Friday, 3 April 2015

Difference between JSPs and Servlets


Both JSPs and Servlets are server side technologies based on Java to generate web content dynamically that is then rendered on the requesting client which is typically your web browser. In this post we will see How JSPs work and what is exactly the difference between JSPs and Servlets.

Java Servlet

Servlet is a Java module that listens for HTTP requests like GET, POST etc. Once request is received by the servlet it will return appropriate response for it. A server code would look like - 

import javax.servlet.*;
import javax.servlet.http.*;
* @author athakur
public class TestServlet extends HttpServlet {

  public void init() throws ServletException
      //Servlet initialization code go here

  public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
      PrintWriter out = response.getWriter();
      out.println("<b>Hello World!</b>");
  public void destroy()
      // Any code to be executed when servlet is destroyed

and your web.xml will have mapping of this servlet and the URL the servlet handles - 



So when ever tomcat server receives a request for URL like http://localhost:8080/projectName/HelloWorld you should see response as Hello World! in bold on a webpage that is rendered on your web browser.

As I said before think of a servlet as Java module that intercepts HTTP request and provides the response for it. It is like HTML in Java .

JSP pages

JSP  (Java server pages) - They are like Java in HTML. A simple JSP would look like below - 

<head><title>Test JSP</title></head>
    String greeting = "Hello World!";
  <b><%= greeting %></b>

As you can see it is  HTML content with embedded Java logic. Lets now see how JSPs actually work?

How JSPs work?

  1. When JSP container gets request for a JSP (Request URL would be something like http://localhost:8080/projectName/greetings.jsp) it checks whether JSP is compiled or not. If it is compiled it check for last modification time. If JSP page is modified after the JSP was previously compiled then JSP will be compiled again.
  2. Before compilation JSP is actually converted to a java servlet. This generated content is in Java just like a normal servlet we saw above. This servlet is then compiled into a class file.
  3. Request is then forwarded to this generated servlet which then handles the request and returns the response.

 The relevant generated servlet content in step 2 above for JSP same page that I have described above would be something like -

out.write("<html>\r\n  ");
String greeting = "Hello World!";
out.print( greeting );
out.write("</b> \r\n");

 You should be able to see the generated java file under 
  • work\Catalina\localhost\ProjectName\

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