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XML and Java
Alex Chaffee, [email protected]
http://www.purpletech.com
©1996-2000 jGuru.com
Overview
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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
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Unicode support in Java
XML Structures map well to Java Objects
Portability
Network friendly
XML Parsers
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Validating/Non-Validating
Tree-based
Event-based
SAX-compliance
Not technically parsers
– XSL
– XPath
Some Java XML Parsers
• DOM
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Sun JAXP
IBM XML4J
Apache Xerces
Resin (Caucho)
DXP (DataChannel)
• SAX
– Sun JAXP
– SAXON
• JDOM
Dom API
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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>
name
<meal name="snack">
<food>Chips</food>
</meal>
"breakfast"
</menu>
menu
meal
meal
food
food
drink
"Scrambled
Eggs"
"Hash
Browns"
"Orange
Juice"
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
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Node
– getNodeType()
– getNodeName()
– getNodeValue()
•Attr
–attributes are not technically child
nodes
–getParent() et al. return null
–getName(), getValue()
• returns null for Elements
– getAttributes()
• returns null for non-Elements
•Document
– getChildNodes()
– getParentNode()
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Element
– getTagName()
• same as getNodeName()
– getElementsByTagName(String tag)
• get all children of this name, recursively
– normalize()
• smooshes text nodes together
–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
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Java from the ground up
Open source
Clean, simple API
Uses Java Collections
JDOM vs. DOM
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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
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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
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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)
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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)
Reading
data
XMLC
HTML Object
(automatically
generated)
Setting
values
Java Class
(e.g. Servlet)
HTML (dynamically-generated)
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
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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
©1996-2000 jGuru.com
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
JSP
JavaBeans
DOM, SAX
Filesystem
EJB
XPath
XML-savvy
RDBMS
XSL
XML
Data Feed
HTML
Servlet
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
©1996-2000 jGuru.com
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
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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 elementonly 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/
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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
Sample XWingML code
<?xml version="1.0"?>
<!DOCTYPE XwingML SYSTEM "file:///c:/XwingML/xml/xwingml.dtd">
<XwingML>
<Classes>
<Instance name="OpenFile" className="XMLOpenFile"/>
<Instance name="SaveFile" className="XMLSaveFile"/>
<Instance name="ParseFile" className="XMLParseFile"/>
<Instance name="About" className="XMLAbout"/>
</Classes>
<JFrame name="MainFrame" title="Bluestone XMLEdit" image="icon.gif"
x="10%" y="10%" width="80%" height="80%">
<JMenuBar>
<JMenu text="File" mnemonic="F">
<JMenuItem icon="open.gif" text="Open..." mnemonic="O"
accelerator="VK_O,CTRL_MASK" actionListener="OpenFile"/>
<JMenuItem icon="save.gif" text="Save" mnemonic="S"
accelerator="VK_S,CTRL_MASK" actionCommand="save"
actionListener="SaveFile"/>
<JMenuItem icon="save.gif" text="Save As..." mnemonic="a"
actionCommand="saveas" actionListener="SaveFile"/>
<Separator/>
<JMenuItem text="Exit" mnemonic="x"
accelerator="VK_X,CTRL_MASK"
actionListener="com.bluestone.xml.swing.XwingMLExit"/>
XML -> Java Implementations
• XML -> Java
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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")
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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
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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/j
sr/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-byexample.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
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John McGann
Daniel Zen
David Orchard
My Mom