Recipe 18.6 Counting the Number of Web Application Requests
Problem
You want to count the number of requests
handled by a web application.
Solution
Use a
javax.servlet.ServletRequestListener to be notified whenever
an HTTP request is initialized.
Discussion
A request listener is a good candidate for tracking requests, because
the web container notifies the listener of new requests by calling
its requestInitialized(
) method. Example 18-8 keeps
track of the request count with a static class variable named
reqCount. The program increments this variable in
a synchronized block within the requestInitialized(
) method.
The
ServletContext is used to log a message about the
request so that you can observe the listener behavior. However, a
busy production application that logs information about every request
typically represents an inefficient use of web container resources.
This type of logging activity should be reserved for development
applications.
Example 18-8. A request listener class for counting application requests
package com.jspservletcookbook;
import javax.servlet.*;
import javax.servlet.http.*;
public class ReqListener implements ServletRequestListener {
private static long reqCount;
public void requestInitialized(ServletRequestEvent sre){
//used for logging purposes
ServletContext context = sre.getServletContext( );
//Used to get information about a new request
ServletRequest request = sre.getServletRequest( );
//The static class variable reqCount is incremented in this block;
//the incrementing of the variable is synchronized so that one
// thread is not reading the variable while another increments it
synchronized (context){
context.log(
"Request for "+
(request instanceof HttpServletRequest ?
((HttpServletRequest) request).getRequestURI( ) :
"Unknown")+ "; Count="+ ++reqCount);
}//synchronized
}
public void requestDestroyed(ServletRequestEvent sre){
//Called when the servlet request is going oout of scope.
}//requestDestroyed
}
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You can access the new ServletRequest in the two
ServletRequestListener methods by calling
ServletRequestEvent.getServletRequest(
)
. You must cast the
ServletRequest return value to an
HttpServletRequest to call the latter
class's methods. Example 18-8
accesses the new HttpServletRequests in order to
call those object's getRequestURI(
)
method, which provides part of the information the code includes in a
logging message.
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You must register the ServletRequestListener in
web.xml:
<listener>
<listener-class>com.jspservletcookbook.ReqListener</listener-class>
</listener>
The web container then creates an instance of the listener when it
starts up. Here is an example of a server-log entry when a request is
made to the application within which the request listener is
registered:
2003-05-30 07:22:21 Request for /home/servlet/com.jspservletcookbook.SessionDisplay;
Count=2
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For Tomcat, this line would be displayed in the log file found in
<Tomcat-installation-directory>/logs.
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See Also
Recipe 18.1 and Recipe 18.2 on examining request
headers in a servlet and a JSP; Recipe 18.3
on using a filter to wrap the request and forward it along the filter
chain; Chapter 7 on handling request parameters
and JavaBean properties with servlets, JSPs, and
filters.
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