public static byte[] hexToBytes(String hexStr) {

public static byte[] hexToBytes(String hexStr) {
   int len = hexStr.length();
   hexStr = hexStr.toUpperCase();
   byte[] des;
   if (len % 2 != 0 || len == 0) {
    return null;
   } else {
    int halfLen = len / 2;
    des = new byte[halfLen];
    char[] tempChars = hexStr.toCharArray();
    for (int i = 0; i < halfLen; ++i) {
     char c1 = tempChars[i * 2];
     char c2 = tempChars[i * 2 + 1];
     int tempI = 0;
     if (c1 >= '0' && c1 <= '9') {
      tempI += ((c1 - '0') << 4);
     } else if (c1 >= 'A' && c1 <= 'F') {
      tempI += (c1 - 'A' + 10) << 4;
     } else {
      return null;
     }
     if (c2 >= '0' && c2 <= '9') {
      tempI += (c2 - '0');
     } else if (c2 >= 'A' && c2 <= 'F') {
      tempI += (c2 - 'A' + 10);
     } else {
      return null;
     }
     des[i] = (byte) tempI;
     // System.out.println(des[i]);
    }
    return des;  }
 }