Hex-encoded String to Byte Array
Java SE 6 or Java EE 5 provides a method to do this now so there is no need for extra libraries.
The method is DatatypeConverter.parseHexBinary
In this case it can be used as follows:
String str = "9B7D2C34A366BF890C730641E6CECF6F";
byte[] bytes = DatatypeConverter.parseHexBinary(str);
The class also provides type conversions for many other formats that are generally used in XML.
I think what the questioner is after is converting the string representation of a hexadecimal value to a byte array representing that hexadecimal value.
The apache commons-codec has a class for that, Hex.
String s = "9B7D2C34A366BF890C730641E6CECF6F";
byte[] bytes = Hex.decodeHex(s.toCharArray());