多线程断点下载原理及其Java实现(2)

上一篇文章用了多线程下载技术,那么这篇文章来说一说续传的问题.

其实,续传很简单,只要把上次程序退出的位置保存起来,启动的时候如果存在配置文件,那么久读取,并且把startIndex设置成相应的值就好了.

但是还有一个问题就是传统的文件rw打开模式是先把文件写入缓冲区,然后缓冲区满了之后再写到磁盘文件,那么如果我们关掉程序的时候缓冲区的内容没被写到磁盘里面,那么就会造成文件的部分字节丢失.还是很幸运,RandomAccessFile类提供了一种实时写到磁盘的打开方式,下面开始编码吧。

package snippet;
import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
public class MutiDownload {
    
    private static final int    THREAD_COUNT    = 5;
    private static final String    DOWNLOAD_URL    = "http://s1.music.126.net/download/osx/NeteaseMusic_1.4.3_452_web.dmg";
    private static final String    fileName        = "/Users/August/Desktop/NeteaseMusic_1.4.3_452_web.dmg";
                                                
    public static void main(String[] args) {
        
        long fileSize;
        HttpURLConnection connection = null;
        try {
            
            connection = (HttpURLConnection) new URL(DOWNLOAD_URL).openConnection();
            connection.setRequestMethod("GET");
            connection.setConnectTimeout(8000);
            connection.setReadTimeout(8000);
            
            if (connection.getResponseCode() == 200) {
                fileSize = connection.getContentLength();
                long eachSize = fileSize / THREAD_COUNT;
                RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");
                
                raf.setLength(fileSize);
                raf.close();
                
                for (int i = 0; i < THREAD_COUNT; i++) {
                    long startIndex = i * eachSize;
                    long endIndex = (i + 1) * eachSize - 1;
                    if (i == THREAD_COUNT - 1) {
                        endIndex = fileSize;
                    }
                    
                    new DownloadThread(DOWNLOAD_URL, fileName, i, startIndex, endIndex).start();
                }
                
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                connection.disconnect();
                connection = null;
            }
        }
    }
    
}
class DownloadThread extends Thread {
    
    private String                url;
    private String                fileName;
    private int                    threadID;
    private long                startIndex;
    private long                endIndex;
    private HttpURLConnection    connection;
    private RandomAccessFile    raf;
    private InputStream            inputStream;
                                
    /**
     * 进度文件
     */
    private RandomAccessFile    progressRaf;
                                
    public DownloadThread(String url, String fileName, int threadID, long startIndex, long endIndex) {
        super();
        this.url = url;
        this.fileName = fileName;
        this.threadID = threadID;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
    }
    
    @Override
    public void run() {
        try {
            HttpURLConnection connection = (HttpURLConnection) new URL(url + "?ts=" + System.currentTimeMillis())
                    .openConnection();
            connection.setRequestMethod("GET");
            connection.setConnectTimeout(8000);
            connection.setReadTimeout(8000);
            connection.setRequestProperty("RANGE", "bytes=" + startIndex + "-" + endIndex);
            
            File progressFile = new File(String.valueOf(threadID));
            if (progressFile.exists()) {
                progressRaf = new RandomAccessFile(progressFile, "rwd");
            } else {
                progressFile.createNewFile();
                progressRaf = new RandomAccessFile(progressFile, "rwd");
                progressRaf.write(String.valueOf(startIndex).getBytes());
            }
            /**
             * 判断进度文件是否存在,不存在则创建,并且存入startIndex的值
             */
            
            progressRaf.seek(0);
            startIndex = Long.valueOf(progressRaf.readLine());
            /**
             * 这时进度文件一定存在,就读取上次结束为止 若为第一次下载,读取的依旧是startIndex的值
             */
            
            if (connection.getResponseCode() == 206) {
                inputStream = connection.getInputStream();
                byte[] bs = new byte[1024];
                int len;
                raf = new RandomAccessFile(fileName, "rwd");
                raf.seek(startIndex);
                
                long total = 0;
                long position = startIndex + total;
                while ((len = inputStream.read(bs)) != -1) {
                    total += len;
                    System.out.println("线程" + threadID + ":" + total);
                    raf.write(bs, 0, len);
                    progressRaf.seek(0);
                    progressRaf.write(String.valueOf(position).getBytes());
                    /**
                     * 实时同步进度 必须写到raf.write(bs, 0, len);之后.
                     * 因为如果先写到进度再去写目标文件的话,那么当写完进度后被停掉程序,目标文件那部分是没有数据的
                     * 下次续传的时候还是从上次结束那里开始的话,数据依旧是丢失的
                     * 
                     * 考虑若先同步目标文件再去同步进度文件,即使同步文目标文件程序被停掉,
                     * 那么下次续传顶多就把没有写入进度那部分再重新下载一次.对整个程序影响不大
                     */
                }
                progressFile.delete();
                /**
                 * 下载完毕就把进度文件删除
                 */
            }
            
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (connection != null) {
                    connection.disconnect();
                    connection = null;
                }
                if (raf != null) {
                    raf.close();
                    raf = null;
                }
                if (inputStream != null) {
                    inputStream.close();
                    inputStream = null;
                }
                if (progressRaf != null) {
                    progressRaf.close();
                    progressRaf = null;
                }
            } catch (Exception e2) {
                e2.printStackTrace();
            }
        }
    }
}