字符串MD5值，适用于大文本的比较

public class Test
{
protected static char hexDigits[] = {'0', '1', '2', '3', '4', '5', '6',
'7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

public static void main(String[] args)
{
MessageDigest messagedigest;
try
{
messagedigest = MessageDigest.getInstance("MD5");
messagedigest.update("aa".getBytes());

System.out.println(bufferToHex(messagedigest.digest()));
}
catch (NoSuchAlgorithmException e)
{
e.printStackTrace();
}
}

private static String bufferToHex(byte bytes[])
{
int m = 0;
int n = bytes.length;
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);
}
}