ITEC 325 2011fall ibarland

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lect14c-ch07-dtd-entities
DTD - Entities
ch07

Entities are named-constants for your XML. With an entity, you define its name and the text it should expand into when referenced in an XML document. The entity reference is included in an XML document or DTD; it's replaced by defined-value upon rendering.

We will discuss “general entities” here. There are other types of entities (“parameter entities”, and parsed/unparsed versions of general entities), but they all work in the same way, and they are all defined through a DTD. The differences lie in where the entity can be expanded, and what kind of data it contains. See the textbook for further info on these other types of entities.

• Creating a General Entity
  
  • In the DTD -
    <!ENTITY entName "content"> -- general form
    <!ENTITY wow "Wonders of <em>the</em> World"> -- example
    
  • In XML, there are five built-in general entities.
    
    • &amp;
    • &lt;
    • &gt;
    • &quot;
    • &apost;
  • Using General Entities
    

    <story> The oldest of the &wow;, the Great Pyramid, is … </story>
    

    will render as “The oldest of the Wonders of the World, the Great Pyramid, is…”.
• Creating an External General Entity
  If you have a larger entity, or one that could be reused in multiple DTD documents, it is often more convenient to save it in a separate, external document.
  • Create the content for the entity in an external file. Save the file as text-only using an extension of .ent.
    actually any file extension can be used, but this is common
  • In the DTD -
    <!ENTITY entName SYSTEM "entity.uri" >
    entity.uri - is the location of the file containing the entity content.
    
    <!ENTITY gardens_story SYSTEM "lect14c-ch07-gardens.ent">
    Now, “&gardens_story;” can be written in a source xml file, and it will be replaced with the contents of the lect14c-ch07-gardens.ent.
    lect14c-ch07-05.dtd
• Using External General Entities
  Once created you can use your entity, share the entity with others, and borrow entities from others.
  • In the XML document that will refer to the DTD, add “standalone="no"” to the XML declaration as in:
    <?xml version="1.0" standalone="no"?>
  • In the XML document, just use &ent_name;
    For example: <story>&gardens_story;</story>
    lect14c-ch07-05.dtd
    lect14c-ch07-06-wonders-noxsl.xml
• Creating and Using External DTD
  
  • Create the DTD in an external file.
  • In the XML document add a DOCTYPE declaration.
    
    <!DOCTYPE sometype SYSTEM "some-uri.dtd">
    "some-uri.dtd" is the URL of the DTD file.
  • In the XML document change the XML declaration.
    <?xml version="1.0" standalone="no"?>
• Validating XML documents against a DTD
  
  • Make sure you include the declaration in the XML document.
  • Make sure the DTD file is publicly available (permissions).
  • Go to:
    http://validator.w3.org/
    Choose the method you want to use to handle the file. Then click check.
  • Be careful -- it takes some experience to properly decipher the error messages.

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(remaining topics are optional — not frequently used:)

• Creating Entities for Unparsed Content
  Entities that contain text are called parsed entities because the XML parser looks at them and analyzes them in the course of going through the XML document. Unparsed entities don't usually contain text and are completely bypassed by the XML parser. They are used to embed non-text or non-XML content into an XML document.
  • Create the data that you want to embed in the XML document. It may be, or contain, virtually anything — plain text, an image file, a video file, a PDF file, etc..
  • In order to create an entity for unparsed content, you must first identify how to process the unparsed content using a “notation”.
    
    <!NOTATION notationName SYSTEM "notation.instr" >
    The notation.instr defines how to process the unparsed content.
  • Note most XML parsers bundled with current Browsers do not handle unparsed entities, even if specified correctly in the DTD.

---

• Embedding Unparsed Content
  After defining an entity for unparsed content, it can be embedded into an XML document. Unparsed entities do not have entity references (that is, you won't actually use them between & and ;). Unparsed entities are instead referred to by giving their name in a tag's attribute-list; that attribute must have been defined as ENTITY. Declare the attribute that will contain the reference to the unparsed entity:
  • Define an element in the DTD, this element will contain the attribute referencing the unparsed entity.
  • create the attribute list tag:
    <!ATTLIST elemName attName ENTITY (ENTITIES) >
    
    

    <!NOTATION jpg SYSTEM "image/jpeg">
    <!ENTITY lighthouse_pic SYSTEM "lighthouse.jpg" NDATA jpg>
    <!ELEMENT photo EMPTY>
    <!ATTLIST photo source ENTITY #REQUIRED>

  • To embed an unparsed entity:
    show the value of the attribute as in --
    attrName = "entName"
    For example: <photo source="lighthouse_pic" />
  • Note most XML parsers bundled with current Browsers do not handle unparsed entities, even if specified correctly in the DTD.
  • This example uses parsed and unparsed general entitities. Here's the raw XML document.
    
    REVIEW classic_cars4_photo.dtd
    
    XML document
    
    Here's the document using XSLT. Classic Cars 4 w/Photo Example
• Parameter Entities are used to reference external information in the DTD itself.

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©2011, Ian Barland, Radford University Last modified 2011.Dec.13 (Tue)

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