基于Http的文件断点续传实现

1:断点续传的介绍

客户端软件断点续传指的是在下载或上传时，将下载或上传任务（一个文件或一个压缩包）人为的划分为几个部分，每一个部分采用一个线程进行上传或下载，如果碰到网络故障，可以从已经上传或下载的部分开始继续上传下载未完成的部分，而没有必要从头开始上传下载。从而达到让用户节省时间，提高速度的目的。

2:断点续传的环境要求

如果是基于http请求与响应实现的断点续传,需要服务器支持"响应一部分"数据的功能;(本案例采用的是tomcat7服务器,而tomcat7服务器是支持这个功能的)

在客户端需要使用RandomAccessFile类对文件任意位置的数据进行随机读写操作;

3:java的RandomAccessFile类介绍

java的API中对RandomAccessFile类的解释如下:

我对RandomAccessFile类的理解是:RandomAccessFile类是java提供的一个可以用于随机读写文件内容的类,我们可以对RandomAccessFile类关联的文件中的任意位置和任意大小的数据进行任意的读写操作;因此要想完成文件的断点续传操作,该类的使用是必不可少的!

4:断点续传的基本实现思路

5:断点续传的代码实现

基础环境搭建:

创建WEB的maven工程;
引入maven的tomcat7插件;
在webapp目录下存放多个文件,以备测试断点续传下载使用;

java客户端代码实现:

public class MyDownLoadClient {
    public static String urlpath = "http://127.0.0.1:80/";
    private static int threadCount = 5;

    public static void main(String[] args) throws Exception {
        // 让用户输入要下载的文件名称
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入要下载的文件名称:");
        String file = sc.next();
        urlpath = urlpath.concat(file);
        // 获取文件总大小
        URL url = new URL(urlpath);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(2000);
        int responseCode = conn.getResponseCode();
        if (responseCode == 200) {
            int contentLength = conn.getContentLength();
            System.out.println("length" + contentLength);
            int part = contentLength / threadCount;
            // 读配置文件
            ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
            CountDownLatch count;
            InputStream in = MyDownLoadClient.class.getClassLoader().getResourceAsStream(file + ".properties");
            if (in != null) {
                // 说明该文件不是第一次下载,需要断点续传
                Properties p = new Properties();
                p.load(in);
                in.close();
                Set keys = p.stringPropertyNames();
                count = new CountDownLatch(keys.size());
                for (String key : keys) {
                    String value = p.getProperty(key);
                    String[] arr = value.split(",");
                    long start = Long.parseLong(arr[0]);
                    long end = Long.parseLong(arr[1]);
                    map.put(key,value);
                    new DownloadThread(start, end, key, map, count, file).start();
                }
                p.clear();
                p = null;
            } else {
                count = new CountDownLatch(threadCount);
                // 说明该文件是第一次下载,直接下载即可
                for (int i = 0; i < threadCount; i++) {
                    long startIndex = i * part; //每个线程起始下载位置

                    long endIndex = (i + 1) * part;//每个线程的结束位置

                    if (i == threadCount - 1) {//最后一个线程的结束位置

                        endIndex = contentLength;

                    }
                    map.put( String.valueOf(i),startIndex+","+endIndex);
                    new DownloadThread(startIndex, endIndex, String.valueOf(i), map, count, file).start();
                }
            }
            // 等待任务完成,删除配置文件
            count.await();

            new File(MyDownLoadClient.class.getClassLoader().getResource("").getPath(),file + ".properties").delete();
            System.out.println("==========================下载任务完成==========================");
        } else {
            System.out.println("连接服务器失败...请检查服务器是否畅通及资源路径是否正确...");
        }
    }
}

下载任务的线程代码实现:

class DownloadThread extends Thread {
    private long startIndex;
    private long endIndex;
    private String threadId;
    private ConcurrentHashMap<String, String> map;
    private CountDownLatch count;
    //private long subTotal = 0;
    private String fileName;

    public DownloadThread(long startIndex, long endIndex, String threadId, ConcurrentHashMap<String, String> map, CountDownLatch count, String fileName) {
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.threadId = threadId;
        this.map = map;
        this.count = count;
        this.fileName = fileName;
    }

    @Override
    public void run() {
        try {
            URL url = new URL(MyDownLoadClient.urlpath);

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();

            conn.setRequestMethod("GET");

            conn.setConnectTimeout(5000);
            //固定写法，表示向服务器请求部分资源
            conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);

            int responseCode = conn.getResponseCode();
            //状态码206表示请求部分资源成功
            if (responseCode == 206) {
                RandomAccessFile rafAccessFile = new RandomAccessFile(fileName, "rw");
                rafAccessFile.seek(startIndex);
                InputStream is = conn.getInputStream();
                int len = -1;
                byte[] buffer = new byte[1024];
                Random r = new Random();
                while ((len = is.read(buffer)) != -1) {
                    FileOutputStream fout = new FileOutputStream(this.getClass().getClassLoader().getResource("").getPath()+""+fileName + ".properties");
                    try {
                        //模拟意外情况导致下载中断的代码

                        /*if (r.nextInt(2) == 0) {
                            int i = 1 / 0;
                        }*/
                        rafAccessFile.write(buffer, 0, len);
                        startIndex += len;
                        map.put(threadId, startIndex + "," + endIndex);
                    } catch (Exception e) {
                        e.printStackTrace();
                        throw new RuntimeException();
                    } finally {
                        Set<Map.Entry<String, String>> entries = map.entrySet();
                        for (Map.Entry<String, String> entry : entries) {
                            fout.write((entry.getKey() + "=" + entry.getValue() + "rn").getBytes());
                        }
                        fout.close();
                    }
                }
                rafAccessFile.close();
                System.out.println("线程" + threadId + "下载完成");
                System.gc();
            }
            count.countDown();
        } catch (Exception e) {
            e.printStackTrace();
            System.gc();
        }
    }
}