In this little blogpost, I’ll try to explain the differences between ServletConfig and ServletContext and when you can use them into a Java Web Application.

ServletConfig

ServletConfig is actually very simple. Like stated above, you can retrieve parameters for the servlet which are set at deployment time.
I’ll explain this with a very simple example.

If you have created a servlet with the name “MyServlet” you can add the following 2 variables into the servlet:

    <servlet>
         <servlet-name>MyServlet</servlet-name>
         <servlet-class>servlets.MyServlet</servlet-class>
         <init-param>
             <param-name>variable1</param-name>
             <param-value>I'm var1</param-value>
         </init-param>
         <init-param>
             <param-name>variable2</param-name>
             <param-value>I'm var2</param-value>
         </init-param>
     </servlet>
     <servlet-mapping>
         <servlet-name>MyServlet</servlet-name>
         <url-pattern>/MyServlet</url-pattern>
     </servlet-mapping>

You can retrieve variable2 with the following code:

getServletConfig().getInitParameter("variable2");

Retrieve all the variables of the servlet and loop over them:

Enumeration allvars = getServletConfig().getInitParameterNames();
while (allvars.hasMoreElements()) {
     String next = (String) allvars.nextElement();
     out.println(next + " has the value " + getServletConfig().getInitParameter(next));
}

It’s also possible to retrieve the servlets name through getServletconfig

out.println("The name of the servlet is: " + getServletConfig().getServletName());

ServletContext

ServletContext can be used to retrieve application-wide parameters, get serverparameters and store/retrieve/remove application-wide attributes.

It is possible to call the ServletContext right on or through the ServletConfig. Both are the same so you can choose between them.

if (getServletContext().equals(getServletConfig().getServletContext())) {
       out.println("both calls retrieve the same Context");
}

Just like with ServletConfig, you can set parameters in the Deployment Descriptor. They are initialized on compile time. It’s not possible to remove or add them on runtime.

<context-param>
        <param-name>variable3</param-name>
        <param-value>Im var 3</param-value>
</context-param>

And call them in your servlet just like the ServletConfig parameters:

getServletContext().getInitParameter("variable3")

Unlike ServletConfig, ServletContext can work with attributes. These are parameters or objects which you can set into the servlet and retrieve in every other servlet of your Web Application:

getServletContext().setAttribute("test", "I'm an application-wide String");
..
out.println(getServletContext().getAttribute("test"));
..
getServletContext().removeAttribute("test");

I’ll end this little short tutorial with an example of how you can retrieve serversettings through the Context:

out.println(getServletContext().getContextPath());
out.println(getServletContext().getMajorVersion());
out.println(getServletContext().getRealPath(""));
out.println(getServletContext().getServerInfo());

If there are any more questions, just shoot them