrl) {
        int index = imageUrl.lastIndexOf("/");
        return imageUrl.substring(index + 1);
    }

    // 下载线程
    public static class FileDownloadThread extends Thread {
        private URL url;  // 下载链接
        private int start; // 线程下载开始位置
        private int end; // 线程下载结束位置
        private File file; // 本地保存的文件
        private String savePath; // 文件保存目录

        public FileDownloadThread(URL url, int start, int end, File file, String savePath) {
            this.url = url;
            this.start = start;
            this.end = end;
            this.file = file;
            this.savePath = savePath;

            if (!file.exists()) {
                try {
                    FileOutputStream out = new FileOutputStream(file);
                    byte[] buf = new byte[end - start + 1];
                    out.write(buf);
                    out.close();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }

        @Override
        public void run() {
            try {
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("GET");
                connection.setRequestProperty("Range", "bytes=" + start + "-" + end);

                InputStream in = connection.getInputStream();
                byte[] buf = new byte[1024];
                int len = 0;
                RandomAccessFile out = new RandomAccessFile(file, "rw");
                out.seek(start);
                while ((len = in.read(buf)) != -1) {
                    out.write(buf, 0, len);
                }
                out.close();
                in.close();
                connection.disconnect();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

以上就是Java多线程下载的实现方法和示例。