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Supporting External DTDs

So far, we've stored DTDs internally in XML documents, using <!DOCTYPE> elements. But we can also store DTDs externally, in entirely separate files (which usually use the extension .dtd). It's often a good idea to use an external DTD with an XML application that is shared by many people. That way, if you want to make changes in the XML application, you only need to change the DTD once, not in dozens of separate files. (In fact, that's the way many XML applications, such as XHTML, are implemented.)

Private and Public DTDs

There are two ways to support external DTDs—as private DTDs for personal or limited use and as public DTDs for public use. We'll start with private DTDs.

Creating Private DTDs

You specify that we're using an external private DTD by using the SYSTEM keyword in the <!DOCTYPE> element, like this:

<!DOCTYPE document SYSTEM "ch04_07.dtd"> 

This example specifies the name of the document element (which is just <document> in this example), the SYSTEM keyword to indicate that the example is using a private external DTD, and the name of the external DTD file. Note that because the XML document now depends on an external file, the external DTD file, we must also change the standalone attribute from "yes" to "no", as shown in ch04_06.xml in Listing 4.6. The external DTD here is in ch04_07.dtd, which is shown in Listing 4.7. Note that the external DTD simply holds the part of the document that was originally between the [ and ] in the earlier versions of the <!DOCTYPE> element.

Listing 4.6 A Sample XML Document That Uses a Private External DTD (ch04_06.xml)

<?xml version = "1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE document SYSTEM "ch04_07.dtd"> 
<document>
  <employee>
    <name>
      <lastname>Kelly</lastname>
      <firstname>Grace</firstname>
    </name>
    <hiredate>October 15, 2005</hiredate>
    <projects>
      <project>
        <product>Printer</product>
        <id>111</id>
        <price>$111.00</price>
      </project>
      <project>
        <product>Laptop</product>
        <id>222</id>
        <price>$989.00</price>
      </project>
    </projects>
  </employee>
    .
    .
    .
</document>

Listing 4.7 An External DTD (ch04_07.dtd)

<!ELEMENT document (employee)*> 
<!ELEMENT employee (name, hiredate, projects)> 
<!ELEMENT name (lastname, firstname)> 
<!ELEMENT lastname (#PCDATA)> 
<!ELEMENT firstname (#PCDATA)> 
<!ELEMENT hiredate (#PCDATA)> 
<!ELEMENT projects (project)*> 
<!ELEMENT project (product,id,price)> 
<!ELEMENT product (#PCDATA)> 
<!ELEMENT id (#PCDATA)> 
<!ELEMENT price (#PCDATA) > 

The example shown in Listing 4.7 assumes that the external DTD is in the same directory as the XML document itself, so you just need to give the name of the external DTD file in the <!DOCTYPE> element:

<?xml version = "1.0" standalone="no"?>
<!DOCTYPE document SYSTEM "ch04_07.dtd"> 

On the other hand, you can place the external DTD anywhere, as long as you give its full URI (in this case, that's just the full URL, as far as most XML processors are concerned) in the <!DOCTYPE> element, as in this example:

<?xml version = "1.0" standalone="no"?>
<!DOCTYPE document SYSTEM "http://www.xmlpowercorp.com/dtds/ch04_07.dtd"> 

You need to supply a URL like this for an external DTD if you want to use an online XML validator.

Creating Public DTDs

As discussed so far today, it's easy to create and use a private external DTD. Creating and using a public external DTD can take a little more work. In this case, you use the PUBLIC keyword instead of SYSTEM in the <!DOCTYPE> DTD. To use the PUBLIC keyword, you must also create a formal public identifier (FPI), which is a quoted string of text, made up of four fields separated by //. For example, the official FPI for transitional XHTML 1.0 is -//W3C//DTD XHTML 1.0 Transitional//EN. Here are the rules for creating the fields in FPIs:

  • The first field indicates whether the DTD is for a formal standard. For DTDs you create on your own, this field should be -. If a non-official standards body has created the DTD, you use +. For formal standards bodies, this field is a reference to the standard itself (such as ISO/IEC 19775:2003).

  • The second field holds the name of the group or person responsible for the DTD. You should use a name that is unique (for example, W3C just uses W3C).

  • The third field specifies the type of the document the DTD is for and should be followed by a unique version number of some kind (such as Version 1.0).

  • The fourth field specifies the language in which the DTD is written (for example, EN for English) .

When you use a public external DTD, we can use the <!DOCTYPE> element like this: <!DOCTYPE rootname PUBLIC FPI URI>. Listing 4.8 shows an example, ch04_08.xml, which uses the made-up FPI -//DTDS4ALL//Custom DTD Version 1.0//EN. This document uses ch04_07.dtd as the external DTD, as in the previous example, but as we can see, it treats that DTD as a public external DTD, complete with its own FPI.

Listing 4.8 A Sample XML Document That Uses a Public External DTD (ch04_08.xml)

<?xml version = "1.0" standalone="no"?>
<!DOCTYPE document PUBLIC "-//DTDS4ALL//Custom DTD Version 1.0//EN"
"http://www.xmlpowercorp.com/dtds/ch04_07.dtd">
<document>
  <employee>
    <name>
      <lastname>Kelly</lastname>
      <firstname>Grace</firstname>
    </name>
    <hiredate>October 15, 2005</hiredate>
    <projects>
      <project>
        <product>Printer</product>
        <id>111</id>
        <price>$111.00</price>
      </project>
      <project>
        <product>Laptop</product>
        <id>222</id>
        <price>$989.00</price>
      </project>
    </projects>
  </employee>
    .
    .
    .
</document>

Using Internal and External DTDs at the Same Time

So far, you've seen these versions of the <!DOCTYPE> element:

  • <!DOCTYPE rootname [DTD]>

  • <!DOCTYPE rootname SYSTEM URI>

  • <!DOCTYPE rootname PUBLIC identifier URI>

However, you can also use both internal and external DTDs if you use these forms of the <!DOCTYPE> element:

  • <!DOCTYPE rootname SYSTEM URI [DTD]>

  • <!DOCTYPE rootname PUBLIC identifier URI [DTD]>

In this case, the external DTD is specified by URL and the internal one by DTD. Listing 4.9 shows an example in ch04_09.xml, where the external DTD—ch04_10.xml in List- ing 4.10—specifies the syntax of all elements in ch04_09.xml except the <price> element, which is specified in the <!DOCTYPE> element in the XML document ch04_09.xml.

Listing 4.9 A Sample XML Document That Uses an Internal DTD and an External DTD (ch04_09.xml)

<?xml version = "1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE document SYSTEM "http://www.lightlink.com/steve/ch04_10.dtd" [ 
<!ELEMENT price (#PCDATA)>           
]>                       
<document>
  <employee>
    <name>
      <lastname>Kelly</lastname>
      <firstname>Grace</firstname>
    </name>
    <hiredate>October 15, 2005</hiredate>
    <projects>
      <project>
        <product>Printer</product>
        <id>111</id>
        <price>$111.00</price>
      </project>
      <project>
        <product>Laptop</product>
        <id>222</id>
        <price>$989.00</price>
      </project>
    </projects>
  </employee>
    .
    .
    .
</document>

Listing 4.10 The External DTD for ch04_09.xml (ch04_10.xml)

<!ELEMENT document (employee)*> 
<!ELEMENT employee (name, hiredate, projects)> 
<!ELEMENT name (lastname, firstname)> 
<!ELEMENT lastname (#PCDATA)> 
<!ELEMENT firstname (#PCDATA)> 
<!ELEMENT hiredate (#PCDATA)> 
<!ELEMENT projects (project)*> 
<!ELEMENT project (product,id,price)> 
<!ELEMENT product (#PCDATA)> 
<!ELEMENT id (#PCDATA) > 

Combining internal and external DTDs like this is a good idea if you have a standard DTD that we share with other XML documents but also want to do some customization in certain XML documents. Theoretically, if you specify the syntax for an element or attribute in both an internal and external DTD, the internal DTD is supposed to take precedence. Unfortunately, however, most XML processors these days just treat conflicts in an internal and external DTD as errors.

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