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This chapter is from the book

Well-Formed vs. Valid Documents

We learned that if a document follows the XML syntax rules discussed in the previous section, the document is said to be well-formed, the minimal requirement to be an XML document. That is, if a document isn't well-formed, it can't even be called XML (excepting XML fragments).

Validity

Even if a document is well-formed, however, it may not be valid. According to the XML specification (see http://www.w3.org/TR/REC-xml#sec-documents and http://www.w3.org/TR/REC-xml#dt-valid),

A data object is an XML document if it is well-formed, as defined in this specification. A well-formed XML document may in addition be valid if it meets certain further constraints. . . . An XML document is valid if it has an associated document type declaration and if the document complies with the constraints expressed in it.

In other words, since a document type declaration (and therefore a DTD) is optional, only documents that refer to a DTD can be checked for validity. This makes sense because following the well-formedness rules only indicates adherence to basic syntactical constraints; it says nothing about meeting the more stringent requirements of a specific structural model.

To reinforce the difference, let's take another look at the Employees example from chapter 2. The DTD is repeated here in Listing 3-4 for convenience.

Listing 3-4 Employees DTD (employee.dtd)

<!ELEMENT Employees ( Employee+ ) >
<!ELEMENT Employee ( Name, Title, Projects, Email, PhoneNumbers, Address ) >
<!ATTLIST Employee sex NMTOKEN #REQUIRED >
<!ELEMENT Name ( First, Last ) >
<!ELEMENT First ( #PCDATA ) >
<!ELEMENT Last ( #PCDATA ) >
<!ELEMENT Title ( #PCDATA ) >
<!ELEMENT Projects ( Project+ ) >
<!ELEMENT Project ( #PCDATA ) >
<!ELEMENT Email ( #PCDATA ) >
<!ELEMENT PhoneNumbers ( Home, Office, Cell ) >
<!ELEMENT Home ( #PCDATA ) >
<!ELEMENT Office ( #PCDATA ) >
<!ELEMENT Cell ( #PCDATA ) >
<!ELEMENT Address ( Street, City, State, Zip ) >
<!ELEMENT Street ( #PCDATA ) >
<!ELEMENT City ( #PCDATA ) >
<!ELEMENT State ( #PCDATA ) >
<!ELEMENT Zip ( #PCDATA ) >

Now consider the document in Listing 3-5. Is it valid?

Listing 3-5 Employee Example 1 (employee-WF.xml)

<?xml version='1.0' standalone='yes'?>
<Employees>
 <Employee sex="female">
  <Name>
   <First>Betty Jo</First>
   <Last>Bialowsky</Last>
   <!-- aka: Melanie Haber, Audrey Farber, Susan Underhill, Nancy -->
  </Name>
  <Title>Project Leader</Title>
  <Projects>
   <Project>MegaProject</Project>
   <Project>SmallProject</Project>
   <Project>others</Project>
  </Projects>
  <Email>BettyJo.Bialowsky@home.com</Email>
  <PhoneNumbers>
   <Home>555-abc-1235</Home>
   <Office>555-xyz-4321</Office>
   <Cell>555-pqr-1267</Cell>
  </PhoneNumbers>
  <Address>
   <Street>321 Carmel Court</Street>
   <City>Columbia</City>
   <State>MD</State>
   <Zip>20777</Zip>
  </Address>
 </Employee>
</Employees>

At first glance, it appears to be valid because it follows the structural rules of the DTD. However, because it does not contain a document type declaration, the parser has no DTD to compare the document instance against in order to determine validity. Therefore, it is well-formed, but not valid, or at least its validity cannot be determined. What about the document in Listing 3-6?

Listing 3-6 Employee Example 2 (employee-Miss-Elt.xml)

<?xml version='1.0' standalone='no'?>
<!DOCTYPE Employees SYSTEM "employee.dtd" >
<Employees>
 <Employee sex="female">
  <Name>
   <First>Betty Jo</First>
   <Last>Bialowsky</Last>
   <!-- aka: Melanie Haber, Audrey Farber, Susan Underhill, Nancy -->
  </Name>
  <Projects>
   <Project>MegaProject</Project>
   <Project>SmallProject</Project>
   <Project>others</Project>
  </Projects>
  <Email>BettyJo.Bialowsky@home.com</Email>
  <PhoneNumbers>
   <Home>555-abc-1235</Home>
   <Office>555-xyz-4321</Office>
  </PhoneNumbers>
  <Address>
   <Street>321 Carmel Court</Street>
   <City>Columbia</City>
   <State>MD</State>
   <Zip>20777</Zip>
  </Address>
 </Employee>
</Employees> 

With the inclusion of a reference to a DTD, a validating parser can check this instance. It will conclude, however, that there are two missing elements, namely Title and Cell, so this document is well-formed but invalid. In fact, most parsers diagnose the problem quite clearly. For example, the free parser XML Validator from ElCel Technology (http://www.elcel.com/products/xmlvalid.html) reports:

employee-Miss-Elts.xml [10:15] : Error: element content invalid. Element 'Projects' is not expected here, expecting 'Title'
employee-Miss-Elts.xml [19:20] : Error: premature end to content of element 'PhoneNumbers'. Expecting child element 'Cell'

This indicates, among other things, that the errors are detected on lines 10 and 19. And what about this one in Listing 3-7?

Listing 3-7 Employee Example 3 (employee-BJB.xml)

<?xml version='1.0' standalone='no'?>
<!DOCTYPE Employees SYSTEM "employee.dtd" >
<Employees>
 <Employee sex="female">
  <Name>
   <First>Betty Jo</First>
   <Last>Bialowsky</Last>
   <!-- aka: Melanie Haber, Audrey Farber, Susan Underhill, Nancy -->
  </Name>
  <Title>Project Leader</Title>
  <Projects>
   <Project>MegaProject</Project>
   <Project>SmallProject</Project>
   <Project>others</Project>
  </Projects>
  <Email>BettyJo.Bialowsky@home.com</Email>
  <PhoneNumbers>
   <Home>555-abc-1235</Home>
   <Office>555-xyz-4321</Office>
   <Cell>555-pqr-1267</Cell>
  </PhoneNumbers>
  <Address>
   <Street>321 Carmel Court</Street>
   <City>Columbia</City>
   <State>MD</State>
   <Zip>20777</Zip>
  </Address>
 </Employee>
</Employees>

This document is well-formed and valid because it matches the structure defined by the DTD, which is referenced in the document type declaration.

Well-Formed or Toast?

Ignoring for a moment the potential importance of validity to data-oriented applications, you might wonder why even when an XML document does not require a DTD (i.e., is standalone), it still must be well-formed. In fact, if a document is not well-formed, it cannot even be called an XML document.

The reason for insisting on well-formedness is to counteract the "browser bloat" syndrome that occurred when the major browser vendors decided they wanted their browser to be able to render the horribly inaccurate HTML developed by graduates (or perhaps flunkies) of the Learn HTML in 2 Days or Less school. Many Web pages contain completely invalid HTML, with improperly nested elements, missing end tags, misspelled element names, missing delimiters, and other aberrations. Browsers such as Netscape Communicator and Internet Explorer do an admirable job of recovering from these errors, but only at the expense of a considerable amount of built-in recovery code.

Fortunately, with XML (and XHTML), parsers do not need to implement recovery code and can therefore stay trim and lightweight. If the parser encounters a well-formedness problem, it should only report the problem to the calling application. It explicitly must not attempt to correct what might be missing, overlapping, or misspelled. Violations of well-formedness constraints are considered fatal errors, according to the XML 1.0 Recommendation. The bottom line here is: either a document is well-formed XML, or it's toast; that is, it's not XML.

The extra code necessary to do the HTML-like corrections might not be a significant problem for a desktop PC with lots of memory. It's more of an issue as XML is fed to handheld PCs and other devices with limited memory and/or processing power.

Validating and Nonvalidating Parsers

The differences between validating and nonvalidating parsers are not quite as clear as you might think. According to the XML 1.0 specification (http://www.w3.org/ TR/REC-xml#proc-types),

Validating processors must, at user option, report violations of the constraints expressed by the declarations in the DTD, and failures to fulfill the validity constraints given in this specification. To accomplish this, validating XML processors must read and process the entire DTD and all external parsed entities referenced in the document. Non-validating processors are required to check only the document entity, including the entire internal DTD subset, for well-formedness.

In other words, validating parsers must read the entire DTD and check the document against the structural constraints it describes. You might conclude, therefore, that nonvalidating parsers do not need to consult the DTD, but that turns out to be incorrect. Even nonvalidating parsers need to supply default values for attributes and to replace text based on internal entities (discussed in chapter 4).

Although there used to be a class of strictly nonvalidating parsers, they tend to be much less popular of late. Most modern parsers (2000 and beyond) can be run in either validating or nonvalidating mode. Why run in nonvalidating mode when a parser is capable of validation? Because validation can significantly impact performance, especially when long and complex DTDs are involved. Some developers find that while enabling validation during development and test phases is crucial, it's sometimes beneficial to surpress validation in production systems where document throughput is most valued and the reliability of the data is already known. Consult the documentation of prospective parsers to determine how to toggle this switch, and which is the default mode. For example, the Apache Xerces parser is nonvalidating by default.

Some of the more highly regarded XML parsers include:

  • Apache XML Project's Xerces

  • IBM's XML Parser for Java (xml4j)

  • JavaSoft's XML Parser

  • MSXML 4.0 Release: Microsoft XML Core Services component (aka MSXML Parser) and SDK

  • Oracle's XML Parser

  • ElCel Technology's XML Validator

URLs for these parsers and many more can be found on the XML Parsers/ Processors list at XMLSoftware.com, http://www.xmlsoftware.com/.

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