web.xml - context & init params

You can specify parameters to your web application in the web.xml, using the tags <context-param> and <init-param>.

The former are "global" - they can be accessed by all servlets.
The later are servlet-specific.

Let us see this with an example.

We will declare a 'color' and a 'size'.

Update your web.xml, like this:

 <web-app>   
      <context-param>  
           <param-name>color</param-name>  
           <param-value>red</param-value>  
      </context-param>  
   
       <servlet>   
             <servlet-name>hello1</servlet-name>   
             <servlet-class>org.confucius.HelloWorld</servlet-class>   
             <init-param>  
                  <param-name>size</param-name>  
                  <param-value>medium</param-value>  
             </init-param>   
       </servlet>   
     
       <servlet>   
             <servlet-name>hello2</servlet-name>   
             <servlet-class>org.confucius.HelloWorld</servlet-class>   
             <init-param>  
                  <param-name>size</param-name>  
                  <param-value>large</param-value>  
             </init-param>   
       </servlet>   
     
       <servlet-mapping>   
             <servlet-name>hello1</servlet-name>   
             <url-pattern>/hello1</url-pattern>   
       </servlet-mapping>   
         
       <servlet-mapping>   
             <servlet-name>hello2</servlet-name>   
             <url-pattern>/hello2</url-pattern>   
       </servlet-mapping>   
               
 </web-app>        

We have declared two servlets, hello1 and hello2 - both of which point to the same class HelloWorld.
One is configured with a size 'medium, the other with size 'large'.

Let us now write HelloWorld.java (in your /src/org/confucius) which will read these parameters, like this:

 package org.confucius;  
   
 import java.io.IOException;  
 import javax.servlet.ServletException;  
 import javax.servlet.http.HttpServlet;  
 import javax.servlet.http.HttpServletRequest;  
 import javax.servlet.http.HttpServletResponse;  
   
 public class HelloWorld extends HttpServlet {  
        
      public void doGet(HttpServletRequest request, HttpServletResponse response)  
                throws ServletException, IOException {  
           String color = getServletContext().getInitParameter("color");  
           String size = getServletConfig().getInitParameter("size");  
           response.getWriter().println("Color is " + color);  
           response.getWriter().println("Size is " + size);            
      }     
 }  

<context-param> is accessed via ServletContext and <init-param> via ServletConfig.

Build HelloWorld.war and deploy it, then point your browser to:
http://localhost:8080/HelloWorld/hello1

You will see the color set to red and the size to medium.

Now point your browser to:
http://localhost:8080/HelloWorld/hello2

You will see the color set to red and the size to large.

Thus, each servlet accesses its own init-param, but they all access the same context-param.