External DTDs

The DTDs I've created in this chapter so far have all been built into the documents they are targeted for. However, you can also create external DTDs, in which the actual DTD is stored in an external file (usually with the extension .dtd).

Using external DTDs makes it easy to create an XML application that can be shared by many people. In fact, that's the way many XML applications are supported. There are two ways to specify external DTDs: as private DTDs or as public DTDs. I'll take a look at private DTDs first.

Private DTDs are intended for use by people or groups privately and are not intended for public distribution. You specify an external private DTD with the SYSTEM keyword in the <!DOCTYPE> element, like this (note also that because this document now depends on an external file, the DTD file ch03_10.dtd, I've changed the value of the standalone attribute from yes to no ):

Listing ch03_09.xml

 <?xml version = "1.0" standalone="no"?>  <!DOCTYPE DOCUMENT SYSTEM "ch03_10.dtd">  <DOCUMENT>     <CUSTOMER>         <NAME>             <LAST_NAME>Smith</LAST_NAME>             <FIRST_NAME>Sam</FIRST_NAME>         </NAME>         <DATE>October 15, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Tomatoes</PRODUCT>                 <NUMBER>8</NUMBER>                 <PRICE>.25</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Oranges</PRODUCT>                 <NUMBER>24</NUMBER>                 <PRICE>.98</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER>     <CUSTOMER>         <NAME>             <LAST_NAME>Jones</LAST_NAME>             <FIRST_NAME>Polly</FIRST_NAME>         </NAME>         <DATE>October 20, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Bread</PRODUCT>                 <NUMBER>12</NUMBER>                 <PRICE>.95</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Apples</PRODUCT>                 <NUMBER>6</NUMBER>                 <PRICE>.50</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER>     <CUSTOMER>         <NAME>             <LAST_NAME>Weber</LAST_NAME>             <FIRST_NAME>Bill</FIRST_NAME>         </NAME>         <DATE>October 25, 2003</DATE>         <ORDERS>             <ITEM>                 <PRODUCT>Asparagus</PRODUCT>                 <NUMBER>12</NUMBER>                 <PRICE>.95</PRICE>             </ITEM>             <ITEM>                 <PRODUCT>Lettuce</PRODUCT>                 <NUMBER>6</NUMBER>                 <PRICE>.50</PRICE>             </ITEM>         </ORDERS>     </CUSTOMER> </DOCUMENT> 

Here's the file ch03_10.dtd that holds the external DTD. Note that it simply holds the part of the document that was originally between the [ and ] in the <!DOCTYPE> element:

Listing ch03_10.dtd

 <!ELEMENT DOCUMENT (CUSTOMER)*> <!ELEMENT CUSTOMER (NAME,DATE,ORDERS)> <!ELEMENT NAME (LAST_NAME,FIRST_NAME)> <!ELEMENT LAST_NAME (#PCDATA)> <!ELEMENT FIRST_NAME (#PCDATA)> <!ELEMENT DATE (#PCDATA)> <!ELEMENT ORDERS (ITEM)*> <!ELEMENT ITEM (PRODUCT,NUMBER,PRICE)> <!ELEMENT PRODUCT (#PCDATA)> <!ELEMENT NUMBER (#PCDATA)> <!ELEMENT PRICE (#PCDATA)>