I wasn't aware of a difference, but a coworker says there is, although he can't back it up. What's the difference if any?
13 Answers
There is a difference, yes.
XML that adheres to the XML standard is considered well formed, while xml that adheres to a DTD is considered valid.
Valid XML is XML that succeeds validation against a DTD.
Well formed XML is XML that has all tags closed in the proper order and, if it has a declaration, it has it first thing in the file with the proper attributes.
In other words, validity refers to semantics, well-formedness refers to syntax.
So you can have invalid well formed XML.
As others have said, well-formed XML conforms to the XML spec, and valid XML conforms to a given schema.
Another way to put it is that well-formed XML is lexically correct (it can be parsed), while valid XML is grammatically correct (it can be matched to a known vocabulary and grammar).
An XML document cannot be valid until it is well-formed. All XML documents are held to the same standard for well-formedness (an RFC put out by the W3). One XML document can be valid against some schemas, and invalid against others. There are a number of schema languages, many of which are themselves XML-based.
Well-Formed XML is XML that meets the syntactic requirements of the language. Not missing any closing tags, having all your singleton tags use <whatever /> instead of just <whatever>, and having your closing tags in the right order.
Valid XML is XML that uses a DTD and complies with all its requirements. So if you use an attribute improperly, you violate the DTD and aren't valid.
All valid XML is well-formed, but not all well-formed XML is valid.
XML is well-formed if meets the requirements for all XML documents set out by the standards - so things like having a single root node, having nodes correctly nested, all nodes having a closing tag (or using the empty node shorthand of a slash before the closing angle bracket), attributes being quoted etc. Being well-formed just means it adheres to the rules of XML and can therefore be parsed properly.
XML is valid if it will validate against a DTD or schema. This obviously differs from case to case - XML that is valid against one schema won't be valid against another schema, even though it is still well-formed.
If XML isn't well-formed it can't be properly parsed - parsers will simply throw an exception or report an error. This is generic and it doesn't matter what your XML contains. Only once it is parsed can it be checked for validity. This domain or context dependent and requires a DTD or schema to validate against. For simple XML documents, you may not have a DTD or schema, in which case you can't know if the XML is valid - the concept or validity simply doesn't apply in this case. Of course, this doesn't mean you can't use it, it just means you can't tell whether or not it's valid.
I'll add that valid XML also implies that it's well-formed, but well-formed XML is not necessarily valid.
Well, XML that isn't well formed, sort of by definition, isn't XML. Poeple usually refer to valid XML as XML that adheres to a certain schema (XSD or DTD).
DTD is the acronym for Document Type Definition. This is a description of the content for a family of XML files. This is part of the XML 1.0 specification, and allows one to describe and verify that a given document instance conforms to the set of rules detailing its structure and content.
Validation is the process of checking a document against a DTD (more generally against a set of construction rules).
The validation process and building DTDs are the two most difficult parts of the XML life cycle. Briefly a DTD defines all the possible elements to be found within your document, what is the formal shape of your document tree (by defining the allowed content of an element; either text, a regular expression for the allowed list of children, or mixed content i.e. both text and children). The DTD also defines the valid attributes for all elements and the types of those attributes.
An XML document with correct syntax is called "Well Formed".
An XML document validated against a DTD is both "Well Formed" and "Valid".
Related