XML Parsing With DOM and Xerces (part 1) - When Laziness Is A Virtue
(Page 5 of 5 )
In the example above, I've manually written code to handle each level of the tree for illustrative purposes - however, in a production environment, doing this is pure insanity, especially since an XML document can have any number of nested levels. A far more professional approach would be to write a recursive function to automatically iterate through the document tree - this results in cleaner, more readable code, and it's also much, much easier to maintain.
In order to understand the difference, consider this next example, which uses a recursive function to parse a more complex XML document. Here's the XML,
<?xml version="1.0"?>
<inventory>
<!-- time to lock and load -->
<item>
<id>758</id>
<name>Rusty, jagged nails for
nailgun</name>
<supplier>NailBarn, Inc.</supplier>
<cost
currency="USD">2.99</cost>
<quantity alert="500">10000</quantity>
</item>
<item>
<id>6273</id>
<name>Power
pack for death ray</name>
<supplier>QuakePower.domain.com</supplier>
<cost currency="USD">9.99</cost>
<quantity alert="20">10</quantity>
</item>
</inventory>
and here's the Java code to parse it:
import org.apache.xerces.parsers.DOMParser;
import org.w3c.dom.*;
import java.io.*;
public
class MyThirdDomApp {
// a counter for keeping track of the "tabs"
private
int TabCounter = 0;
// constructor
public MyThirdDomApp (String
xmlFile) {
// create a Xerces DOM parser
DOMParser parser
= new DOMParser();
// parse the document and
// access
the root node with its children
try {
parser.parse(xmlFile);
Document document = parser.getDocument();
NodeDetails(document);
} catch (IOException e) {
System.err.println (e);
}
}
// this is a recursive function to traverse the document
tree
private void NodeDetails (Node node) {
String Content = "";
int type = node.getNodeType();
// check if element
if (type == Node.ELEMENT_NODE) {
FormatTree(TabCounter);
System.out.println ("Element: " + node.getNodeName() );
//
check if the element has any attributes
if(node.hasAttributes())
{
// if it does, store it in a NamedNodeMap object
NamedNodeMap AttributesList = node.getAttributes();
// iterate
through the NamedNodeMap and get the attribute
names and
values
for(int j = 0; j < AttributesList.getLength(); j++) {
FormatTree(TabCounter);
System.out.println("Attribute: "
+
AttributesList.item(j).getNodeName() + " = " +
AttributesList.item(j).getNodeValue());
}
}
} else if (type == Node.TEXT_NODE) {
// check if text node and print value
Content = node.getNodeValue();
if (!Content.trim().equals("")){
FormatTree(TabCounter);
System.out.println ("Character data: " + Content);
}
} else if (type == Node.COMMENT_NODE) {
// check if comment
node and print value
Content = node.getNodeValue();
if (!Content.trim().equals("")){
FormatTree(TabCounter);
System.out.println ("Comment: " + Content);
}
}
// check if current node has any children
NodeList children
= node.getChildNodes();
if (children != null) {
// if it does,
iterate through the collection
for (int i=0; i< children.getLength();
i++) {
TabCounter++;
// recursively call function
to proceed to next level
NodeDetails(children.item(i));
TabCounter--;
}
}
}
// this formats the
output for the generated tree
private void FormatTree (int TabCounter) {
for(int j = 1; j < TabCounter; j++) {
System.out.print("t");
}
}
// the main method to create an instance of our DOM application
public static void main (String[] args) {
MyThirdDomApp MyThirdDomApp
= new MyThirdDomApp (args[0]);
}
}
Here's the output:
Element: inventory
Comment: time to lock and load
Element: item
Element: id
Character data: 758
Element: name
Character data: Rusty, jagged nails for nailgun
Element: supplier
Character data: NailBarn, Inc.
Element: cost
Attribute:
currency = USD
Character data: 2.99
Element: quantity
Attribute: alert = 500
Character data: 10000
Element: item
Element: id
Character data: 6273
Element: name
Character data: Power pack for death ray
Element: supplier
Character data: QuakePower.domain.com
Element: cost
Attribute:
currency = USD
Character data: 9.99
Element: quantity
Attribute: alert = 20
Character data: 10
Now, wasn't that easier than manually writing code for each level of the document
tree?
This should be easily understandable if you're familiar with the concept of recursion. Most of the work happens in the NodeDetails() function, which now includes additional code to iterate through the different levels of the document tree automatically, and to make intelligent decisions about what to do with each node type found.
// this is a recursive function to traverse the document tree
private void NodeDetails
(Node node) {
// snip
// check if element
if (type == Node.ELEMENT_NODE)
{
FormatTree(TabCounter);
System.out.println ("Element: " + node.getNodeName()
);
// check if the element has any attributes
if(node.hasAttributes())
{
// if it does, store it in a NamedNodeMap object
NamedNodeMap
AttributesList = node.getAttributes();
// iterate through the NamedNodeMap
and get the attribute names
and
values
for(int j = 0; j < AttributesList.getLength();
j++) {
FormatTree(TabCounter);
System.out.println("Attribute:
" +
AttributesList.item(j).getNodeName()
+ " = " + AttributesList.item(j).getNodeValue());
}
}
}
// snip
}
// snip
}
If the node is an element, the element name is printed to the standard output
device. A check is then performed for element attributes; if they exist, they are returned as a NamedNodeMap object (essentially, an array whose elements can be accessed either by integer or string) and can be processed and displayed using methods exposed by that object. If you know the name of the attribute, the getNamedItem() method can be used to retrieve the corresponding value; if you don't (as in the example above), the getLength() and item() methods can be used in combination with a loop to iterate through the list of attributes.
// this is a recursive function to traverse the document tree
private void NodeDetails
(Node node) {
// snip
// check if element
if (type == Node.ELEMENT_NODE)
{
// snip
} else if (type == Node.TEXT_NODE) {
// check if
text node and print value
Content = node.getNodeValue();
if
(!Content.trim().equals("")){
FormatTree(TabCounter);
System.out.println ("Character data: " + Content);
}
} else if (type
== Node.COMMENT_NODE) {
// check if comment node and print value
Content = node.getNodeValue();
if (!Content.trim().equals("")){
FormatTree(TabCounter);
System.out.println ("Comment: "
+ Content);
}
}
// snip
}
In a similar manner, it's also possible to check for text nodes, comments and
any other node type, and write code to process each type individually. The example above handles text nodes and comments, printing each one to the standard output device as they are encountered. Note that, again, the getNodeValue() function is used to extract the raw value of the node - it must be nice to be so popular!
Finally, once the node has been processed, it's time to see if it has any children, and proceed to the next level of the tree if so.
// this is a recursive function to traverse the document tree
private void NodeDetails
(Node node) {
// snip
// check if current node has any children
NodeList children = node.getChildNodes();
if (children != null) {
// if it does, iterate through the collection
for (int i=0; i< children.getLength();
i++) {
TabCounter++;
// recursively call function to proceed
to next level
NodeDetails(children.item(i));
TabCounter--;
}
}
}
In the event that the node does have children, the children are stored in a NodeList,
and the NodeDetails() function is recursively called for each of these nodes. And so on, and so on, ad infinitum...or at least until the entire tree has been processed.
Finally, the very simple FormatTree() method checks the value of the tab counter to determine the current depth within the XML tree, and displays that many spaces in the output in a primitive attempt to represent the data as a tree.
// this formats the output for the generated tree
private void FormatTree
(int TabCounter) {
for(int j = 1; j < TabCounter; j++) {
System.out.print("t");
}
}
As with most things - easy when you know how.
Obviously, this is just one illustration of the applications of the Xerces DOM parser. This is probably enough to get you started with simple Java/XML applications...but you can do a lot more with Xerces than just this.
In the second part of this article, I'll build on everything you just learnt to demonstrate how the Xerces DOM parser can be combined with JSP to format XML documents for a Web browser. I'll also take a look at the error-handling functions built into the parser, demonstrating how they can be used to trap and catch errors in XML processing. Make sure you come back for that one!
Note: All examples in this article have been tested with JDK 1.3.0, Apache 1.3.11, mod_jk 1.1.0, Xerces 1.4.4 and Tomcat 3.3. Examples are illustrative only, and are not meant for a production environment. YMMV!
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