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Chapter 16
How to work with XML
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 1
Objectives
Applied
 Use servlets to work with XML files whenever you need to do that.
Knowledge
 Describe two common types of uses for XML files.
 Distinguish between storing data in an attribute and storing data as
content for an element.
 In general terms, describe the use of a Document Type Definition.
 In general terms, describe the use of the API for the Document
Object Model.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 2
Tabular data for three users
First name
Andrea
Joel
Alexandra
Java Servlets and JSPCH16
Last name
Steelman
Murach
White
Email address
[email protected]
[email protected]
[email protected]
© 2003, Mike Murach & Associates, Inc.
Contact
no
yes
yes
Slide 3
Data stored in an XML document
<?xml version="1.0" encoding="UTF-8"?>
<!--User Information-->
<Users>
<User contact="no">
<firstName>Andrea</firstName>
<lastName>Steelman</lastName>
<emailAddress>[email protected]</emailAddress>
</User>
<User contact="yes">
<firstName>Joel</firstName>
<lastName>Murach</lastName>
<emailAddress>[email protected]</emailAddress>
</User>
<User contact="yes">
<firstName>Alexandra</firstName>
<lastName>White</lastName>
<emailAddress>[email protected]</emailAddress>
</User>
</Users>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 4
Common uses of XML
 Control files
 Files for exchanging data between two different systems
 Business data files that contain small amounts of data, especially
if they are used by two different types of systems.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 5
An introduction to XML
 XML, which stands for Extensible Markup Language, is a
method of structuring information.
 An XML document uses XML tags to identify the data elements.
When an XML document is stored in an XML file, the file
usually has an extension of XML.
 Unlike HTML tags, XML tags are case-sensitive.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 6
A User element and its child elements
<?xml version="1.0" encoding="UTF-8" ?>
User element
<!--User information-->
firstName
<Users>
element
<User contact="no">
<firstName> Andrea</firstName>
lastName
<lastName>Steelman</lastName>
element
<emailAddress>[email protected]</emailAddress>
emailAddress
</User>
element
</Users>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 7
A User element that adds another level of child
elements
<User contact="no">
<firstName>Andrea</firstName>
<lastName>Steelman</lastName>
<emailAddresses>
<emailAddress>[email protected]</emailAddress>
<emailAddress>[email protected]</emailAddress>
</emailAddresses>
</User>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 8
XML declarations and comments
 The first line in an XML document is an XML declaration that
indicates which version of the XML standard is being used for the
document.
 The XML declaration usually identifies the standard character set
that’s being used (UTF-8 is the character set that’s commonly used
for XML documents in English-speaking countries).
 XML comments are coded the same way HTML comments are
coded.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 9
XML elements
 An XML element begins with a starting tag and ends with an
ending tag.
 The starting tag provides the name of the element and contains any
attributes assigned to the element. The ending tag repeats the
element name, but is prefixed with a slash (/).
 The content for an element is coded within the element’s starting
and ending tags.
 An element can contain one or more other elements. An element
that’s contained within another element is the child element. An
element that contains a child element is a parent element.
 The highest-level parent element in an XML document is the root
element. An XML document must have one and only one root
element.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 10
An attribute of the User element
<?xml version="1.0" encoding="UTF-8" ?>
<!--User information-->
the contact attribute
<Users>
<User contact="no" >
<firstName> Andrea</firstname>
<lastName>Steelman</lastname>
<emailAddress>[email protected]</emailAddress>
</User>
</Users>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 11
XML attributes
 You can include one or more XML attributes in the starting tag for
an element.
 An attribute consists of an attribute name, an equals sign, and a
string value in single or double quotes. Unlike HTML, the quotes
around the attribute are required.
 If an element has more than one attribute, the attributes must be
separated by one or more spaces.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 12
When to use attributes instead of child elements
 When you design an XML document, you can use child elements
or attributes to identify the data items of an element. The choice of
whether to implement a data item as an attribute or as a child
element is often a matter of preference.
 Although attributes require less code, an element that contains too
many attributes can become unwieldy.
 Although child elements require more code, they’re usually easier
to read, they’re better for working with long string values, and
they can store additional child elements.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 13
The rules for an XML document
 The document must contain one and only one Users element.
 The document can contain multiple User elements. Each of these
User elements must contain three elements named firstName,
lastName, and emailAddress.
 Each User element can contain one attribute named contact that can
hold either a default “yes” value or a “no” value.
 The firstName, lastName, and emailAddress elements can contain
any text data. They can’t contain child elements.
A DTD the implements these rules
<!DOCTYPE Users [
<!ELEMENT User (firstName, lastName, emailAddress)>
<!ATTLIST User contact (yes|no) "yes">
<!ELEMENT firstName (#PCDATA)>
<!ELEMENT lastName (#PCDATA)>
<!ELEMENT emailAddress (#PCDATA)>
]>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 14
How to refer to a DTD file in an XML document
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE Users SYSTEM
"c:/tomcat/webapps/murach/web-inf/etc/users.dtd">
<!--User information-->
<Users>
<User contact="no">
<firstName>Andrea</firstName>
<lastName>Steelman</lastName>
<emailAddress>[email protected]</emailAddress>
</User>
</Users>
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 15
DTD declarations
Keyword
DOCTYPE
ELEMENT
ATTLIST
Java Servlets and JSPCH16
Defines
The root element
The names of the child elements or the element name
and the type of data it will contain
The names of the attributes and the types of data they
will contain
© 2003, Mike Murach & Associates, Inc.
Slide 16
An introduction to DTDs
 To define the conditions of an XML document, you use a schema
language to create a schema. These conditions can then be
enforced for any XML document that’s based on the schema.
 Document Type Definition, or DTD, is a schema language that’s
part of standard XML. It is supported by SDK 1.4.
 XML Schema Definition, or XSD, is a newer schema language
that’s part of standard XML. It is more flexible than DTD, but it is
also more difficult to work with and it isn’t supported by SDK 1.4.
 An XML document can refer to a DTD that’s stored in a file or it
can contain an inline DTD at the start of the XML document.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 17
SAX is
 Memory efficient
 Read-only
 Typically used for working with documents larger than 10
megabytes
DOM is
 Memory intensive
 Read-write
 Typically used for working with documents smaller than 10
megabytes
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 18
An introduction to XML APIs
 Version 1.4 of the SDK includes two APIs that you can use to
parse XML documents to get the data you want.
 Two types of APIs exist for processing XML documents with
Java. The Simple API for XML (SAX) and the Document Object
Model (DOM).
 SAX reads one line at a time and lets you access the tags in the
line immediately.
 DOM reads an entire XML document before you can access the
individual tags.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 19
The structure of the DOM tree
Node A
Document
User
Node B
Element
Users
Node C
Comment
Node D
Element
User
Node F
Attr
contact
Java Servlets and JSPCH16
Node E
Element
User
Node G
Element
firstName
Node H
Element
lastName
Node I
Element
emailAddress
Node J
Text
Andrea
Node K
Text
Steelman
Node L
Text
[email protected]
© 2003, Mike Murach & Associates, Inc.
Slide 20
The DOM tree
 You can use the DOM API to load an XML document into a
DOM tree. Then, you can use the DOM API to work with the
data in the DOM tree.
 A DOM tree is a collection of nodes. Each node may have a
parent node, one or more child nodes, and one or more sibling
nodes.
 Each node has a node type that indicates whether the node
represents the document, an element, a comment, an attribute, or
a text value.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 21
The Node interface hierarchy
Node
Document
Element
CharacterData
Text
Java Servlets and JSPCH16
Attr
Comment
© 2003, Mike Murach & Associates, Inc.
Slide 22
Interfaces for working with the DOM tree
 The Node interface represents a single node in the DOM tree.
 The Document, Element, Text, Comment, and Attr interfaces
represent the different types of nodes available in a DOM tree.
 You can use the methods of these interfaces to work with the nodes
of a DOM tree.
 Since the Document, Element, CharacterData, Attr, Text, and
Comment interfaces inherit the Node interface, you can use any of
the methods in the Node interface to work with nodes.
 These interfaces are located in the org.w3c.dom package.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 23
Code that creates an empty DOM tree
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.newDocument();
Packages
javax.xml.parsers
org.w3c.dom
Exceptions
javax.xml.parsers.ParserConfigurationException
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 24
Code that creates and appends the root element
Element rootElement = doc.createElement("Users");
doc.appendChild(rootElement);
Code that creates and appends a comment
Comment comment = doc.createComment("User Information");
doc.appendChild(comment);
Code that creates and appends an element
Element userElement = doc.createElement("User");
rootElement.appendChild(userElement);
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 25
Code that sets an attribute
userElement.setAttribute("contact", "yes");
Code that creates and appends an element that
contains text
Element firstNameElement = doc.createElement("firstName");
userElement.appendChild(firstNameElement);
Text firstNameText = doc.createTextNode("firstName");
firstNameText.setNodeValue("Alexandra");
firstNameElement.appendChild(firstNameText);
Packages
org.w3c.dom
Exceptions
org.w3c.dom.DOMException
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 26
Code that writes a DOM tree to a file
//Prepare the DOM document as the input source
DOMSource in = new DOMSource(doc);
//Prepare the output file
File xmlFile = new File("C:/tomcat/webapps/murach/WEBINF/etc/users.xml");
StreamResult out = new StreamResult(xmlFile);
// Write the DOM document to the file
Transformer transformer =
TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty(
"{http://xml.apache.org/xslt}indent-amount", "4");
transformer.transform(in, out);
Exception
javax.xml.transform.TransformerException
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 27
How to write a DOM tree to a file
 To specify the DOM tree as the input source, you create a
DOMSource object from the Document object.
 To specify an XML file as the output, you create a StreamResult
object that points to an XML file.
 To write the DOM tree to the XML file, you create a Transformer
object by calling the newTransformer method of the
TransformerFactory class. Then, you can call the transform
method of the Transformer object.
 If you don’t specify any output properties in the Transformer
object, the XML document will appear as one continuous string
and may be difficult to read.
 To specify indentation and other formatting, you can use the
setOutputProperty methods.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 28
How to write a DOM tree to a file (continued)
 The DOMSource class is located in the javax.xml.transform.dom
package, the StreamResult interface is located in the
javax.xml.transform.stream package, and the Tranformer and
TransformerFactory classes are located in the javax.xml.transform
package.
 Both the newTransformer method of the TransformerFactory class
and the transform method of the Transformer class throw a
TransformerException.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 29
Code that reads a DOM tree from a file
InputSource in = new InputSource(
"c:/tomcat/webapps/murach/web-inf/etc/users.xml");
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(in);
Code that reads a DOM tree and validates it
against its DTD schema
InputSource in = new InputSource(
"c:/tomcat/webapps/murach/web-inf/etc/users.xml");
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
dbf.setValidating(true);
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(in);
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 30
Methods of the DocumentBuilderFactory class
setIgnoringComments(boolean b)
setIgnoringElementContentWhitespace(boolean b)
setValidating(boolean b)
Packages
javax.xml.parsers
org.w3c.dom
Exceptions
java.io.IOException
org.xml.sax.SAXException
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 31
How to read a DOM tree from a file
 To get a Document object for an existing XML document, you can
call the parse method of the DocumentBuilder object with an
InputSource object that points to the XML file as its argument.
 By default, the three set methods above are set to false. If that’s
not what you want, you can call them from the DocumentBuilder
object and change them so they’re true.
 The parse method of the DocumentBuilder class throws a
SAXException and an IOException.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 32
Code that reads the root element of the DOM tree
Element root = doc.getDocumentElement();
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 33
Code that reads all EmailAddress elements into a
vector
Vector emailVector = new Vector();
NodeList emailList =
doc.getElementsByTagName("emailAddress");
for (int i = 0; i < emailList.getLength(); i++){
Element emailElement = (Element) emailList.item(i);
Text emailText = (Text) emailElement.getFirstChild();
String emailAddress = emailText.getNodeValue();
emailVector.add(emailAddress);
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 34
Code that reads all User elements into a vector
Vector users = new Vector();
NodeList usersList = doc.getElementsByTagName("User");
for (int i = 0; i < usersList.getLength(); i++){
Element userElement = (Element) usersList.item(i);
String contact = userElement.getAttribute("contact");
NodeList firstList =
userElement.getElementsByTagName("firstName");
Element firstElement = (Element) firstList.item(0);
Text firstText = (Text) firstElement.getFirstChild();
String firstName = firstText.getNodeValue();
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 35
The code (continued)
NodeList lastList =
userElement.getElementsByTagName("lastName");
Element lastElement = (Element) lastList.item(0);
Text lastText = (Text) lastElement.getFirstChild();
String lastName = lastText.getNodeValue();
NodeList emailList =
userElement.getElementsByTagName("emailAddress");
Element emailElement = (Element) emailList.item(0);
Text emailText = (Text) emailElement.getFirstChild();
String emailAddress = emailText.getNodeValue();
User user = new User(firstName, lastName,
emailAddress);
users.add(user);
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 36
How to read the nodes of a DOM tree
 The NodeList interface stores a collection of Node objects.
 You can use the getLength and item methods to loop through the
Node objects stored in a NodeList object.
 You can use the getElementsByTagName method to return a
NodeList object that contains a collection of nodes for the
specified tag.
 You can use the getDocumentElement method to return the root
element of the XML document.
 You can use the getAttribute method to return the value of the
specified attribute.
 You can use the getFirstChild method to return the first child
node.
 You can use the getNodeValue method to return the value of a
text node.
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 37
Code that appends an element after all other
child nodes
root.appendChild(userElement);
Code that inserts an element before another child
node
root.insertBefore(userElement1, userElement2);
Code that updates an element
root.replaceChild(userElement1, userElement2);
Code that removes an element
root.removeChild(userElement);
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 38
Methods of the Node interface for adding,
updating, and deleting elements
appendChild(Node child)
insertBefore(Node newChild, Node refChild)
replaceChild(Node newChild, Node oldChild)
removeChild(Node child)
Exceptions
org.w3c.dom.DOMException
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 39
The XMLUtil class
package util;
import
import
import
import
import
import
import
org.xml.sax.*;
org.w3c.dom.*;
javax.xml.transform.*;
javax.xml.transform.dom.*;
javax.xml.transform.stream.*;
javax.xml.parsers.*;
java.io.*;
public class XMLUtil{
public static Document getDocumentFromFile(
String xmlFile) throws IOException,
ParserConfigurationException, SAXException{
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
InputSource in = new InputSource(xmlFile);
Document doc = db.parse(in);
return doc;
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 40
The XMLUtil class (continued)
public static String getTextNodeValue(Element parent,
String tagName){
NodeList list =
parent.getElementsByTagName(tagName);
Element element = (Element) list.item(0);
Text text = (Text) element.getFirstChild();
String value = text.getNodeValue();
return value;
}
public static void addTextNode(Document doc,
Element parent, String elementName,
String elementValue){
Element element = doc.createElement(elementName);
parent.appendChild(element);
Text text = doc.createTextNode(elementName);
text.setNodeValue(elementValue);
element.appendChild(text);
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 41
The XMLUtil class (continued)
public static void writeDocumentToFile(String xmlFile,
Document doc) throws TransformerException {
DOMSource in = new DOMSource(doc);
File xmlFileObj = new File(xmlFile);
StreamResult out = new StreamResult(xmlFileObj);
Transformer transformer =
TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT,
"yes");
transformer.setOutputProperty(
"{http://xml.apache.org/xslt}indent-amount",
"4");
transformer.transform(in, out);
}
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 42
The UserXML class
package data;
import
import
import
import
import
import
import
import
import
import
org.xml.sax.*;
org.w3c.dom.*;
javax.xml.transform.*;
javax.xml.transform.dom.*;
javax.xml.transform.stream.*;
javax.xml.parsers.*;
java.io.*;
java.util.Vector;
business.User;
util.XMLUtil;
public class UserXML{
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 43
The UserXML class (continued)
public synchronized static boolean isMatch(
String emailAddress, String xmlFile)
throws IOException, ParserConfigurationException,
SAXException {
Document doc = XMLUtil.getDocumentFromFile(xmlFile);
NodeList emailElements =
doc.getElementsByTagName("emailAddress");
for (int i = 0; i < emailElements.getLength(); i++){
Text textNode =
(Text) emailElements.item(i).getFirstChild();
if (textNode.getNodeValue().equalsIgnoreCase(
emailAddress)){
return true;
}
}
return false;
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 44
The UserXML class (continued)
public synchronized static void addRecord(User user,
String xmlFile) throws IOException, SAXException,
ParserConfigurationException, TransformerException{
Document doc = XMLUtil.getDocumentFromFile(xmlFile);
Element userElement = doc.createElement("User");
userElement.setAttribute("contact", "yes");
XMLUtil.addTextNode(doc, userElement, "firstName",
user.getFirstName());
XMLUtil.addTextNode(doc, userElement, "lastName",
user.getLastName());
XMLUtil.addTextNode(doc, userElement,
"emailAddress", user.getEmailAddress());
Element root = doc.getDocumentElement();
root.appendChild(userElement);
XMLUtil.writeDocumentToFile(xmlFile, doc);
}
}//contains other methods that delete and update records
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 45
The doGet method of the EmailServlet class
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException{
String file =
"../webapps/murach/WEB-INF/etc/users.xml";
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress =
request.getParameter("emailAddress");
User user = new User(firstName, lastName,
emailAddress);
HttpSession session = request.getSession();
session.setAttribute("user", user);
String message = "";
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 46
The doGet method (continued)
try{
//check whether the email address already exist
boolean emailExists =
UserXML.isMatch(emailAddress, file);
if (emailExists){
message = "This email address already "
+ "exists. <br>Please enter "
+ "another email address.";
session.setAttribute("message", message);
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(
"/email16/join_email_list.jsp");
dispatcher.forward(request, response);
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 47
The doGet method (continued)
else{
UserXML.addRecord(user, file);
session.setAttribute("message", "");
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(
"/email16/show_email_entry.jsp");
dispatcher.forward(request, response);
}
}
catch(Exception e){
message = "EmailServlet Exception: " + e;
session.setAttribute("message", message);
RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(
"/email16/join_email_list.jsp");
dispatcher.forward(request, response);
}
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 48
The code for the XMLServlet class
package email16;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class XMLServlet extends HttpServlet{
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException{
response.setContentType("text/xml");
File file = new File(
"../webapps/murach/WEB-INF/etc/users.xml");
FileInputStream in = new FileInputStream(file);
PrintWriter out = response.getWriter();
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 49
The XMLServlet class (continued)
int i;
while ((i = in.read()) != -1)
out.write(i);
in.close();
}
}
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 50
The XML file displayed in a browser
Java Servlets and JSPCH16
© 2003, Mike Murach & Associates, Inc.
Slide 51