java 多线程下载

   import java.io.ByteArrayOutputStream;
 import java.io.InputStream;

 public class StreamTool {
     /**
      * 把一个inputstream里面的内容转化成一个byte[]
      */
     public static byte[] getBytes(InputStream is) throws Exception {
         ByteArrayOutputStream bos = new ByteArrayOutputStream();
         byte[] buffer = new byte[1024];
         int len = 0;
         while ((len = is.read(buffer)) != -1) {
             bos.write(buffer, 0, len);
         }
         is.close();
         bos.flush();
         byte[] result = bos.toByteArray();
         System.out.println(new String(result));
         return result;
     }
 }

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

 public class TestDownload {
     public static final String path = "http://192.168.1.100:8080/aaa.exe";

     public static void main(String[] args) throws Exception {
         URL url = new URL(path);
         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
         conn.setRequestMethod("GET");
         conn.setConnectTimeout(5000);
         conn.setRequestProperty("User-Agent",
            "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
         int code = conn.getResponseCode();
         if (code == 200) {
             int len = conn.getContentLength();
             RandomAccessFile file = new RandomAccessFile(
                "C:/Users/DELL/Desktop/" + getFilenName(path), "rwd");
             // 1.设置本地文件大小跟服务器的文件大小一致
             file.setLength(len);

             // 2 .假设开启3 个线程
             int threadnumber = 3;
             int blocksize = len / threadnumber;
             /**
              * 线程1 0~ blocksize 线程2 1*bolocksize ~ 2*blocksize 线程3 2                   *blocksize ~
              * 文件末尾
              */
             for (int i = 0; i < threadnumber; i++) {
                 int startposition = i * blocksize;
                 int endpositon = (i + 1) * blocksize;
                 if (i == (threadnumber - 1)) {
                     // 最后一个线程
                     endpositon = len;
                 }
                 DownLoadTask task = new DownLoadTask(
                       i, path, startposition,endpositon);
                 task.start();
             }
         }
     }

     public static String getFilenName(String path) {
         int start = path.lastIndexOf("/") + 1;
         return path.substring(start, path.length());
     }

 }

 class DownLoadTask extends Thread {
     public static final String path = "http://192.168.1.100:8080/aaa.exe";
     int threadid;
     String filepath;
     int startposition;
     int endpositon;

     public DownLoadTask(int threadid, String filepath, 
                int startposition, int endpositon) {
         this.threadid = threadid;
         this.filepath = filepath;
         this.startposition = startposition;
         this.endpositon = endpositon;
     }

     @Override
     public void run() {
         try {
             File postionfile = new File(threadid + ".txt");
             URL url = new URL(filepath);
             HttpURLConnection conn = (HttpURLConnection) url.openConnection();
             System.out.println("线程" + threadid + "正在下载 " + "开始位置 : "
                     + startposition + "结束位置 " + endpositon);

             // 断点操作
             if (postionfile.exists()) {
                 FileInputStream fis = new FileInputStream(postionfile);
                 byte[] result = StreamTool.getBytes(fis);
                 int newstartposition = Integer.parseInt(new String(result));
                 if (newstartposition > startposition) { // 如果新的位置 > 开始位置。
                     startposition = newstartposition;
                 }
             }

             // "Range", "bytes=2097152-4194303")
             conn.setRequestProperty("Range", "bytes=" + startposition + "-"
                     + endpositon);
             conn.setRequestMethod("GET");
             conn.setConnectTimeout(5000);
             conn.setRequestProperty("User-Agent",
                     "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
             InputStream is = conn.getInputStream();
             RandomAccessFile file = new RandomAccessFile(
                     "C:/Users/DELL/Desktop/" + getFilenName(path), "rwd");
             // 设置 数据从文件哪个位置开始写
             file.seek(startposition);
             byte[] buffer = new byte[1024];
             int len = 0;

             // 代表当前读到的服务器数据的位置 ,同时这个值已经存储的文件的位置
             int currentPostion = startposition;

             // 创建一个文件对象 ,记录当前某个文件的下载位置
             while ((len = is.read(buffer)) != -1) {
                 file.write(buffer, 0, len);

                 currentPostion += len;
                 // 需要把currentPostion 信息给持久化到存储设备
                 String position = currentPostion + "";
                 System.out.println("线程 .." + threadid + "....开始位置..."
                         + startposition + "..当前位置..." + currentPostion
                         + "...结束位置...." + endpositon);
                 FileOutputStream fos = new FileOutputStream(postionfile);
                 fos.write(position.getBytes());
                 fos.flush();
                 fos.close();
             }
             file.close();
             System.out.println("线程" + threadid + "下载完毕");
             // 当线程下载完毕后 把文件删除掉
             if (postionfile.exists()) {
                 postionfile.delete();
             }
         } catch (Exception e) {
             e.printStackTrace();
         }
         super.run();
     }

     public static String getFilenName(String path) {
         int start = path.lastIndexOf("/") + 1;
         return path.substring(start, path.length());
     }

 }

 public class test1 {
     public static void main(String[] args) throws Exception {
         RandomAccessFile file = new RandomAccessFile("haha.txt", "rwd");
         file.setLength(1024 * 1024);
     }
 }