It is always difficult to testing/debugging a servlets. Servlets tend to involve a large amount of client/server interaction, making errors likely but hard to reproduce.

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

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. Additionally −

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.
Stopping at breakpoints technique stops the normal execution hence takes more time. Whereas writing to System.out doesn’t interfere much with the normal execution flow of the application, which makes it very valuable when timing is crucial.

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

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

All the messages generated by above syntax would be logged in web server log file.

Message Logging

It is always great idea to use proper logging method to log all the debug, warning and error messages using a standard logging method. I use log4J to log all the messages.

The Servlet API also provides a simple way of outputting information by using the log() method as follows −

// Import required java libraries

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;




public class ContextLog extends HttpServlet {

   public void doGet(HttpServletRequest request, HttpServletResponse response)

      throws ServletException, java.io.IOException {

   

      String par = request.getParameter("par1");

     

      //Call the two ServletContext.log methods

      ServletContext context = getServletContext( );




      if (par == null || par.equals(""))

         //log version with Throwable parameter

         context.log("No message received:", new IllegalStateException("Missing parameter"));

      else

         context.log("Here is the visitor's message: " + par);

     

      response.setContentType("text/html");

      java.io.PrintWriter out = response.getWriter( );

      String title = "Context Log";

      String docType =

         "<!doctype html public \"-//w3c//dtd html 4.0 " + "transitional//en\">\n";

     

      out.println(docType +

         "<html>\n" +

            "<head><title>" + title + "</title></head>\n" +

            "<body bgcolor = \"#f0f0f0\">\n" +

               "<h1 align = \"center\">" + title + "</h1>\n" +

               "<h2 align = \"center\">Messages sent</h2>\n" +

            "</body>

         </html>"

      );

   } //doGet

}

The ServletContext logs its text messages to the servlet container’s log file. With Tomcat these logs are found in <Tomcat-installation-directory>/logs.

The log files do give an indication of new emerging bugs or the frequency of problems. For that reason it’s good to use the log() function in the catch clause of exceptions which should normally not occur.

Using JDB Debugger

You can debug servlets with the same jdb commands you use to debug an applet or an application.

To debug a servlet, we debug sun.servlet.http.HttpServer and carefully watch as HttpServer executes servlets in response to HTTP requests made from browser. This is very similar to how applets are debugged. The difference is that with applets, the actual program being debugged is sun.applet.AppletViewer.

Most debuggers hide this detail by automatically knowing how to debug applets. Until they do the same for servlets, you have to help your debugger by doing the following −

Set your debugger’s classpath so that it can find sun.servlet.http.Http-Server and associated classes.
Set your debugger’s classpath so that it can also find your servlets and support classes, typically server_root/servlets and server_root/classes.

You normally wouldn’t want server_root/servlets in your classpath because it disables servlet reloading. This inclusion, however, is useful for debugging. It allows your debugger to set breakpoints in a servlet before the custom servlet loader in HttpServer loads the servlet.

Once you have set the proper classpath, start debugging sun.servlet.http.HttpServer. You can set breakpoints in whatever servlet you’re interested in debugging, then use a web browser to make a request to the HttpServer for the given servlet (http://localhost:8080/servlet/ServletToDebug). You should see execution being stopped at your breakpoints.

Using Comments

Comments in your code can help the debugging process in various ways. Comments can be used in lots of other ways in the debugging process.

The Servlet uses Java comments and single line (// …) and multiple line (/* … */) comments can be used to temporarily remove parts of your Java code. If the bug disappears, take a closer look at the code you just commented and find out the problem.

Client and Server Headers

Sometimes when a servlet doesn’t behave as expected, it’s useful to look at the raw HTTP request and response. If you’re familiar with the structure of HTTP, you can read the request and response and see exactly what exactly is going with those headers.

Important Debugging Tips

Here is a list of some more debugging tips on servlet debugging −

Remember that server_root/classes doesn’t reload and that server_root/servlets probably does.
Ask a browser to show the raw content of the page it is displaying. This can help identify formatting problems. It’s usually an option under the View menu.
Make sure the browser isn’t caching a previous request’s output by forcing a full reload of the page. With Netscape Navigator, use Shift-Reload; with Internet Explorer use Shift-Refresh.
Verify that your servlet’s init() method takes a ServletConfig parameter and calls super.init(config) right away.

So, this brings us to the end of blog. This Tecklearn ‘Debugging in Servlets’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Servlets and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

https://www.tecklearn.com/course/java-and-jee-training/

Java and JEE Training

About the Course

Java and JEE Certification Training is designed by professionals as per the industrial requirements and demands. This training encompasses comprehensive knowledge on basic and advanced concepts of core Java & J2EE along with popular frameworks like Hibernate, Spring & SOA. In this course, you will gain expertise in concepts like Java Array, Java OOPs, Java Function, Java Loops, Java Collections, Java Thread, Java Servlet, and Web Services using industry use-cases and this will help you to become a certified Java expert.

Why Should you take Java and JEE Training?

Java developers are in great demand in the job market. With average pay going between $90,000/- to $120,000/- depending on your experience and the employers.
Used by more than 10 Million developers worldwide to develop applications for 15 Billion devices.
Java is one of the most popular programming languages in the software world. Rated #1 in TIOBE Popular programming languages index (15th Consecutive Year)

What you will Learn in this Course?

Introduction to Java

Java Fundamentals
Introduction to Java Basics
Features of Java
Various components of Java language
Benefits of Java over other programming languages
Key Benefits of Java

Installation and IDE’s for Java Programming Language

Installation of Java
Setting up of Eclipse IDE
Components of Java Program
Editors and IDEs used for Java Programming
Writing a Simple Java Program

Data Handling and Functions

Data types, Operations, Compilation process, Class files, Loops, Conditions
Using Loop Constructs
Arrays- Single Dimensional and Multi-Dimensional
Functions
Functions with Arguments

OOPS in Java: Concept of Object Orientation

Object Oriented Programming in Java
Implement classes and objects in Java
Create Class Constructors
Overload Constructors
Inheritance
Inherit Classes and create sub-classes
Implement abstract classes and methods
Use static keyword
Implement Interfaces and use it

Polymorphism, Packages and String Handling

Concept of Static and Run time Polymorphism
Function Overloading
String Handling –String Class
Java Packages

Exception Handling and Multi-Threading

Exception handling
Various Types of Exception Handling
Introduction to multi-threading in Java
Extending the thread class
Synchronizing the thread

File Handling in Java

Input Output Streams
io Package
File Handling in Java

Java Collections

Wrapper Classes and Inner Classes: Integer, Character, Boolean, Float etc
Applet Programs: How to write UI programs with Applet, Java.lang, Java.io, Java.util
Collections: ArrayList, Vector, HashSet, TreeSet, HashMap, HashTable

Java Database Connectivity (JDBC)

Introduction to SQL: Connect, Insert, Update, Delete, Select
Introduction to JDBC and Architecture of JDBC
Insert/Update/Delete/Select Operations using JDBC
Batch Processing Transaction
Management: Commit and Rollback

Java Enterprise Edition – Servlets

Introduction to J2EE
Client Server architecture
URL, Port Number, Request, Response
Need for servlets
Servlet fundamentals
Setting up a web project in Eclipse
Configuring and running the web app with servlets
GET and POST request in web application with demo
Servlet lifecycle
Servlets Continued
Session tracking and filter
Forward and include Servlet request dispatchers

Java Server Pages (JSP)

Fundamentals of Java Server Page
Writing a code using JSP
The architecture of JSP
JSP Continued
JSP elements: Scriptlets, expressions, declaration
JSP standard actions
JSP directives
Introduction to JavaBeans
ServletConfig and ServletContext
Servlet Chaining
Cookies Management
Session Management

Hibernate

Introduction to Hibernate
Introduction to ORM
ORM features
Hibernate as an ORM framework
Hibernate features
Setting up a project with Hibernate framework
Basic APIs needed to do CRUD operations with Hibernate
Hibernate Architecture

POJO (Plain Old Java Object)

POJO (Plain Old Java Object)
Persistent Objects
Lifecycle of Persistent Object

Spring

Introduction to Spring
Spring Fundamentals
Advanced Spring

Got a question for us? Please mention it in the comments section and we will get back to you.

487