讯雷下载的核心思想是校验文件的md5值，两个文件若md5相同则为同一文件。

当得到用户下载某个文件的请求后它根据数据库中保留的文件md5比对出拥有此文件的url，

将用户请求挂接到此url上并仿造一个虚假的断点续传请求，从若干url上将一个正常文件拼接出来。

查了下资料，java也可以实现文件md5校验码获取，代码如下:

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MD5Util {

protected static char hexDigits[] = { ‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,‘a‘, ‘b‘, ‘c‘, ‘d‘, ‘e‘, ‘f‘ };
protected static MessageDigest messagedigest = null;
static{
   try{
    messagedigest = MessageDigest.getInstance("MD5");
   }catch(NoSuchAlgorithmException nsaex){
    System.err.println(MD5Util.class.getName()+"初始化失败，MessageDigest不支持MD5Util。");
    nsaex.printStackTrace();
   }
}

public static String getFileMD5String(File file) throws IOException {
   FileInputStream in = new FileInputStream(file);
   FileChannel ch = in.getChannel();
   MappedByteBuffer byteBuffer = ch.map(FileChannel.MapMode.READ_ONLY, 0, file.length());
   messagedigest.update(byteBuffer);
   return bufferToHex(messagedigest.digest());
}

public static String getMD5String(String s) {
   return getMD5String(s.getBytes());
}

public static String getMD5String(byte[] bytes) {
   messagedigest.update(bytes);
   return bufferToHex(messagedigest.digest());
}

private static String bufferToHex(byte bytes[]) {
   return bufferToHex(bytes, 0, bytes.length);
}

private static String bufferToHex(byte bytes[], int m, int n) {
   StringBuffer stringbuffer = new StringBuffer(2 * n);
   int k = m + n;
   for (int l = m; l < k; l++) {
    appendHexPair(bytes[l], stringbuffer);
   }
   return stringbuffer.toString();
}

private static void appendHexPair(byte bt, StringBuffer stringbuffer) {
   char c0 = hexDigits[(bt & 0xf0) >> 4];
   char c1 = hexDigits[bt & 0xf];
   stringbuffer.append(c0);
   stringbuffer.append(c1);
}

public static boolean checkPassword(String password, String md5PwdStr) {
   String s = getMD5String(password);
   return s.equals(md5PwdStr);
}

public static void main(String[] args) throws IOException {
   long begin = System.currentTimeMillis();

   //2EA3E66AC37DF7610F5BD322EC4FFE48 670M 11s kuri双核1.66G 2G内存
   File big = new File("e:/新建文件夹.rar");

   String md5=getFileMD5String(big);

   long end = System.currentTimeMillis();
   System.out.println("md5:"+md5+" time:"+((end-begin)/1000)+"s");
}

}

转载自：http://blog.sina.com.cn/s/blog_53d3c24a0100mpm1.html

Java 获取 文件md5校验码