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This chapter is from the book

This chapter is from the book

17.2 Java Xerces on Your Computer

We will now take you through the process of downloading and exploring the Xerces2 package. By the end of this section, you should know the basic layout of the package and have successfully run several Xerces example applications.

17.2.1 Downloading Java Xerces 2 Parser

The download location for the Java Xerces2 parser is at

http://xml.apache.org/dist/xerces-j/

or you can click Xerces Java 2 on http://xml.apache.org, then click Download. A menu of options appears as shown in Figure 17.1.

Figure 17.1 FIGURE 17.1 The Xerces download page.

Figuring out which file to download is easy. The working parser, packaged with supporting API documentation, is found in bin packages (bin being short for binaries or executables). Next, match the version you want to employ. There should be one 1.x.x version and one 2.x.x version. Only the latest of each parser version is available through this main page; older versions can be found at the bottom of the page (or with a little digging, if this page has changed).

There are many download options once you have selected your type and version of Xerces. The two of interest end with '.zip' and '.tar.gz'. Windows users recognize '.zip', just as Unix users know well the tared, "GunZipped" format. The latter format is actually a double-compressed file and almost always smaller (and quicker to download) than its .zip counterpart. What many Windows users do not realize is that they too can open .tar.gz files, using the familiar WinZip utility, so there is no reason not to download the smallest package for your version.

The bin package is only one of three options. src packages contain the full source code for each version. The tools package includes the ant source-code compiler, the junit test package, and xerces and xalan JAR files. If you intend to customize code (by downloading the src package), you might find these tools quite helpful. Again, the version numbering applies, just as for the bin packages.

17.2.2 Exploring the Xerces Package

After you have downloaded and unzipped your Xerces package, you will see a top-level layout of files and folders (Xerces version 2.0) as in Figure 17.2.

Figure 17.2 FIGURE 17.2 The Xerces2 project folder.

Table 17.2 contains a brief summary of contents. Explore these folders and files for a few minutes.

Table 17.2  Xerces2 Download Package, Top-level Files and Folders

Xerces2 Top-level Files and Directories

Contents

docs/

Contains the API documentation, including Javadocs

samples/

-A plethora of source code samples demonstrating DOM and SAX as well as XML UI and IO and the custom XNI Xerces features

data/

-Contains XML, Document Type Definition (DTD), and XML schema document files for the sample apps

License

The Apache-Xerces license

Readme.html

Links to the official API documentation

xercesImpl.jar

All the parser class files

xmlParserAPIs.jar

-Implementation for all the standard APIs (DOM Level 2, SAX 2.0 R2 PR1, and the javax.xml.parsers of JAXP 1.1)

xercesSamples.jar

-Compiled samples (class files) from the samples/folder


For experienced users of Xerces who might have expected to see a xerces.jar file (from the official documentation): "Xerces formerly came with a file called xerces.jar. This file contained all of the parser's functionality. Two files are now included: xercesImpl.jar, our implementation of various APIs, and xmlParserAPIs.jar, the APIs themselves. This was done so that, if your XSLT [XSL Transformations] processor ships with APIs at the same level as those supported by Xerces-J, you can avoid putting xmlParserAPIs.jar on your classpath."

17.2.3 -Running the Samples

Installation and execution of Java applications has always been a breeze. Certainly you know by now that you must have Java installed on your system, which simply means that a virtual machine is accessible on your path. Visit http://java.sun.com to locate and download a Java virtual machine/Java Runtime Environment (JRE) or a full Java Development Kit (JDK); most packages have an extractor or installation program to take care of all details. After you have Java installed, running Xerces samples is a piece of cake.

Xerces Java samples run in JDK 1.x and Java 2 (Java 1.2.x, 1.3, and later) except those with a user interface (the samples in the samples\ui folder), which require Java 2 because they were written in Swing 1.1. The Xerces parser includes over a dozen complete code samples. This is code you can reuse immediately to see quick results, and these samples reveal techniques that go even beyond this chapter. The following list contains some of the more useful sample classes, with brief descriptions summarized from the official documentation:

dom.Counter: Shows the duration and count of elements, attributes, ignorable whitespaces, and characters appearing in the document. Three durations are shown: the parse time, the first traversal of the document, and the second traversal of the tree.

dom.DOMAddLines: Illustrates how to add lines to the DOM Node and override methods from DocumentHandler, how to turn off ignorable whitespaces by overriding ignorableWhitespace, how to use the SAX Locator to return row position (line number of DOM element), and how to attach user-defined objects to Nodes by using the setUserData method.

dom.GetElementsByTagName: Illustrates how to use the Document#getElementsByTagName() method to quickly and easily locate elements by name. This sample is also a DOM filter.

dom.Writer: Illustrates how to traverse a DOM tree to print a document that is parsed.

dom.ASBuilder: Illustrates how to preparse XML schema documents and build schemas, and how to validate XML instances against those schemas, using DOM Level 3 classes.

sax.Counter: Illustrates how to register a SAX2 ContentHandler and receive the callbacks to print information about the document. The output of this program shows the time and count of elements, attributes, ignorable whitespaces, and characters appearing in the document.

sax.DocumentTracer: Provides a complete trace of SAX2 events for files parsed. This is useful for making sure a SAX parser implementation faithfully communicates all information in the document to the SAX handlers.

sax.Writer: Illustrates how to register a SAX2 ContentHandler and receive the callbacks to print a document that is parsed.

socket.DelayedInput: Delays the input to the SAX parser to simulate reading data from a socket where data is not always immediately available.

socket.KeepSocketOpen: Provides a solution to the problems of sending multiple XML documents over a single socket connection or sending other types of data after the XML document without closing the socket connection.

ui.TreeView: Allows exploration of XML nodes via a tree panel in a Java Swing application. This sample also reveals document messages sent as the tree is traversed.

The names of the samples in the preceding list are used to invoke the corresponding code in the xercesSamples.jar package. If all three Xerces JAR files are placed in the classpath, running the sax.DocumentTracer sample, for example, is as simple as typing the following anywhere on the command line:

java sax.DocumentTracer my.xml

in which my.xml is any XML file (in the current path). If you do not want to add all three JAR files to your classpath, use the -cp option, with each JAR file listed afterward (separated by semicolons), as shown in Figure 17.3.

Figure 17.3 FIGURE 17.3 Execution and output of first Xerces samples.

If you have a Java virtual machine version 1.3 or later, find the ext folder. For Windows users, this is typically:

C:\Program Files\JavaSoft\JRE\1.4\lib\ext\

Here you can place JAR files for global access, just as if you had placed each JAR file in your classpath. This makes using the -cp option unnecessary.

We conclude this section with one more sample, just for fun (and also because it might come in useful for message tracing in the future). This is the UI example from the sample set, called "TreeView," which we are going to feed our address.xml file (make sure the address.xsd file is also in the current path). At the command line, enter the following line (presuming, this time, that you do not have the Xerces JAR files in the classpath):

java -cp xercesImpl.jar;xmlParserAPIs.jar;xercesSamples.jar

ui.TreeView address.xml

Within a few seconds, a brand-new window appears, similar to the window shown in Figure 17.4.

Figure 17.4 FIGURE 17.4 Output of Xerces TreeView sample.

In this window, you can expand the entire tree, refresh the view, and load new documents. (You must load an XML file from the command line initially, though; otherwise, the window will not appear.)

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