Servlet App Event Listeners
Figure 1 shows the console for Tomcat when it starts up and the
above web application is deployed.
Figure 1. Tomcat's console when it starts up.
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Figure 2 is a snapshot of the directory structure to give you a
good idea of how the demo web application is set up.
Figure 2. A snapshot of Tomcat's working directory.
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Listening For Changes To The Servlet Context
The Servlet 2.3 specification allows for more interaction between
the web application programmer and ServletContext. The
programmer can now write an application lifecycle event listener that
executes when the attributes of the ServletContext object
have been modified. If you want to execute some code when the
attributes of the ServletContext object has been
modified, write a Java class that implements the
javax.Servlet.ServletContextAttributesListener
interface. This interface defines three methods with the following
signatures (this is taken from the JavaDocs):
void attributeAdded (ServletContextAttributeEvent
scab)
Notification that a new attribute was
added to the servlet context.
void attributeRemoved
(ServletContextAttributeEvent scab)
Notification that an existing attribute has been removed
from the servlet context.
void attributeReplaced
(ServletContextAttributeEvent scab)
Notification that an attribute on the servlet context has
been replaced.
Listing 3 is an example of writing an application event
listener for getting notifications of changes to the
ServletContext object.
Listing 3: An application event
listener that writes messages to the console when attributes of the
ServletContext object have been
modified. (ServletContextAttribListener.java)
package com.listeners;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributesListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class ServletContextAttribListener
implements ServletContextAttributesListener {
//This method is invoked when an attribute
//is added to the ServletContext object
public void attributeAdded (ServletContextAttributeEvent scab)
{
System.out.println("An attribute was added to the " +
"ServletContext object");
}
//This method is invoked when an attribute
//is removed from the ServletContext object
public void attributeRemoved (ServletContextAttributeEvent scab)
{
System.out.println("An attribute was removed from " +
"the ServletContext object");
}
//This method is invoked when an attribute
//is replaced in the ServletContext object
public void attributeReplaced (ServletContextAttributeEvent scab)
{
System.out.println("An attribute was replaced in the " +
"ServletContext object");
}
}
This code is well and good, but what about testing it? Listing 4 is
for an HTML document that allows the user to add name-value pairs as
an object to the ServletContext object. You can also
remove items and replace items in the ServletContext
object. Listing 5 is the code for the Servlet that parses the request
from the HTML document and actually does the manipulation of the
ServletContext object.
Listing 4: An HTML document that
allows users to play with the ServletContext
object. (servletcontextattrib.html)
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>Working With Attributes Of the ServletContext Object</title>
</head>
<body>
<center>
<font size="6" color="navy">
Playing With The ServletContext Object
</font>
</center>
<form name="MyForm" action="./servletcontextattrib" method="POST">
<table width="75%" bgcolor="silver" align="center">
<tr><td rowspan="3">Select an Action</td>
<td><input type="radio" name="action" value="add">
Add Item To ServletContext
</td>
</tr>
<tr>
<td><input type="radio" name="action" value="remove">
Remove Item From ServletContext
</td>
</tr>
<tr>
<td><input type="radio" name="action" value="replace">
Replace Item In ServletContext
</td>
</tr>
<tr><td>Servlet Context Attribute Name</td>
<td><input type="text" name="name" value=""></td>
</tr>
<tr><td>Servlet Context Attribute Value</td>
<td><input type="text" name="value" value=""></td>
</tr>
<tr><td colspan="2" align="center">
<input type="submit" name="btnSubmit" value="Submit">
</td>
</tr>
</table>
</form>
</body>
</html>
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