Package org.eigenbase.xom

Provides an object layer for reading and writing XML documents.

See: Description

Interface Summary

Interface	Description
Any	An element which has 'Any' content.
DOMWrapper	DOMWrapper implements a Wrapper around the Element class from any DOM-style XML parser.
Location	Represents the location of a node within its document.
Locator	Callback to find the location of a node within its document.
MetaDef.Content	The Content class contains all entities which represent types of content which may appear within an Element.
MetaDef.Definition	The Definition class represents a generic type of element definition.
MetaDef.FullDefinition	The FullDefinition class represents a fully-specified definition that may include content.
NodeDef	NodeDef represents a node in a parse tree.
Parser	The Parser interface abstracts the behavior which the org.eigenbase.xom package needs from an XML parser.

Class Summary

Class	Description
CdataDef	A CdataDef represents a CDATA element.
CommentDef	Comment element.
DefWalker	DefWalker is a class designed to help users of plugin elements and elements with content type ANY.
DOMElementParser	DOMElementParser is a utility wrapper around DOMWrapper.
ElementDef	ElementDef is the base class for all element definitions.
GenericDef	A GenericDef is a ElementDef whose attributes and children are stored in collections, not generated members.
MetaDef	This model is the XOM Meta Model.
MetaDef.Any	The Any content is a special marker which allows an Element to contain any type of data.
MetaDef.Array	An Array is a set of multiple instances of Elements.
MetaDef.Attribute	The Attribute entity appears within any Element definition.
MetaDef.CData	The CData content is a special marker which allows an Element to contain a single CDATA section as its only content, yet still have attributes.
MetaDef.Class	A Class Definition defines a class of entities.
MetaDef.Element	An Element Definition defines a basic entity of the meta model.
MetaDef.Import	An Import as a special type of element that represents another element stored in an external model.
MetaDef.Model	Model is the top-level element for a model description.
MetaDef.Object	An Object is a single instance of an Element type.
MetaDef.Plugin	A Plugin in a special type of element whose content may be derived from a different model.
MetaDef.StringElement	A StringElement is a simple type of element which has no attributes and whose content is a single String (usually represented as a CDATA section).
MetaGenerator	MetaGenerator is a utility class which reads an XOM Meta Model description in XML and generates the corresponding .dtd and .java definition files.
StringEscaper	StringEscaper is a utility for replacing special characters with escape sequences in strings.
TextDef	A TextDef represents piece of textual data in an XML document.
WrapperElementDef	WrapperElementDef is an ElementDef which retains the underlying XML DOMWrapper.
XMLAttrVector	XMLAttrVector is an class which assists in writing XML attributes to a stream.
XMLOutput	XMLOutput is a class which implements streaming XML output.
XMLUtil	Utilities for dealing with XML data.
XOMGenTask	XOMGenTask is an ANT task with which to invoke MetaGenerator.
XOMUtil	Utility functions for the org.eigenbase.xom and org.eigenbase.xom.wrappers packages.

Exception Summary

Exception	Description
AssertFailure	Exception indicating that an assertion has failed.
XOMException	XOMException extends Exception and provides detailed error messages for xom-specific exceptions.

Package org.eigenbase.xom Description

Provides an object layer for reading and writing XML documents. The XML-Object Mapping layer generates a class for each entity in an XML schema, a member property for each attribute, collections for sub-objects, and methods to serialize to and from XML.

Schema format

The schema is defined in an XML file, whose format is somewhat similar to an XML Schema. The (self-describing) meta-schema is meta.xml (as you can see, an XSL style-sheet formats each schema into a javadoc-like web page).

Other schemas include resource.xml.

Generated Java Classes

The utilities in this package enable conversion of XML (stored in a DOM-style model) into a typesafe Java representation. Roughly, conversion occurs as follows:

• Each defined Element becomes a Java Class. The Java Class name matches the Element name. All classes representing Elements descend from ElementDef. All classes from the same model are implemented as static inner classes of a single enclosure class. By convention, enclosure classes always end in "Def".
• Each Attribute becomes a public member of its enclosing Element class.
• Text (PCDATA) content becomes a single public String called cdata.
• Element content becomes a series of public members. The members are of the appropriate types. The members' names are defined in the schema.
• ANY content becomes a single array of ElementDef called children.

Converting an XML Document to its XOM representation is a two-step process. First, you parse the XML into a DOM representation using your favorite W3C DOM Level 1-compliant parser (note: MSXML is supported as well for backward compatibility). Then, you instantiate the XOM ElementDef subclass corresponding to the root element in the document, passing a portion of the DOM as input to the constructor. After this, the fully-constructed root element provides complete typesafe access to the whole document!

Specific instructions for parsing using a DOM-compliant parser (such as XERCES):

1. Instantiate your parser, parse the document (validation against the DTD is optional), and retrieve the parsed Document object.
2. Call Document.getDocumentElement() to retrieve the toplevel Node in the document.
3. Wrap the toplevel node in an instance of W3CDOMWrapper.
4. Construct the appropriate ElementDef subclass from the wrapped node.

Specific instructions for parsing using Microsoft's XML Parser:

1. Instantiate your parser, parse the document (validation against the DTD is optional), and retrieve com.ms.xml.om.Document object representing the document.
2. Call com.ms.xml.om.Document#getRoot() to retrieve the toplevel Element in the document.
3. Wrap the toplevel element in an instance of org.eigenbase.xom.wrappers.MSXMLWrapper.
4. Construct the appropriate ElementDef subclass from the wrapped node.

Generator

There is now an ANT target, XOMGen, implemented by XOMGenTask.

Tester

There is another helpful utility we use to verify that the stuff we generate works. It is class MetaTester, and you invoke it as follows:

jview <classpath_options> org.eigenbase.xom.MetaTester [-debug]
        [-msxml | -xerces] <XML model file> <output dir> [<tests>...]

All the arguments are the same as the ones for MetaGenerator, except for tests. <tests> is a list of test .xml files that should be valid according to the generated DTD. The tester will validate the java files against the DTD and also against the java class. It also runs some basic checks on the java class to make sure that it works.

Generation

For all XML files except meta.xml, we generate corresponding DTD and NameDef.java at build time. MetaDef is tricky, because it depends upon itself; use the ant all target to rebuild it.

Guidelines for writing models

1. Note that within an <Entity>, all <Attribute>s must occur before all <Object>, <Array> or <CData> elements.

Known Issues

Dependencies

This package is dependent upon the following other packages:

• java.lang, java.io, java.util

Class XOMGenTask only is dependent upon org.apache.tools.ant. You therefore require ant.jar to build, but not to run.

Class XercesDOMParser only is dependent upon org.xml.sax and DOMParser. You therefore require xerces.jar to build, but not to run.