In this blog , we will discuss Debugging a JSP. It is always difficult testing/debugging a JSP and servlets. JSP and Servlets tend to involve a large amount of client/server interaction, making errors likely but hard to reproduce.

The following are a few hints and suggestions that may aid you in your debugging.

Using System.out.println()

System.out.println() is easy to use as a marker to test whether a certain piece of code is being executed or not. We can print out variable values as well. Consider the following additional points −

Since the System object is part of the core Java objects, it can be used everywhere without the need to install any extra classes. This includes Servlets, JSP, RMI, EJB’s, ordinary Beans and classes, and standalone applications.
Compared to stopping at breakpoints, writing to System.out doesn’t interfere much with the normal execution flow of the application, which makes it very valuable when the iming is crucial.

Following is the syntax to use System.out.println() −

System.out.println(“Debugging message”);

Following example shows how to use System.out.print() −

<%@taglib prefix = "c" uri = "http://java.sun.com/jsp/jstl/core" %>




<html>

   <head><title>System.out.println</title></head>

   <body>

      <c:forEach var = "counter" begin = "1" end = "10" step = "1" >

     

         <c:out value = "${counter-5}"/></br>

         <% System.out.println( "counter = " + pageContext.findAttribute("counter") ); %>

      </c:forEach>

     

   </body>

</html>

Access the above JSP, the browser will show the following result −

If you are using Tomcat, you will also find these lines appended to the end of stdout.log in the logs directory.

counter = 1

counter = 2

counter = 3

counter = 4

counter = 5

counter = 6

counter = 7

counter = 8

counter = 9

counter = 10

This way you can bring variables and other information into the system log which can be analyzed to find out the root cause of the problem or for various other reasons.

Using the JDB Logger

The J2SE logging framework is designed to provide logging services for any class running in the JVM. We can make use of this framework to log any information.

Let us re-write the above example using JDK logger API −

<%@taglib prefix = "c" uri = "http://java.sun.com/jsp/jstl/core" %>

<%@page import = "java.util.logging.Logger" %>




<html>

   <head><title>Logger.info</title></head>

  

   <body>

      <% Logger logger = Logger.getLogger(this.getClass().getName());%>




      <c:forEach var = "counter" begin = "1" end = "10" step = "1" >

      <c:set var = "myCount" value = "${counter-5}" />

      <c:out value = "${myCount}"/></br>

         <% String message = "counter = "

            + pageContext.findAttribute("counter") + "myCount = "

            + pageContext.findAttribute("myCount");

            logger.info( message );

         %>

      </c:forEach>

     

   </body>

</html>

The above code will generate similar result on the browser and in stdout.log, but you will have additional information in stdout.log. We will use the info method of the logger because and log the message just for informational purpose. Following is a snapshot of the stdout.log file −