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As the name implies, the Java API for XML Processing (JAXP) is an
API designed
to help you write programs for processing XML documents. JAXP is
very important for many reasons, not the least of which is the
fact that it is a critical part of the Java Web Services Developer Pack
(Java WSDP).
This is the second lesson in a series designed to initially help you
understand how to use JAXP,
and to eventually help you understand how to use the Java WSDP.
XML is an acronym for the eXtensible Markup Language.
I will not attempt to teach XML in this series of
tutorial lessons. Rather, I will assume that you already
understand
XML, and I will teach you how to use JAXP to write programs for
creating and processing XML documents.
I have published numerous tutorial lessons on XML at Gamelan.com and www.DickBaldwin.com.
You may find it useful to refer to those lessons. In addition, I
provided
a review of the salient aspects of XML in the first lesson in this
series. From time to time, I will also provide background
information regarding XML in the lessons in this series. For
example, I will provide background information on XSL and XSL
Transformations (XSLT) later in this document under General Background
Information on XSLT.
Viewing tip
You may find it useful to open another copy of this lesson in a
separate browser window. That will make it easier for you to
scroll back and forth among the different listings and figures while
you are reading about them.
Supplementary material
I recommend that you also study the other lessons in my extensive
collection of online Java tutorials. You will find those lessons
published at Gamelan.com.
However, as of the date of this writing, Gamelan doesn't maintain a
consolidated index of my Java tutorial lessons, and sometimes
they are difficult to locate there. You will find a consolidated
index at www.DickBaldwin.com.
As of this writing, to my knowledge, Microsoft IE is the only
widely-used web browser that has the ability to do a good job of
rendering XML documents. IE can render XML documents using either
Cascading Style Sheets (CSS) or XSL. Thus, IE provides a good
vehicle for testing XSLT files that you intend to use with JAXP.
What is the W3C?
For purposes of this lesson, the W3C is a
governing body that has published many important documents on XSL and
XSLT, two of which will be referenced later in this document.
What is XSL?
XSL is an acronym for Extensible Stylesheet Language.
According to the W3C, XSL
is
a language for expressing stylesheets. It consists of two parts:
A language for transforming XML documents, and
An XML vocabulary for specifying formatting semantics.
Again, according to the W3C,
"An XSL stylesheet specifies the presentation of a class
of XML documents by describing how an instance of the class is
transformed into an XML document that uses the formatting vocabulary."
Separating content from presentation
As you are probably aware by now, one of the primary virtues of XML
is the ability to separate content from presentation.
In other words, an XML document contains structured information, but
does not provide any hints as to how that information should be
rendered for the benefit of a consumer.
"This specification defines the syntax and semantics of
XSLT, which is a language for transforming XML documents into other XML
documents.
XSLT is designed for use as part of XSL, which is a stylesheet
language for XML. In addition to XSLT, XSL
includes an XML vocabulary for specifying formatting. XSL specifies
the styling of an XML document by using XSLT to describe how the
document
is transformed into another XML document that uses the formatting
vocabulary.
XSLT is also designed to be used independently of XSL. However,
XSLT is not intended as a completely general-purpose XML transformation
language. Rather it is designed primarily for the kinds of
transformations that are needed when XSLT is used as part of XSL."
Transforming XML to other formats
Because an HTML document can be represented as an XML document, XSLT
can be used to transform XML documents into HTML documents. This
makes it possible to render the information contained in an XML
document using a common HTML Web browser. Thus, one useful way to
view the contents of an XML document is to transform it into an HTML
document and view it using a standard Web browser.
Where does the transformation take place?
When transforming information from an XML document for rendering on
an HTML browser, the transformation can take place anywhere between the
XML document and the browser.
Transforming on the server
For example, an XSLT engine could be written in Java and run as a
servlet, or it could be written as a JavaBeans component and accessed
from a scriptlet in a JavaServer page (JSP).
Transforming at the browser
Or, the transformation could be performed at the browser. For
example, Microsoft IE can be used for this purpose.
As you learned in the previous lesson, a DOM parser can be used to
create a tree structure in memory that represents an XML
document. In Java, that tree structure is encapsulated in an
object of the interface type Document. Document
and its superinterface Node declare numerous methods. As
is always the case, classes that implement Document must
provide concrete definitions of those methods.
Many operations are possible
Thus, given an object of type Document, there are many
methods that
can be invoked on the object to perform a variety of operations.
For example, it is possible to move nodes from one location in the tree
to another location in the tree, thus rearranging the structure of the
XML document represented by the Document object. It is
also possible to delete nodes, and to insert new nodes. As you
saw in the sample program in the previous lesson, it is also possible
to
recursively traverse the tree, extracting information about the nodes
along
the way.
I showed you ...
In the previous lesson, I showed you how to:
Use JAXP, DOM, and an input XML file to create a Document object
that represents the XML file.
Recursively traverse the DOM tree, gathering information about
each node in the tree along the way.
Use the information about the nodes to create a new XML file that
represents the Document object.
The unmodified Document object represented the original XML
file in the previous lesson. The DOM tree was not modified in
that example. The final XML file represented the unmodified Document
object, which represented the original XML file. Therefore, the
final XML file was functionally equivalent to the original XML file.
Something of an overkill
The things that you learned in the previous lesson about traversing
the tree structure and gathering information about each node in the
tree will serve you well in the future. However, if all you need
to do is to write an output XML file that represents a DOM tree, there
is an easier way to do that using XSLT. That is the primary topic
of this lesson.
For simplicity, I elected not to show you how to write exception
handlers that produce meaningful output in the event of parser errors
in the previous lesson. I will also cover that topic in this
lesson.
Nothing fancy intended
The sample program that I will explain in this lesson is not
intended to do anything fancy. It is intended simply to introduce
you to the
use of XSLT to transform DOM objects in Java programs.
Discussion
and Sample Code
The sample program consists of a single class named Xslt01.
For purposes of illustration, the program operates on two XML
files. One of the XML files is named Xslt01.xml.
The other XML file is named Xslt01bad.xml. The first XML
file is well formed, and is used to illustrate the behavior of the
program in the absence of parser errors. The second XML file is
not well formed, and is used to illustrate the behavior of the program
in the face of parser errors.
(You could, of course, use the program to operate on
other XML files of your own design.)
As is often the case, I will discuss the program code in
fragments. Complete listings of all three files are shown in
Listings 9, 10, and 11 near the end of the lesson.
The XML file named Xslt01.xml
I will begin my discussion with the XML file named Xslt01.xml.
A complete listing of this file is shown in Listing 10 near the
end of the lesson. This is a relatively simple XML file.
Assuming that you understood the material in the previous lesson, there
should be no surprises in the file named Xslt01.xml. This
file will be used to test the program for the case where there are no
parser errors.
The XML file named Xslt01bad.xml
A complete listing of the file named Xslt01bad.xml is shown in
Listing 11 near the end of the lesson. This file is not well
formed. It is missing a right angle bracket at the end of line 6,
resulting in a bad end tag for the element named line.
Again, assuming that you understood the material in the previous
lesson, there should be no surprises in the file named Xslt01bad.xml.
This file will be used to test the program for the case where there are
parser errors.
The class named Xslt01
The entire program in contained in a class named Xslt01.
A complete listing of the program is shown in Listing 9 near the
end of the lesson.
Behavior of the program
This program is a modification of the program named Dom02 that
was discussed in the previous lesson. The program was modified to
use
an identity XSL Transformer object to format an output XML
file in
place of a call to Dom02Writer, as was the case in the previous
program.
This modification resulted in a much simpler and probably more reliable
program.
The program was also modified to display the output XML on the Standard
Output Device (typically the screen) as well as to provide
meaningful output in the event of a parsing error.
This program shows you how to:
Create a Document object using JAXP, DOM, and an input
XML file.
Create an identity XSL Transformer object.
Use the identity Transformer object to display the XML
represented by the Document object on the Standard Output
Device.
Use the identity Transformer object to write the XML
represented by the Document object into an output file.
Provide meaningful output in the case of a parser error.
Operation of the program
The program requires two command-line arguments. The input XML
file name is provided by the user as the first command-line
argument.
The output XML file name is provided by the user as the second
command-line
argument.
Get a DOM parser object
The program begins by instantiating a DOM parser object of type DocumentBuilder
based on JAXP. The parser is configured as a non-validating
parser.
Create a DOM tree as a Document object
The program uses the parse method of the parser object to parse
an XML file specified on the command line. The parse method
returns an object of type Document that represents the parsed
XML file.
Get an identity Transformer object
Then the program gets a TransformerFactory object and uses
that object to get an identity Transformer object capable of
performing a copy of a source to a result.
Get a Source object
Following this, the program uses the Document object to get a DOMSource
object that implements the Source interface, and acts as a
holder for a transformation source tree in the form of a DOM tree.
Get a Result object
Then the program gets a StreamResult object that implements
the Result interface, and points to the standard output
device. This object acts as a holder for a transformation result.
Transform the DOM tree
Having gone through the preparation steps, the program uses the Transformer
object, the DOMSource object, and the StreamResult object
to transform the DOM tree to text and display it on the standard output
device (the screen).
Having transformed the Document on the screen, the program gets
another StreamResult object that points to an output
file.
Then it transforms the DOM tree to XML text, and writes it into the
output
file.
Handle errors and exceptions
The program catches and handles a variety of different types of errors
and exceptions and provides meaningful output in the event of parser
errors. An XML document that is not well formed is used to
illustrate the ability
to display meaningful information in the event of a parser error.
Miscellaneous comments about the program
The program was tested using SDK 1.4.2 and WinXP with two different XML
files. The XML file named Xslt01.xml is well formed, and
is shown in Listing 10 near the end of the lesson.
The XML file named Xslt01bad.xml is not well formed and is
shown in Listing 11 near the end of the lesson. This file was
purposely corrupted, and is missing a right angle bracket in the
closing tag of a line
element. This file is used to test for parser errors. I
will show you the output produced by this file later in the lesson
under
the discussion of the catch block for exceptions of type SAXParseException.
Let's see some code
The program named Xslt01 begins in Listing 1, which shows the
beginning of the class definition and the beginning of the main
method.
public class Xslt01{
public static void main(String argv[]){ if (argv.length != 2){ System.err.println( "usage: java Xslt01 fileIn fileOut"); System.exit(0); }//end if
Listing 1
The code in Listing 1 simply checks to confirm that the user has
entered the correct number of command-line arguments, and aborts if the
user has failed to enter the correct number.
Steps for creating a Document object
As you will recall from the previous lesson, three steps
are required to create a Document object:
Create a DocumentBuilderFactory object
Use the DocumentBuilderFactory object to create a DocumentBuilder
object
Use the parse method of the DocumentBuilder object
to create a Document object
These three steps are illustrated by the three statements in Listing
2.
(We will probably see these three statements in many different
programs
in this series of lessons.)
Document document = parser.parse( new File(argv[0]));
Listing 2
The DocumentBuilderFactory Class
Reviewing some of what you learned in the previous lesson, the DocumentBuilderFactory
class
"Defines a factory API that enables applications to
obtain a parser that produces DOM object trees from XML documents."
The DocumentBuilderFactory class extends Object, and
defines about fifteen methods, one of which is a static method
named newInstance. The newInstance method is used
to create an object of the DocumentBuilderFactory class (as
shown in Listing 2).
The class also defines the newDocumentBuilder instance method,
which is used to create objects of the DocumentBuilder class (also
shown in Listing 2).
The DocumentBuilder Class
The DocumentBuilder class
"Defines the API to obtain DOM Document instances from
an
XML document."
This class also extends Object, and defines about
ten methods, which include several overloaded versions of the parse
method. When the parse method is invoked and passed an
input source containing XML, the method returns a Document
object (DOM tree) that represents the XML.
(In Listing 2, the parse method is passed a
reference to a File object that represents the input XML file.)
The Document interface
Document is an interface in the org.w3c.dom package,
which extends the Node interface belonging to the same
package. When we invoke the parse method, it returns a
reference to an object instantiated from a class that implements the Document
interface.
(The reference is returned as type Document, not
as the name of the class from which the object was actually
instantiated. Because Document extends Node,
that object could also be treated as type Node when
appropriate.)
According to Sun:
"The Document interface represents the entire HTML or
XML
document. Conceptually, it is the root of the document tree, and
provides
the primary access to the document's data."
Steps for creating a Transformer object
This information is new to this lesson. The following two steps
are required to create an identity Transformer object.
Create a TransformerFactory object by invoking the
static newInstance method of the TransformerFactory class.
Invoke the newTransformer method on the TransformerFactory
object.
These two steps are illustrated by the code in Listing 3.
//Get a TransformerFactory object TransformerFactory xformFactory = TransformerFactory.newInstance(); //Get an XSL Transformer object Transformer transformer = xformFactory.newTransformer();
Listing 3
The TransformerFactory class
A TransformerFactory instance can be used to create Transformer and
Templates objects.
(This lesson does not discuss Templates
objects.
That is a topic for a future lesson.)
In a programming style that should by now be familiar, this class
provides a static method named newInstance. Invocation of
the newInstance method returns a reference to a new instance of
TransformerFactory.
The newTransformer method
A TransformerFactory object provides two overloaded versions
of the newTransformer method. Invocation of the version
of newTransformer that takes no parameters (on an instance
of TransformerFactory) returns a reference to a new Transformer
object that performs a
copy of a source to a result. Some authors refer
to
this as the identity transform.
The code in Listing 3 produces such a Transformer object, and
saves the object's reference in a variable named transformer.
The other overloaded
version of the newTransformer method takes a parameter that
represents an XSL
stylesheet, and returns a Transformer object that implements
the instructions
in the stylesheet. I will show you how to use that version is a
future
lesson.
The Transformer class
Here is some of what Sun has to say about an object of the Transformer
class:
"An instance of this abstract class can transform a
source tree into a result tree.
An instance of this class can be obtained with the
TransformerFactory.newTransformer method. This instance may then be
used to process XML from a variety of
sources and write the transformation output to a variety of sinks."
The transform method
The transform method of the Transformer class is
partially described in Figure 1.
public abstract void transform( Source xmlSource, Result outputTarget) throws TransformerException
Process the source tree to the output result.
Parameters: xmlSource - The input for the source tree. outputTarget - The output target.
Figure 1
As you can see, this method requires two parameters:
A reference to an object of type Source
A reference to an object of type Result
The method processes the Source to produce the Result.
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