JDOM and XML Parsing, Part 1
By Jason Hunter
JDOM makes XML manipulation in Java easier than ever.
Chances are, you've probably used one of a number of Java libraries to manipulate XML data structures in the past. So what's the point of JDOM (Java Document Object Model), and why do developers need it?
JDOM is an open source library for Java-optimized XML data manipulations. Although it's similar to the World Wide Web Consortium's (W3C) DOM, it's an alternative document object model that was not built on DOM or modeled after DOM. The main difference is that while DOM was created to be language-neutral and initially used for JavaScript manipulation of HTML pages, JDOM was created to be Java-specific and thereby take advantage of Java's features, including method overloading, collections, reflection, and familiar programming idioms. For Java programmers, JDOM tends to feel more natural and "right." It's similar to how the Java-optimized RMI (remote method invocation) library feels more natural than the language-neutral CORBA (Common Object Request Broker Architecture).
You can find JDOM at jdom.org under an open source Apache-style (commercial-friendly) license. It's collaboratively designed and developed and has mailing lists with more than 3,000 subscribers. The library has also been accepted by Sun's Java Community Process (JCP) as a Java Specification Request (JSR-102) and is on track to become a formal Java specification.
The articles in this series will provide a technical introduction to JDOM. This article provides information about important classes. The next article will give you a feel for how to use JDOM inside your own Java programs.
ORACLE XML TOOLS
The XML Developer Kit (XDK) is a free library of XML tools that Oracle provides for developers. It includes an XML parser and an XSLT transformation engine that can be used with JDOM. You can find lots of information about these tools at the Oracle XML home page, oracle.com/xml.
To download the parser, look for the XML Developer Kit names "XDK for Java." Click on "Software" in the left column for the download links. Once you unpack the distribution, the file xalparserv2.jar contains the parser.
To configure JDOM and other software to use the Oracle parser by default, you need to set the JAXP javax.xml.parsers.SAXParserFactory system property to oracle.xml.jax.JXSAXParserFactory. This tells JAXP that you prefer the Oracle parser. The easiest way is at the command line:
java -Djavax.xml.parsers.SAXParserFactory= oracle.xml.jaxp.JXSAXParserFactory
You can also set this programmatically:
System.setProperty("jaxax.xml.parsers .SAXParserFactory", "oracle.xml.jaxp.JXSAXParserFactory");
In addition to XDK, Oracle provides a native XML repository with Oracle9i Database Release 2. Oracle9i XML Database (XDB) is a high-performance, native XML storage and retrieval technology. It fully absorbs the W3C XML data model into Oracle9i Database and provides new standard access methods for navigating and querying XML. With XDB, you get all the advantages of relational database technology plus the advantages of XML technology.
The JDOM Package Structure
The JDOM library consists of six packages. First, the org.jdom package holds the classes representing an XML document and its components: Attribute, CDATA, Comment, DocType, Document, Element, EntityRef, Namespace, ProcessingInstruction, and Text. If you're familiar with XML, the class names should be self-explanatory.
Next is the org.jdom.input package, which holds classes that build XML documents. The main and most important class is SAXBuilder. SAXBuilder builds a document by listening to incoming Simple API for XML (SAX) events and constructing a corresponding document. When you want to build from a file or other stream, you use SAXBuilder. It uses a SAX parser to read the stream and then builds the document according to the SAX parser callbacks. A good part of this design is that as SAX parsers get faster, SAXBuilder gets faster. The other main input class is DOMBuilder. DOMBuilder builds from a DOM tree. This class comes in handy when you have a preexisting DOM tree and want a JDOM version instead.
There's no limit to the potential builders. For example, now that Xerces has the Xerces Native Interface (XNI) to operate at a lower level than SAX, it may make sense to write an XNIBuilder to support some parser knowledge not exposed via SAX. One popular builder that has been contributed to the JDOM project is the ResultSetBuilder. It takes a JDBC result set and creates an XML document representation of the SQL result, with various configurations regarding what should be an element and what should be an attribute.
The org.jdom.output package holds the classes that output XML documents. The most important class is XMLOutputter. It converts documents to a stream of bytes for output to files, streams, and sockets. The XMLOutputter has many special configuration options supporting raw output, pretty output, or compressed output, among others. It's a fairly complicated class. That's probably why this capability still doesn't exist in DOM Level 2.
Other outputters include the SAXOutputter, which generates SAX events based on the document content. Although seemingly arcane, this class proves extremely useful in XSLT transforms, because SAX events can be a more efficient way than bytes to transfer document data to an engine. There's also a DOMOutputter, which builds a DOM tree representation of the document. An interesting contributed outputter is the JTreeOutputter, which—with just a few dozen lines of code—builds a JTree representation of the document. Combine that with the ResultSetBuilder, and you can go from a SQL query to a tree view of the result with just a couple of lines of code.
Note that, unlike in DOM, documents are not tied to their builder. This produces an elegant model in which you have classes to hold data, various classes to construct data, and various other classes to consume the data. Mix and match as desired!
The org.jdom.transform and org.jdom.xpath packages have classes that support built-in XSLT transformations and XPath lookups.
Finally, the org.jdom.adapters package holds classes that assist the library in DOM interactions. Users of the library never need to call upon the classes in this package. The classes are there because each DOM implementation has different method names for certain bootstrapping tasks, so the adapter classes translate standard calls into parser-specific calls. The Java API for XML Processing (JAXP) provides another approach to this problem and actually reduces the need for these classes, but these classes remain because not all parsers support JAXP, nor is JAXP installed everywhere, due to license issues.
Creating a Document
Documents are represented by the org.jdom.Documentclass. You can construct a document from scratch:
// This builds: <root/> Document doc = new Document(new Element("root"));
Or you can build a document from a file, stream, system ID, or URL:
// This builds a document of whatever's in the given resource SAXBuilder builder = new SAXBuilder(); Document doc = builder.build(url);
Putting together a few calls makes it easy to create a simple document in JDOM:
// This builds: <root>This is the root</root> Document doc = new Document(); Element e = new Element("root"); e.setText("This is the root"); doc.addContent(e);
If you're a power user, you may prefer to use "method chaining," in which multiple methods are called in sequence. This works because the set methods return the object on which they acted. Here's how that looks:
Document doc = new Document( new Element("root").setText("This is the root"));
For a little comparison, here's how you'd create the same document, using JAXP/DOM:
// JAXP/DOM DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = factory.newDocumentBuilder(); Document doc = builder.newDocument(); Element root = doc.createElement("root"); Text text = doc.createText("This is the root"); root.appendChild(text); doc.appendChild(root);
Building with SAXBuilder
As shown earlier, SAXBuilder presents a simple mechanism for building documents from any byte-oriented resource. The default no-argument SAXBuilder() constructor uses JAXP behind the scenes to select a SAX parser. If you want to change parsers, you can set the javax.xml.parsers.SAXParserFactory system property to point at the SAXParser Factory implementation provided by your parser. For the Oracle9i Release 2 XML parser, you would do this:
java -Djavax.xml.parsers.SAXParserFactory= oracle.xml.jaxp.JXSAXParserFactory YourApp
For the Xerces parser, you would do this instead:
java -Djavax.xml.parsers.SAXParserFactory=org.apache.xerces.jaxp .SAXParserFactoryImpl YourApp
If JAXP isn't installed, SAXBuilder defaults to Apache Xerces. Once you've created a SAXBuilder instance, you can set several properties on the builder, including:
setValidation(boolean validate)
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This method tells the parser whether to validate against a Document Type Definition (DTD) during the build. It's false (off) by default. The DTD used is the one referenced within the document's DocType. It isn't possible to validate against any other DTD, because no parsers support that capability yet.
setIgnoringElementContentWhitespace(boolean ignoring)
This method tells the parser whether to ignore what's called ignorable whitespace in element content. Per the XML 1.0 spec, whitespace in element content must be preserved by the parser, but when validating against a DTD it's possible for the parser to know that certain parts of a document don't declare support for whitespace, so any whitespace in that area is "ignorable." It's false (off) by default. It's generally good to turn this on for a little performance savings, unless you want to "round trip" a document and output the same content as was input. Note that this flag has an effect only if validation is on, and validation causes a performance slowdown, so this trick is useful only when validation is already in use.
setFeature(String name, String value)
This method sets a feature on the underlying SAX parser. This is a raw pass-through call, so be very careful when using this method, because setting the wrong feature (such as tweaking namespaces) could break JDOM behavior. Furthermore, relying on any parser-specific features could limit portability. This call is most useful for enabling schema validation.
setProperty(String name, Object value)
This method sets a property on the underlying SAX parser. It's also a raw pass-through call, with the same risks and the same usefulness to power users, especially for schema validation. Putting together the methods, the following code uses the JAXP-selected parser to read a local file, with validation turned on and ignorable whitespace ignored.
SAXBuilder builder = new SAXBuilder(); builder.setValidation(true); builder.setIgnoringElementContentWhitespace(true); Document doc = builder.build(new File("/tmp/foo.xml"));
Writing with XMLOutputter
A document can be output to many different formats, but the most common is a stream of bytes. In JDOM, the XMLOutputter class provides this capability. Its default no-argument constructor attempts to faithfully output a document exactly as stored in memory. The following code produces a raw representation of a document to a file.
// Raw output XMLOutputter outp = new XMLOutputter(); outp.output(doc, fileStream);
If you don't care about whitespace, you can enable trimming of text blocks and save a little bandwidth:
// Compressed output outp.setTextTrim(true); outp.output(doc, socketStream);
If you'd like the document pretty-printed for human display, you can add some indent whitespace and turn on new lines:
outp.setTextTrim(true); outp.setIndent(" "); outp.setNewlines(true); outp.output(doc, System.out);
When pretty-printing a document that already has formatting whitespace, be sure to enable trimming. Otherwise, you'll add formatting on top of formatting and make something ugly.
Navigating the Element Tree
JDOM makes navigating the element tree quite easy. To get the root element, call:
Element root = doc.getRootElement();
To get a list of all its child elements:
List allChildren = root.getChildren();
To get just the elements with a given name:
List namedChildren = root.getChildren("name");
And to get just the first element with a given name:
Element child = root.getChild("name");
The List returned by the getChildren() call is a java.util.List, an implementation of the List interface all Java programmers know. What's interesting about the List is that it's live. Any changes to the List are immediately reflected in the backing document.
// Remove the fourth child allChildren.remove(3); // Remove children named "jack" allChildren.removeAll(root.getChildren("jack")); // Add a new child, at the tail or at the head allChildren.add(new Element("jane")); allChildren.add(0, new Element("jill"));
Using the List metaphor makes possible many element manipulations without adding a plethora of methods. For convenience, however, the common tasks of adding elements at the end or removing named elements have methods on Element itself and don't require obtaining the List first:
root.removeChildren("jill"); root.addContent(new Element("jenny"));
One nice perk with JDOM is how easy it can be to move elements within a document or between documents. It's the same code in both cases:
Element movable = new Element("movable"); parent1.addContent(movable); // place parent1.removeContent(movable); // remove parent2.addContent(movable); // add
With DOM, moving elements is not as easy, because in DOM elements are tied to their build tool. Thus a DOM element must be "imported" when moving between documents.
With JDOM the only thing you need to remember is to remove an element before adding it somewhere else, so that you don't create loops in the tree. There's a detach() method that makes the detach/add a one-liner:
parent3.addContent(movable.detach());
If you forget to detach an element before adding it to another parent, the library will throw an exception (with a truly precise and helpful error message). The library also checks Element names and content to make sure they don't include inappropriate characters such as spaces. It also verifies other rules, such as having only one root element, consistent namespace declarations, lack of forbidden character sequences in comments and CDATA sections, and so on. This feature pushes "well-formedness" error checking as early in the process as possible.
Handling Element Attributes
Element attributes look like this:
<table width="100%" border="0"> ... </table>
With a reference to an element, you can ask the element for any named attribute value:
String val = table.getAttributeValue("width");
You can also get the attribute as an object, for performing special manipulations such as type conversions:
Attribute border = table.getAttribute("border"); int size = border.getIntValue();
To set or change an attribute, use setAttribute():
table.setAttribute("vspace", "0");
To remove an attribute, use removeAttribute():
table.removeAttribute("vspace");
Working with Element Text Content An element with text content looks like this:
<description> A cool demo </description>
In JDOM, the text is directly available by calling:
String desc = description.getText();
Just remember, because the XML 1.0 specification requires whitespace to be preserved, this returns "\n A cool demo\n". Of course, as a practical programmer you often don't want to be so literal about formatting whitespace, so there's a convenient method for retrieving the text while ignoring surrounding whitespace:
String betterDesc = description.getTextTrim();
If you really want whitespace out of the picture, there's even a getTextNormalize() method that normalizes internal whitespace with a single space. It's handy for text content like this:
<description> Sometimes you have text content with formatting space within the string. </description>
To change text content, there's a setText() method:
description.setText("A new description");
Any special characters within the text will be interpreted correctly as a character and escaped on output as needed to maintain the appropriate semantics. Let's say you make this call:
element.setText("<xml/> content");
The internal store will keep that literal string as characters. There will be no implicit parsing of the content. On output, you'll see this:
<xml/> content<elt>
This behavior preserves the semantic meaning of the earlier setText() call. If you want XML content held within an element, you must add the appropriate JDOM child element objects.
Handling CDATA sections is also possible within JDOM. A CDATA section indicates a block of text that shouldn't be parsed. It is essentially a "syntactic sugar" that allows the easy inclusion of HTML or XML content without so many < and > escapes. To build a CDATA section, just wrap the string with a CDATA object:
element.addContent(new CDATA("<xml/> content"));
What's terrific about JDOM is that a getText() call returns the string of characters without bothering the caller with whether or not it's represented by a CDATA section.
Dealing with Mixed Content
Some elements contain many things such as whitespace, comments, text, child elements, and more:
<table> <!-- Some comment --> Some text <tr>Some child element</tr> </table>
When an element contains both text and child elements, it's said to contain "mixed content." Handling mixed content can be potentially difficult, but JDOM makes it easy. The standard-use cases—retrieving text content and navigating child elements—are kept simple:
String text = table.getTextTrim(); // "Some text" Element tr = table.getChild("tr"); // A straight reference
For more advanced uses needing the comment, whitespace blocks, processing instructions, and entity references, the raw mixed content is available as a List:
List mixedCo = table.getContent(); Iterator itr = mixedCo.iterator(); while (itr.hasNext()) { Object o = i.next(); if (o instanceof Comment) { ... } // Types include Comment, Element, CDATA, DocType, // ProcessingInstruction, EntityRef, and Text }
As with child element lists, changes to the raw content list affect the backing document:
// Remove the Comment. It's "1" because "0" is a whitespace block. mixedCo.remove(1);
If you have sharp eyes, you'll notice that there's a Text class here. Internally, JDOM uses a Text class to store string content in order to allow the string to have parentage and more easily support XPath access. As a programmer, you don't need to worry about the class when retrieving or setting text-only when acce—sing the raw content list.
For details on the DocType, ProcessingInstruction, and EntityRef classes, see the API documentation at jdom.org.
Coming in Part 2
In this article, I began examining how to use JDOM in your applications. In the next article, I examine XML Namespaces, ResultSetBuilder, XSLT, and XPath. Part 2 of this series follows in the November/December 2002 issue of Oracle Magazine.
Jason Hunter ([email protected]) is a consultant, publisher of Servlets.com, and vice president of the Apache Software Foundation. He holds a seat on the JCP Executive Committee.
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