Reuse XML element from other file

Question

Is it possible to use a XML element from other file in another XML?
For instance, instead of having:

<document>
   <a><!-- huge content --></a>
   <b/>
</document>

I would like to have:

<document>
    <a ref="aDef"/>
    <b/>
</document>

Where is defined in its own XML and reused where needed. I would like this to be done by the parser and transparent to the application, the app would not be able to know if the element is a reference or a copy.
How can i get this done?

Answer 1

This is one option:

Fragment XML file (frag.xml):

<a><!-- huge content --></a>

Main XML file:

<!DOCTYPE document [
<!ENTITY aDef SYSTEM "frag.xml">
]>
<document>
   &aDef;
   <b/>
</document>

Answer 2

This is what the xinclude W3C standard is for. Similar to the external entities approach (as in above answer), you can encode the content-to-be-included in a separate file, like e.g. (frag.xml):

<a><!-- huge content --></a>

In the main XML file, an xinclude instruction refers to this external content:

<document>
  <xi:include href="frag.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/&gt;
  <b/>
</document>

When processed with an (xinclude-capable) XML processor (like e.g. Xerces http://xerces.apache.org/), the parser will expand this xinclude instruction with the contents it points at. The inclusion target in the @href attribute is interpreted as a URI, so you can equally point to fragments using fragment identifiers (e.g. href="frag.xml#fragment1).

Besides simple URIs in @href, the xinclude standard supports a very fine-grained vocabulary for expressing the inclusion target in a @xpointer attribute. However, support for complex XPointer expressions depends on the processor's XPointer compliance, which is generally underused. However, there's a (minimal) XSLT implementation as well: XIPr (http://dret.net/projects/xipr/).