java多线程下载&断点续传功能原理解析+demo

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/ITzhongzi/article/details/88389805

原理解析： 在开启多个线程下载文件的同时，将下载的进度保存在一个临时文件里，每次运行程序查看有没有临时文件，有的话就取临时文件中的进度继续下载。多线程下载的原理 见上一篇 博客。

示例demo

	import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class MutilDownload {
    private static String path = "http://192.168.80.85:8080/test.pdf";
    private static final int threadCount = 3;
    private static int runningThread; //标识 正在运行的线程数量

    public static void main(String[] args) {
        try {
            URL url = new URL(path);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setConnectTimeout(5000);
            int responseCode = conn.getResponseCode();
            if (responseCode == 200) {
                int contentLength = conn.getContentLength();

                runningThread = threadCount;
                System.out.println("length" + contentLength);

                RandomAccessFile rafAccessFile = new RandomAccessFile("test.pdf", "rw");
                rafAccessFile.setLength(contentLength);

                int blockSize = contentLength / threadCount;
                for (int i = 0; i < threadCount; i++) {
                    int startIndex = i * blockSize; //每个现成下载的开始位置
                    int endIndex = (i + 1) * blockSize - 1;// 每个线程的结束位置
                    if (i == threadCount - 1) {
                        //最后一个线程
                        endIndex = contentLength - 1;
                    }

                    new DownloadThread(startIndex, endIndex, i).start();
                }

            }
        } catch (Exception e) {

        }
    }

    private static class DownloadThread extends Thread {
        private int startIndex;
        private int endIndex;
        private int threadId;

        public DownloadThread(int startIndex, int endIndex, int threadId) {
            this.startIndex = startIndex;
            this.endIndex = endIndex;
            this.threadId = threadId;
        }

        @Override
        public void run() {
            try {
                URL url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setConnectTimeout(5000);

                File file = new File(threadId + ".txt");
                if(file.exists() && file.length() > 0) {
                    FileInputStream fis = new FileInputStream(file);
                    BufferedReader buff = new BufferedReader(new InputStreamReader(fis));
                    String lastPosition = buff.readLine();// 读取出来的内容就是上次下载的位置
                    int lastPos = Integer.parseInt(lastPosition);

                    System.out.println("线程id：" + threadId + "当前线程下载的位置：-----"+ lastPos);

                    startIndex = lastPos;
                    fis.close();
                    buff.close();
                }

                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex); //固定写法，请求部分资源
                int responseCode = conn.getResponseCode();  // 206表示请求部分资源
                if (responseCode == 206) {
                    RandomAccessFile rafAccessFile = new RandomAccessFile("test.pdf", "rw");
                    rafAccessFile.seek(startIndex);
                    InputStream is = conn.getInputStream();
                    int len = -1;
                    byte[] buffer = new byte[1024];
                    int total = 0; // 代表当前线程下载的大小
                    while ((len = is.read(buffer)) != -1) {
                        rafAccessFile.write(buffer, 0, len);

                        total += len;
                        //断点续传， 保存当前线程下载的位置
                        int currentThreadPosition = startIndex + total; //当前线程下载的位置
                        // 存储当线程的下载五位置
                        RandomAccessFile raff = new RandomAccessFile(threadId +".txt", "rwd");
                        raff.write(String.valueOf(currentThreadPosition).getBytes());
                        raff.close();
                    }
                    rafAccessFile.close();

                    System.out.println("线程" + threadId + "下载完成");
                    //删除临时文件
//                    File tempFile = new File(threadId + ".txt");
//                    if(tempFile.exists()) {
//                        file.delete();
//                    }
                    synchronized (MutilDownload.class) {
                        runningThread--;
                        if(runningThread == 0) {
                            for (int i = 0; i < threadCount; i++) {
                                File deleteFile = new File(i + ".txt");
                                deleteFile.delete();
                            }
                        }
                    }
                }
            } catch (Exception e) {

            }
        }
    }
}