- Introducing Java
- Installing the Xerces-J Parser
- Well-Formed Versus Valid Documents
- Document Type Definitions
- External DTDs
- Referential Integrity
- XML Schema
- Next Steps
Installing the Xerces-J Parser
Now that Java is installed and working, we need to install the actual parser that we'll use to check our data.
Development of applications, particularly those involving XML, revolves around the idea of an API, or Application Programming Interface. This is sort of like a minilanguage, in that it has a vocabulary where each command, or word, means something specific. Applications that use the same API can talk to each other more easily. Because of this, many XML-related programs are modular, in that different applications can be used for the same purpose, depending on the developer's preference.
API—An Application Programming Interface is a set of commands that programmers use to accomplish certain tasks. A program is said to implement an API if it can accomplish those tasks with those commands.
Modular—A modular program is one in which sections of code can be replaced without affecting the rest of the program.
This is especially true for parsers. Many parsers are available, but one of the most commonly used is Xerces, from the Apache Project.
The Apache Project is an open-source project that has produced the Apache Web Server and many other foundation products for the Web. Several corporations, including IBM, Sun, and Microsoft, have donated projects to Apache, making them freely available for the public use and to work on.
Xerces, and in particular its Java flavor, Xerces-J, is the base parser used with most of the more popular XML parsing applications, such as IBM's XML Parser for Java (XML4J, from which it actually originated) and Sun's Java API for XML Parsing (JAXP). We're going to use it directly to simplify installation issues, on the theory that every layer adds potential for problems that are out of the scope of this book to cover.
Installing Xerces-J
The Xerces-J parser can be found at
http://xml.apache.org/xerces-j/index.html
The current version as of this writing is 1.2.3, but we need to note a reality of XML development right now, and that is that things change. Specifically, the standards Xerces implements have not been finalized yet, so if there are changes, Xerces will have to keep up. Also, shortly before this writing, the group working on the Xerces-J project decided to change much of the focus of its work to add more simplicity and a new API, called JDOM. It is likely that by the time you read this, much of this new functionality will have been implemented, but for now we'll use the existing APIs.
Warning - Version numbers can change quickly in XML development. For instance, just during the writing of this book, Xerces has moved from version 1.1.3 through 1.2.0 to 1.2.3. Download the latest version of Xerces 1, but NOT Xerces 2. This refactoring is likely to be extremely different from version 1, and although you may eventually want to change over, it's better to learn the ropes on version 1, which is still an active project, first.
The important thing to remember is that although specific syntax may change, the concepts remain the same. If you're using a new version and something that looks right isn't working, check the documentation to see whether it's been changed.
To get the parser itself, click the Download button and look for the listings noted as "Latest Binaries," as shown in Figure 3.4. Download the zip file and put it in the same directory where you installed the Java SDK, but not in the Java itself. So, if the Java directory is c:/jdk1.3, put the zip file in c:\.
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Figure 3.4 Download the latest binaries.
To unpack the files, we're going to use a Java tool called jar. Go to the directory where you've placed the zip file and type
jar xf Xerces-J-bin.1.2.3.zip
This takes a minute or so and creates a directory called xerces-1_2_3 in that location.
Setting the CLASSPATH
Xerces requires that the CLASSPATH be set in order to function properly. (It also requires the PATH, but we set that already.) Add the following to your CLASSPATH the same way we added the current directory during the installation of the Java SDK:
c:\xerces-1_2_3\xerces.jar;c:\xerces-1_2_3\xercesSamples.jar;
For instance, in Windows 95/98, this line in the AUTOEXEC.bat file should now read
SET CLASSPATH=.;c:\xerces-1_2_3\xerces.jar;c:\xerces-1_2_3\xercesSamples.jar;
Remember, if you installed to a different location, make adjustments accordingly!
In Windows 95/98, this change won't take effect until you run AUTOEXEC.bat. If you run it manually, the changes will apply only to that window. After you restart, the changes will affect all windows.
Note - If you are installing on UNIX, you need to change the ";" to ":" and the "\" to "/". You also need to remove the reference to drive C:, of course.
Testing the Installation
If all has gone well, you now have a functioning parser installed on your system. We're nowhere near ready to start programming to it, however, so, as we did with Java itself, we'll test the installation with one of the samples.
In this case, however, it's a sample that is actually useful. The SAXCount demo analyzes an XML file to determine certain things about it, such as how many elements or spaces it contains, and outputs it to the screen.
Although this is helpful, it also gives us a chance to validate our data, as we'll see in a little while. To test the installation, run the AUTOEXEC.bat file (or reboot), and then type the following two lines:
cd c:\xerces-1_2_3 java sax.SAXCount data/personal.xml -v
This command tells Java to run the SAXCount program to analyze the personal.xml file in the data directory and to check it against the defined structure while it's at it. You should see output similar to
data/personal.xml: 2250 ms (37 elems, 18 attrs, 150 spaces, 128 chars)
If you get an error telling you the class is not defined or can't be found, make sure that CLASSPATH for that window is referencing the Xerces files. If necessary, make the changes to AUTOEXEC.bat and restart your machine.
If the program appears to function properly but the XML file itself seems to come up with errors, the parser is not validating properly. This can be caused by old class files in your CLASSPATH. If you can't remove them, at least make sure that they appear after the new ones.