# Using SCD for customizations

When using an external customization file, the JAXB spec requires that you use XPath as a means to specify what your customization is attached to. For example, if you want to change the class name generated from a complex type, you'd write something like:

External customization example

    <bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <bindings schemaLocation="../path/to/my.xsd" node="/xs:schema/xs:complexType[@name='foo']">
        <class name="FooType"/>
      </bindings>
    </bindings>

While the above process works, the problem with this is that the XPath+ `schemaLocation` combo tends to be verbose and error prone. It's verbose, because often a trivial target schema component like this "global complex type foo" takes up a lot of characters. The xs namespace declaration also takes up some space, although in this case we managed to avoid declaring the "tns" namespace (that represents the namespace that the schema defines.)

It's also error prone, because it relies on the way schema documents are laid out, because the schemaLocation attribute needs to point to the right schema document file. When a schema is split into multiple files for modularity (happens especially often with large schemas), then you'd have to find which schema file it is. Even though you can use relative paths, this hard-coding of path information makes it hard to pass around the binding file to other people.

JAXB RI 2.1 and onward offers a better way to do this as a vendor extension.

The key technology to solve this problem is a "`schema component designator`" (SCD.) This is a path language just like XPath, but whereas XPath is designed to refer to XML infoset items like elements and attributes, SCD is designed to refer to schema components like element declarations or complex types.

With SCD, the above binding can be written more concisely as follows:

External customization by SCD

    <bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.1" xmlns:tns="http://my.namespace/">
      <bindings scd="/type::tns:foo">
        <class name="FooType"/>
      </bindings>
    </bindings>

`/type::tns:foo` can be written more concisely as `/~tns:foo`, too. If you are interested in more about the syntax of SCDs, read the example part of the spec, and maybe EBNF. If you know XPath, I think you'll find this fairly easy to learn.

Another benefit of an SCD is that tools will have easier time generating SCDs than XPath, as XPaths are often vulnerable to small changes in the schema document, while SCDs are much more robust. The downside of using SCD is as of JAXB 2.1, this feature is a vendor extension and not defined in the spec.