Xml Java

XML and Java Alex Chaffee, alex@jguru.com http://www.purpletech.com

Overview
Why Java and XML?
Parsers: DOM, JDOM, SAX
Using XML from JSP
Java/XML Object Mapping
Resources

Why Java/XML?
XML maps well to Java
late binding
hierarchical (OO) data model
Unicode support in Java
XML Structures map well to Java Objects
Portability
Network friendly

XML Parsers
Validating/Non-Validating
Tree-based
Event-based
SAX-compliance
Not technically parsers
XSL
XPath

Some Java XML Parsers
DOM
Sun JAXP
IBM XML4J
Apache Xerces
Resin (Caucho)
DXP (DataChannel)
SAX
Sun JAXP
SAXON
JDOM

Dom API
Tree-based
Node classes
Part of W3C spec
Sorting/Modifying of Elements
Sharing document with other applications

XML is a Tree
<?xml version="1.0"?>
<!DOCTYPE menu SYSTEM "menu.dtd">
<menu>
<meal name="breakfast">
<food>Scrambled Eggs</food>
<food>Hash Browns</food>
<drink>Orange Juice</drink>
</meal>
<meal name="snack">
<food>Chips</food>
</meal>
</menu>
menu meal name "breakfast" food "Scrambled Eggs" food "Hash Browns" drink "Orange Juice" meal

DOM API (cont’d)
Based on Interfaces
Good design style - separate interface from implementation
Document, Text, Processing Instruction, Element - ALL are interfaces
All extend interface Node
Including interface Attr (parentNode is null, etc)

DOM Example public void print(Node node) { //recursive method call using DOM API... int type = node.getNodeType(); case Node.ELEMENT_NODE: // print element with attributes out.print('<'); out.print(node.getNodeName()); Attr attrs[] = node.getAttributes(); for (int i = 0; i < attrs.length; i++) { Attr attr = attrs[i]; out.print(' '); out.print(attr.getNodeName());out.print("=""); out.print(normalize(attr.getNodeValue())); out.print('"'); } out.print('>'); NodeList children = node.getChildNodes(); if (children != null) { int len = children.getLength(); for (int i = 0; i < len; i++) { print(children.item(i)); } } break; case Node.ENTITY_REFERENCE_NODE: // handle entity reference nodes // ...

DOM API Highlights
Node
getNodeType()
getNodeName()
getNodeValue()
returns null for Elements
getAttributes()
returns null for non-Elements
getChildNodes()
getParentNode()
Element
getTagName()
same as getNodeName()
getElementsByTagName(String tag)
get all children of this name, recursively
normalize()
smooshes text nodes together
Attr
attributes are not technically child nodes
getParent() et al. return null
getName(), getValue()
Document
has one child node - the root element
call getDocumentElement()
contains factory methods for creating attributes, comments, etc.

DOM Level 2
Adds namespace support, extra methods
Not supported by Java XML processors yet

The Trouble With DOM
Written by C programmers
Cumbersome API
Node does double-duty as collection
Multiple ways to traverse, with different interfaces
Tedious to walk around tree to do simple tasks
Doesn't support Java standards (java.util collections)

JDOM: Better than DOM
Java from the ground up
Open source
Clean, simple API
Uses Java Collections

JDOM vs. DOM
Classes / Interfaces
Java / Many languages
Java Collections / Idiosyncratic collections
getChildText() and other useful methods / getNextSibling() and other useless methods

JDOM: The Best of Both Worlds
Clean, easy to use API
document.getRootElement().getChild("book"). getChildText("title")
Random-access tree model (like DOM)
Can use SAX for backing parser
Open Source, not Standards Committee
Allowed benevolent dictatorship -> clean design

JDOM Example XMLOutputter out = new XMLOutputter(); out.output( element, System.out ); Or… public void print(Element node) { //recursive method call using JDOM API... out.print('<'); out.print(node.getName()); List attrs = node.getAttributes(); for (int i = 0; i < attrs.size(); i++) { Attribute attr = (Attribute)attrs.get(i); out.print(' '); out.print(attr.getName());out.print("=""); out.print(attr.getValue() ); out.print('"'); } out.print('>'); List children = node.getChildren(); if (children != null) { for (int i = 0; i < children.size(); i++) { print(children.item(i)); } }

JDOM Example
public Element toElement(User dobj) throws IOException
{
User obj = (User)dobj;
Element element = new Element("user");
element.addAttribute("userid", ""+user.getUserid());
String val;
val = obj.getUsername();
if (val != null) {
element.addChild(new Element("username").setText(val));
}
val = obj.getPasswordEncrypted();
if (val != null) {
element.addChild(new Element("passwordEncrypted").setText(val));
}
return element;
}

JDOM Example
public User fromElement(Element element) throws DataObjectException
{
List list;
User obj = new User();
String value = null;
Attribute userid = element.getAttribute("userid");
if (userid != null) {
obj.setUserid( userid.getIntValue() );
}
value = element.getChildText("username");
if (value != null) {
obj.setUsername( value );
}
value = element.getChildText("passwordEncrypted");
if (value != null) {
obj.setPasswordEncrypted( value );
}
return obj;
}

DOMUtils
DOM is clunky
DOMUtils.java - set of utilities on top of DOM
http://www.purpletech.com/code
Or just use JDOM

Event-Based Parsers
Scans document top to bottom
Invokes callback methods
Treats XML not like a tree, but like a list (of tags and content)
Pro:
Not necessary to cache entire document
Faster, smaller, simpler
Con:
must maintain state on your own
can't easily backtrack or skip around

SAX API
Grew out of xmldev mailing list (grassroots)
Event-based
startElement(), endElement()
Application intercepts events
Not necessary to cache entire document

Sax API (cont’d)
public void startElement(String name, AttributeList atts) {
// perform implementation
out.print(“Element name is “ + name);
out.print(“, first attribute is “ + atts.getName(0)
+ “, value is “ + atts.getValue(0));
}

XPath
The stuff inside the quotes in XSL
Directory-path metaphor for navigating XML document
"/curriculum/class[4]/student[first()]"
Implementations
Resin (Caucho) built on DOM
JDOM has one in the "contrib" package
Very efficient API for extracting specific info from an XML tree
Don't have to walk the DOM or wait for the SAX
Con: yet another syntax / language, without full access to Java libraries

XSL
eXtensible Stylesheet Language
transforms one XML document into another
XSL file is a list of rules
Java XSL processors exist
Apache Xalan
(not to be confused with Apache Xerces)
IBM LotusXSL
Resin
SAXON
XT

Trouble with XSL
It's a programming language masquerading as a markup language
Difficult to debug
Turns traditional programming mindset on its head
Declarative vs. procedural
Recursive, like Prolog
Doesn't really separate presentation from code

JSP
JavaServer Pages
Outputting XML
<%
User = loadUser(request.getParameter("username"));
response.setContentType("text/xml");
%>
<user>
<username><%=user.getUsername()%></username>
<realname><%=user.getRealname()%></realname>
</user>
Can also output HTML based on XML parser, naturally (see my "JSP and XML" talk, or http://www.purpletech.com)

XMLC
A radical solution to the problem of how to separate presentation template from logic…
…to actually separate the presentation template from the logic!

XMLC Architecture HTML (with ID tags) Java Class (e.g. Servlet) HTML Object (automatically generated) XMLC HTML (dynamically-generated) Setting values Data Reading data

XMLC Details
Open-source (xmlc.enhydra.org)
Uses W3C DOM APIs
Generates "set" methods per tag
Source: <H1 id="title">Hello</H1>
Code: obj.setElementTitle("Goodbye")
Output: <H1>Goodbye</H1>
Allows graphic designers and database programmers to develop in parallel
Works with XML source too

XML and Java in 2001
Many apps' config files are in XML
Ant
Tomcat
Servlets
Several XML-based Sun APIs
JAXP
JAXM
ebXML
SOAP (half-heartedly supported  )

Java XML Documentation
Jdox
Javadoc -> single XML file
http://www.componentregistry.com/
Ready for transformation (e.g. XSL)
Java Doclet
http://www.sun.com/xml/developers/doclet
Javadoc -> multiple XML files (one per class)
Cocoon
Has alpha XML doclet

Soapbox: DTDs are irrelevant
DTDs describe structure of an unknown document
But in most applications, you already know the structure – it's implicit in the code
If the document does not conform, there will be a runtime error, and/or corrupt/null data
This is as it should be! GIGO.
You could have a separate "sanity check" phase, but parsing with validation "on" just slows down your app
Useful for large-scale document-processing applications, but not for custom apps or transformations

XML and Server-Side Java

Server-Side Java-XML Architecture
Many possible architectures
XML Data Source
disk or database or other data feed
Java API
DOM or SAX or XPath or XSL
XSL
optional transformation into final HTML, or HTML snippets, or intermediate XML
Java Business Logic
JavaBeans and/or EJB
Java Presentation Code
Servlets and/or JSP and/or XMLC

Server-Side Java-XML Architecture Java UI Java Business Logic XML Processors XML Data Sources HTML JSP JavaBeans Servlet EJB DOM, SAX XPath XSL Filesystem XML-savvy RDBMS XML Data Feed

Server-Side Architecture Notes
Note that you can skip any layer, and/or call within layers
e.g. XML->XSL->DOM->JSP, or
JSP->Servlet->DOM->XML

Cache as Cache Can
Caching is essential
Whatever its advantages, XML is slow
Cache results on disk and/or in memory

XML <-> Java Object Mapping

XML and Object Mapping
Java -> XML
Start with Java class definitions
Serialize them - write them to an XML stream
Deserialize them - read values in from previously serialized file
XML -> Java
Start with XML document type
Generate Java classes that correspond to elements
Classes can read in data, and write in compatible format (shareable)

Java -> XML Implementations
Java -> XML
BeanML
Coins / BML
Sun's XMLOutputStream/XMLInputStream
XwingML (Bluestone)
JDOM BeanMapper
Quick?
JSP (must roll your own)

BeanML Code (Extract)
<?xml version="1.0"?>
<bean class="java.awt.Panel">
<property name="background" value="0xeeeeee"/>
<property name="layout">
<bean class="java.awt.BorderLayout"/>
</property>
<add>
<bean class="demos.juggler.Juggler" id="Juggler">
<property name="animationRate" value="50"/>
<call-method name="start"/>
</bean>
<string>Center</string>
</add>
… </bean>

Coins
Part of MDSAX
Connect XML Elements and JavaBeans
Uses Sax Parser, Docuverse DOM to convert XML into JavaBean
Uses BML - (Bindings Markup Language) to define mapping of XML elements to Java Classes

JDOM BeanMapper
Written by Alex Chaffee 
Default implementation outputs element-only XML, one element per property, named after property
Also goes other direction (XML->Java)
Doesn't (yet) automatically build bean classes
Can set mapping to other custom element names / attributes

XMLOutputStream/XMLInputStream
From some Sun engineers
http://java.sun.com/products/jfc/tsc/articles/persistence/
May possibly become core, but unclear
Serializes Java classes to and from XML
Works with existing Java Serialization
Not tied to a specific XML representation
You can build your own plug-in parser
Theoretically, can be used for XML->Java as well

XMLOutputStream/XMLInputStream

XML -> Java Implementations
XML -> Java
Java-XML Data Binding (JSR 31 / Adelard)
IBM XML Master (Xmas)
Purple Technology XDB
Breeze XML Studio (v2)

Adelard (Java-XML Data Binding)
Java Standards Request 31
Still vapor! (?)

Castor
Implementation of JSR 31
http://castor.exolab.org
Open-source

IBM XML Master ("XMas")
Not vaporware - it works!!!
Same idea as Java-XML Data Binding
From IBM Alphaworks
Two parts
builder application
visual XML editor beans

Brett McLaughlin's Data Binding Package
See JavaWorld articles

Purple Technology XDB
In progress (still vapor)
Currently rewriting to use JDOM
JDOMBean helps
Three parts
XML utility classes
XML->Java data binding system
Caching filesystem-based XML database (with searching)

Conclusion
Java and XML are two great tastes that taste great together

Resources
XML Developments:
Elliot Rusty Harold:
Café Con Leche - metalab.unc.edu/xml
Author, XML Bible
Simon St. Laurent
www.simonstl.com
Author, Building XML Applications
General
www.xmlinfo.com
www.oasis-open.org/cover/xml.html
www.xml.com
www.jdm.com
www.purpletech.com/xml

Resources: Java-XML Object Mapping
JSR 31
http://java.sun.com/aboutJava/communityprocess/jsr/jsr_031_xmld.html
http://java.sun.com/xml/docs/binding/DataBinding.html
XMas
http://alphaworks.ibm.com/tech/xmas

Resources
XSL:
James Tauber:
xsl tutorial: www.xmlsoftware.com/articles/xsl-by-example.html
Michael Kay
Saxon
home.iclweb.com/icl2/mhkay/Saxon.html
James Clark
XP Parser, XT
editor, XSL Transformations W3C Spec

Resources:
JDOM
www.jdom.org

Thanks To
John McGann
Daniel Zen
David Orchard
My Mom

Xml Java

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