java 多线程断点下载demo

源码链接

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;


public class MutileThreadDownload {
    /**
     * 线程的数量
     */
    private static int threadCount = 3;
    /**
     * 每个下载区块的大小
     */
    private static long blocksize;
    /**
     * 正在运行的线程的数量
     */
    private static int runningThreadCount;
    /**
     * 
     * @param args
     * @throws Exception 
     */
    public static void main(String[] args) throws Exception {
        //服务器路径
        String path = "http://download.java.net/jdk/jdk-api-localizations/jdk-api-zh-cn/publish/1.6.0/chm/JDK_API_1_6_zh_CN.CHM";
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setReadTimeout(5000);
        int code = conn.getResponseCode();
        if(code == 200) {
            long size = conn.getContentLength();//得到服务器端返回的文件的大小
            System.out.println("服务器文件的大小：" + size);
            blocksize = size / threadCount;
            //1.首先在本地创建一个大小跟服务器一模一样的空白文件
            File file = new File("temp.CHM");
            RandomAccessFile raf = new RandomAccessFile(file, "rw");
            raf.setLength(size);
            //2.开启若干个子线程分别去下载对应的资源
            runningThreadCount = threadCount;
            for(int i = 1;i <= threadCount; i++) {
                long startIndex = (i - 1) * blocksize;
                long endIndex = i * blocksize - 1;
                if(i == threadCount) {
                    //最后一个线程
                    endIndex = size - 1;
                }
                System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~" + endIndex);
                new DownloadThread(i, startIndex, endIndex, path).start();
            }
        }
        conn.disconnect();
    }
    
    private static class DownloadThread extends Thread {
        private int threadId;
        private long startIndex;
        private long endIndex;
        private String path;
        
        public DownloadThread(int threadId, long startIndex, long endIndex,
                String path) {
            super();
            this.threadId = threadId;
            this.startIndex = startIndex;
            this.endIndex = endIndex;
            this.path = path;
        }
        
        public void run() {
            try {
                //当前线程下载的总大小
                int total = 0;
                File positionFile = new File(threadId + ".txt");
                URL url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                //接着从上一次的位置继续下载数据
                if(positionFile.exists() && positionFile.length() > 0) {  //判断是否有记录
                    FileInputStream fis = new FileInputStream(positionFile);
                    BufferedReader br = new BufferedReader(new InputStreamReader(fis));
                    //获取当前线程上次下载的总大小是多少
                    String lasttotalstr = br.readLine();
                    int lastTotal = Integer.valueOf(lasttotalstr);
                    System.out.println("上次线程" + threadId + "下载的总大小：" + lastTotal);
                    startIndex += lastTotal;  //加上上次下载的总大小
                    fis.close();
                }
                
                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" +endIndex);
                conn.setConnectTimeout(5000);
                int code = conn.getResponseCode();
                System.out.println("code=" + code);
                InputStream is = conn.getInputStream();
                File file = new File("temp.CHM");
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                raf.seek(startIndex);
                System.out.println("第" + threadId + "个线程：写文件的开始位置：" + String.valueOf(startIndex));
                
                int len = 0;
                byte[] buffer = new byte[1024 * 1024];
                while((len = is.read(buffer)) != -1) {
                    RandomAccessFile rf = new RandomAccessFile(positionFile, "rwd");//"rwd"   打开以便读取和写入，对于 "rw"，还要求对文件内容的每个更新都同步写入到底层存储设备。  
                    raf.write(buffer, 0, len);
                    total += len;
                    rf.write(String.valueOf(total).getBytes());
                    rf.close();
                }
                is.close();
                raf.close();
                //conn.disconnect();
                
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                //只有所有线程都下载完毕后，才可以删除记录文件
                synchronized (MutileThreadDownload.class) {   //线程安全
                    System.out.println("线程" + threadId + "下载完毕了");
                    runningThreadCount--;
                    if(runningThreadCount < 1) {
                        System.out.println("所有线程都工作完毕了。删除临时记录的文件");
                        for (int i = 1; i <= threadCount; i++) {
                            File f = new File(i + ".txt");
                            System.out.println(f.delete());
                        }
                    }
                }
            }
        }
    }
}