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Parsing XML to Decipher XML Documents
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We've already seen what an XML document looks like, such as this representative example (we'll see how to construct XML documents like this one in this chapter): < XML version = "10" >
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<!DOCTYPE DOCUMENT [
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<!ELEMENT DOCUMENT (CUSTOMER)*>
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<!ELEMENT CUSTOMER (NAME,DATE,ORDERS)>
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<!ELEMENT NAME (LASTNAME,FIRSTNAME)>
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<!ELEMENT LASTNAME (#PCDATA)>
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<!ELEMENT FIRSTNAME (#PCDATA)>
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<!ELEMENT DATE (#PCDATA)>
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<!ELEMENT ORDERS (ITEM)*>
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<!ELEMENT ITEM (PRODUCT,NUMBER,PRICE)>
<!ELEMENT PRODUCT (#PCDATA)>
<!ELEMENT NUMBER (#PCDATA)>
<!ELEMENT PRICE (#PCDATA)>
<DOCUMENT>
<CUSTOMER>
<NAME>
<LASTNAME>Edwards</LASTNAME>
<FIRSTNAME>Britta</FIRSTNAME>
</NAME>
<DATE>April 17, 1998</DATE>
<ORDERS>
<ITEM>
<PRODUCT>Cucumber</PRODUCT>
<NUMBER>5</NUMBER>
<PRICE>$125</PRICE>
</ITEM>
<ITEM>
<PRODUCT>Lettuce</PRODUCT>
<NUMBER>2</NUMBER>
<PRICE>$98</PRICE>
</ITEM>
</ORDERS>
</CUSTOMER>
<CUSTOMER>
<NAME>
<LASTNAME>Thompson</LASTNAME>
<FIRSTNAME>Phoebe</FIRSTNAME>
</NAME>
<DATE>May 27, 1998</DATE>
<ORDERS>
<ITEM>
<PRODUCT>Banana</PRODUCT>
<NUMBER>12</NUMBER>
<PRICE>$295</PRICE>
</ITEM>
<ITEM>
<PRODUCT>Apple</PRODUCT>
<NUMBER>6</NUMBER>
<PRICE>$150</PRICE>
</ITEM>
</ORDERS>
</CUSTOMER>
</DOCUMENT>
As you can see, all the information in an XML document like the one above is packed pretty densely To make use of that information, we have to unpack it, and that process is known as parsing Parsing an XML document is the first step in working with that document To work with an XML document in code, we have to be able to access the elements in that document, and we do that by parsing the document, which breaks the XML document up into a logical tree This tree represents the structure of the XML document, as we'll see It is this tree that we'll work with when we work with XML documents For example, we can parse this small XML document: <CUSTOMER>
<!-- I am a comment -->
<NAME>Britta Edwards</NAME>
</CUSTOMER>
When we parse this document, this is the tree generated[md]note that the text Britta Edwards is considered a child of the <NAME> element (as mentioned in the last chapter, PCDATA stands for parsed character data[md]XML data that has no child data beneath it): Document
----Element(type=ELEMENT, name=CUSTOMER, numChildren=2)
|---Element(type=COMMENT, text="I am a comment")
---Element(type=ELEMENT, name=NAME, numChildren=1)
----Element(type=PCDATA, text="Britta Edwards")
After parsing an XML document, we can work through it element by element, looping over each element's child elements, as we'll see
Using the Microsoft XML Parser
We are ready to start loading XML documents into Java programs and parsing those documents We'll use the Microsoft XML parser in this book; you can get a copy of this parser at http://www microsoftcom/standards/xml/xmlparsehtm (Please be aware that net addresses change, so you might have to search the Microsoft site, http://microsoftcom for the XML parser) We'll refer to this parser as the MSXML parser Installing the MSXML parser depends on your installation of Java and the version of Java you're using You download the file xmlinst exe and run it, which extracts the Microsoft COM Java package Typically, you place this package (eg, drag the COM folder in the Windows 95 Explorer) into your java\classes directory, so the Java programs you write can find the parser class files easily Then, for example, the MSXML Document class, commsxmlDocument, will be stored as java\classes\com\xml\Documentclass Here are the major MSXML classes and what they do: Microsoft Parser Class Does This + Handles XML elements + Internal to the parser + Stores an element's attributes + Loads and handles XML documents + Handles parsing exceptions To load in an XML document, you use the Document class' + method Let's put that to work at once as we see how to load in an XML document into a working Java program
The showtext Application
Our first MSXML parser application will get us started with using the MSXML parser In this example, we'll see how to load in an XML document and print the character data (ie, the text) of that document We'll write this program as a console-oriented program first and then as a window-oriented program This will give us experience with the MSXML parser in both console-oriented and window-oriented programming (we'll use window-oriented programming for the most part in this book) Start the new file, showtextjava, now, and add a main() method this way: class showtext
public static void main(String args[])
The next step is to load in an XML document, and to do that, we'll need an MSXML Document object
Creating a Document Object
The XML document we'll work with is named +: < XML version = "10" >
<!DOCTYPE DOCUMENT [
<!ELEMENT DOCUMENT (CUSTOMER)*>
<!ELEMENT CUSTOMER (NAME,DATE,ORDERS)>
<!ELEMENT NAME (LASTNAME,FIRSTNAME)>
<!ELEMENT LASTNAME (#PCDATA)>
<!ELEMENT FIRSTNAME (#PCDATA)>
<!ELEMENT DATE (#PCDATA)>
<!ELEMENT ORDERS (ITEM)*>
<!ELEMENT ITEM (PRODUCT,NUMBER,PRICE)>
<!ELEMENT PRODUCT (#PCDATA)>
<!ELEMENT NUMBER (#PCDATA)>
<!ELEMENT PRICE (#PCDATA)>
<DOCUMENT>
<CUSTOMER>
<NAME>
<LASTNAME>Edwards</LASTNAME>
<FIRSTNAME>Britta</FIRSTNAME>
</NAME>
<DATE>April 17, 1998</DATE>
<ORDERS>
<ITEM>
<PRODUCT>Cucumber</PRODUCT>
<NUMBER>5</NUMBER>
<PRICE>$125</PRICE>
</ITEM>
<ITEM>
<PRODUCT>Lettuce</PRODUCT>
<NUMBER>2</NUMBER>
<PRICE>$98</PRICE>
</ITEM>
</ORDERS>
</CUSTOMER>
<CUSTOMER>
<NAME>
<LASTNAME>Thompson</LASTNAME>
<FIRSTNAME>Phoebe</FIRSTNAME>
</NAME>
<DATE>May 27, 1998</DATE>
<ORDERS>
<ITEM>
<PRODUCT>Banana</PRODUCT>
<NUMBER>12</NUMBER>
<PRICE>$295</PRICE>
</ITEM>
<ITEM>
<PRODUCT>Apple</PRODUCT>
<NUMBER>6</NUMBER>
<PRICE>$150</PRICE>
</ITEM>
</ORDERS>
</CUSTOMER>
</DOCUMENT>
We will load this document with the MSXML Document class' load() method and display the text in the document with the Document class' getText() method The MSXML Document class' methods appear in Table 21 Table 21 The Document Class' Methods Method addChild(Element, Element) Does This Override of the Element method that makes it possible to find the root node via Document getRoot Creates a new element for a given tag and element type using the ElementFactory for this document Construct a new empty document Construct a new empty document and use the given ElementFactory when loading XML documents Returns null Returns an enumeration that will never have any attributes because the Document class doesn't support attributes Returns enumeration for root node Returns the external id specified in the <! DOCTYPE> tag Returns the name specified in the <!DOCTYPE> tag Returns null
createElement(String, int) Document() Document(ElementFactory) getAttribute(String) getAttributes() getChildren() getId() getName() getParent()
getRoot() getTagName() getText() getType() getURL() getXML() load(InputStream) load(URL) numAttributes() numElements() removeAttribute(String) removeChild(Element) reportError(ParseException, OutputStream) save(OutputStream) setAttribute(String, String) setText(String)
Returns the root node of the XML parse tree Returns null Returns unmarked-up text representation of entire document Returns ElementDOCUMENT Returns the URL specified in the <!DOCTYPE> tag or null Returns the information stored in the < XML > tag as an Element hierarchy Load the document using the given input stream Load the document from the given URL Returns zero Returns zero or one The Document class doesn't support attributes Removes the root node, if given element is the root node Return information about the given parse exception that was generated during load Save document to given output stream Returns null Passes the text through to the root node, if any
We start the document-loading process by storing the document's path and name in a new string named +: class showtext
static String filename;
public static void main(String args[])
filename = "file:////c://xml//showtext//showtextxml";
Here we store the reference to the file as an URL (If the file was on the Internet, we would have started the URL with http://)[md]note that because forward slashes have special formatting meaning in Java strings, we need to store each forward slash, as two forward slashes, //, in the filename string: filename = "file:////c://xml//showtext//showtextxml"; <--
Note: Note the path of this example: c:\xml\showtext\showtextxml In this book, we'll give each example its own subdirectory of the c:\xml directory
The next step is to construct a Java URL object for this file We do that with the Java URL class, which is in the javanet package: import javanet*; <--
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