Tutorials : The Power of Three - Eclipse, Tomcat, and Struts :

Debugging servlets

To show you how debugging a servlet works, we'll create a very simple servlet:

A single breakpoint is already established. Before debugging can start we'll have to define the servlet to Tomcat. This can be done by creating a web.xml file and placing it in the WEB-INF folder:

<!DOCTYPE web-app 
  PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" 
  "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>

  <servlet>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>HelloWorldServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

</web-app>

The servlet can now be started with this URL:

http://localhost:8080/Tom1/hello

Another possibility is to define a generic rule for all servlets in Tomcat. This can be done in the web.xml file in Tomcat's conf directory. Remove the comments around the invoker servlet definitions:

<servlet>
  <servlet-name>invoker</servlet-name>
  <servlet-class>org.apache.catalina.servlets.InvokerServlet</servlet-class>
  <init-param>
    <param-name>debug</param-name>
    <param-value>0</param-value>
  </init-param>
  <load-on-startup>2</load-on-startup>
</servlet>

...and:

<servlet-mapping>
  <servlet-name>invoker</servlet-name>
  <url-pattern>/servlet/*</url-pattern>
</servlet-mapping>

You'll have to restart Tomcat if you do this modification. The "Hello World" application can now be started like this:

http://localhost:8080/Tom1/servlet/HelloWorldServlet

Having set the breakpoint above Eclipse will switch to the debug perspective and show the breakpoint:

In debug mode Eclipse offers many features, for example the possibility to inspect or even change the values of variables and objects. More information can be found in the Eclipse Help section: "Java Development User Guide".

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