Quick Recap: Chapters 31 to 37

Let us quickly go through what we learnt in the previous few chapters about the JSP Technology.

Introduction to JSPs

• JSPs are an integral part of any J2EE Application and are used extensively
• CGI, Perl and Active Server Pages (ASP) are all predecessors of the JSP Technology
• A JSP page gets converted/translated into a Servlet before it is executed
• You can have the following in a JSP Page
     o HTML Code
     o Java Code
     o XML Code
     o JSP Scriptlets

JSP Tags:

• JSP Tags begin with a “<” symbol and end with a “>” symbol
• There are many directives that can be used in a JSP Page
• The include directive is a placeholder that lets you include the text in another file
• The page directive gives directions to the servlet engine about the general setup of the page
• The scriptlet is a way to include Java code directly in a JSP page
• JSP Tags can be used in two ways. One is the regular JSP Tag construct and the other is the XML Equivalent.

The Page Directive:

• The Page Directive is one of the important components of any JSP Page. It can help us define page specific properties like Buffer size or location of an error page etc
• In JSP, you do not have to use the page directive for the default import list of java.lang.*, javax.servlet.*, javax.servlet.jsp.*, and javax.servlet.http.*. These are automatically available to you
• The page directive can be used to define an error page to which the control will be re-directed in case there is an issue with the current page
• The page directive also defines if the current page is an error page that would be invoked by any other JSP page

JSP Lifecycle:

• Just like Servlets, JSPs too have a life cycle
• The following are the life cycle stages of a JSP Page
     o Page translation
     o Page compilation
     o Load class
     o Create instance
     o Call jspinit
     o Call _jspService
     o Call jspDestroy


JSP Implicit Objects:

In any JSP Page, there are a bunch of implicit objects that are available for the programmer to use. It contains a variety of information that can be used to display stuff on the page. The following Implicit Objects are available for the programmer inside the JSP Page
• request
• response
• out
• session
• config
• application
• page
• pageContext
• exception

Key Terms Learnt in the previous Chapters:
• JSP
• servlet
• directive
• expression
• scriptlet
• implicit object
• JSP container
• JSP page lifecycle
• tag library
• Web application

Previous Chapter: Chapter 37 - JSP Scriptlets

Next Chapter: Self Test - Chapters 31 to 37

Chapter 5: JSP to Servlet Conversion

In the previous chapter, we took a look at how a JSP file looks like and the contents that can be present inside a typical JSP file.

As you might already know (If you have J2EE programming experience) a JSP file gets converted into a Servlet at runtime and then gets executed. Well, if you did not know this, don't worry. That is what this chapter is for. To tell you the fact that JSPs get converted into Servlets for execution and also to tell you how that happens.

So, lets get started!!!

JSP to Servlet Conversion

JSPs are converted to servlets before the container runs them. This is actually cool because you don't need hardcore java programming skills to create a JSP page whereas you’ll need them to write a servlet. Moreover, all you’ll need to write a JSP is some expertise in creating HTML files and in using JavaScript. You can create front-end JSP pages without having much expertise in Java at all. Although JSP reduces the required skill level, JSP becomes a servlet, with the nice performance and portability benefits.

Below is how the conversion happens.

First lets look at a sample JSP page that we will consider for this conversion process. It's the same sample JSP we saw in the previous chapter. Lets name this guy my_first_jsp.jsp
Sample JSP File Code:

< html >
< body >
I Like Cars, Especially Ferrari .
< / body >
< / html >

This JSP file has to be placed in the …\jakarta-tomcat-4.0.1\webapps\examples\jsp folder in our system. To access this JSP through the tomcat server we can use the below URL:

http://localhost:8080/examples/jsp/my_first_jsp.jsp.

When you hit enter after typing the contents above in the browsers address bar, tomcat covnerts this JSP into a servlet, compiles it and then invokes it.

The servlet that gets created will be placed in …\jakarta-tomcat-4.0.1\work\localhost\examples\jsp as my_0005fservlet$jsp.java.

The contents of this converted Servlet would be as below:

package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import org.apache.jasper.runtime.*;

public class my_0005fservlet$jsp extends HttpJspBase {

static {
}
public my_0005fservlet$jsp( ) {
}

private static boolean _jspx_inited = false;

public final void _jspx_init()
throws org.apache.jasper.runtime.JspException {
}

public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws java.io.IOException, ServletException {

JspFactory _jspxFactory = null;
PageContext pageContext = null;
HttpSession session = null;
ServletContext application = null;
ServletConfig config = null;
JspWriter out = null;
Object page = this;
String _value = null;
try {

if (_jspx_inited == false) {
synchronized (this) {
if (_jspx_inited == false) {
_jspx_init();
_jspx_inited = true;
}
}
}
_jspxFactory = JspFactory.getDefaultFactory();
response.setContentType("text/html;charset=" +
"ISO-8859-1");
pageContext = _jspxFactory.getPageContext(this,
request, response, "",
true, 8192, true);

application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();

// HTML // begin [file="/jsp/my_first_jsp.jsp"]
out.write(">
\r\n< html >\r\n< body >"+
"\r\nI Like Cars, Especially Ferrari ."+
"\r\n\r\n\r\n");

// end

} catch (Throwable t) {
if (out != null && out.getBufferSize() != 0)
out.clearBuffer();
if (pageContext != null)
pageContext.handlePageException(t);
} finally {
if (_jspxFactory != null)
jspxFactory.releasePageContext(pageContext);
}
}
}

A point to note here is that, the exact code that gets generated for your Servlet might vary slightly and may not exactly match what is given above.

As you can see, Tomcat does a lot of work when it converts our JSP into a servlet. If you look at the source that is sent to your browser, you will see the original HTML in the JSP file.
Well, the above example was a little too easy and in reality we will have some Java code too in our JSP. So, lets take a look at how the conversion happens if we put some java code into our earlier example.

Our Modified JSP:


< html >
< body >
I Like Cars, Especially Ferrari .

< % ! int val1 = 10, val2=5; % >
< % = val1 * val2 % >

//Close the html and body tags here too

Tomcat will now convert the Java embedded in the JSP to the following:

// begin [file="/jsp/my_first_jsp.jsp";from=(7,3);to=(7,31)]
int val1 = 10, val2=5;
// end

It also generates this version of the try block, which differs slightly from the previous servlet code:

try {

if (_jspx_inited == false) {
synchronized (this) {
if (_jspx_inited == false) {
_jspx_init();
_jspx_inited = true;
}
}
}
_jspxFactory = JspFactory.getDefaultFactory();
response.setContentType("text/html;charset=ISO-8859-1");
pageContext = _jspxFactory.getPageContext(this,
request, response,
"", true, 8192, true);

application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();

// HTML // begin [file="/jsp/my_first_jsp.jsp"...]
out.write("" +
"\r\n< html >\r\n< body >\r\n" +
"I Like Cars, Especially < b > Ferrari < / b >.
\r\n");

// end
// HTML // begin [file="/jsp/my_first_jsp.jsp";from=...]
out.write("\r\n ");

// end
// begin [file="/jsp/my_first_jsp.jsp";from=...]
out.print(val1 * val2);
// end
// HTML // begin [file="/jsp/my_first_jsp.jsp";from=...]
out.write("\r\n\r\n\r\n\r\n");

// end

}

As you can see, Tomcat now takes our val1 and val2 variables that were declared at the top of our JSP page and generates declarations as class variables in the servlet.

So, < % - val1 & val2 % > becomes

out.print(val1*val2);

Once the conversion is complete, this servlet will be compiled and loaded to memory. Every call to invoke this JSP will make Tomcat compare the modification date of the loaded servlet with the date of the JSP. If it is the same, the compiled servlet is executed and contents displayed on screen. Else, if it sees that the JSP has changed, it will recompile the JSP and load the newly converted Servlet instead of the older version.

Previous Chapter: Chapter 4 - A Sample JSP

Next Chapter: Quick Recap - Chapters 1 to 5

Chapter 3: A Sample Servet

We saw what the purpose of a Servlet Container and a Web server is, in the previous chapter. In this chapter, we are going to look at how a Servlet code would look like.

So, lets get started!!!

Servlet Skeleton

If I ask you, what are the components of a Java class, you’ll happily tell me that, there are first package statements and then imports and then the class declaration. Within the class brackets, we have constructors, instance variables, methods etc. That was easy, wasn’t it?

The same way, every Servlet has a certain set of components that are mandatory for its well-being. (I just got carried away a bit) Or I must say, for its proper functioning.

A Servlets skeleton would look like below:

/*
* servlet name
*
* servlet description
* All other stuff that is part of a Standard Class comment section
*/

//package declarations

//import statements

public class ServletName extends HttpServlet {

// Instance Variables

/**
* Code to Initialize the Servlet
*/
public void init() throws ServletException
{
// Servlet Initialization Code goes here
}

/**
* The Service Method
* Gets invoked for every request submitted to the Servlet
* This method is optional. We mostly use doGet, doPost Methods
*/
protected void service(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
// Code for the Service Method goes here
}


/**
* Process a GET request
*
*/
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
// Code for the doGet() method goes here
}

/**
* Process a POST request
*
*/
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
// Code for the doPost() method goes here
}

/**
* Process a PUT request
*
*/
protected void doPut(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
//Code for the doPut() method goes here
}

/**
* You can have any number of methods for your processing
* here. There is no limit as to the number of methods or
* any restrictions on what you can name them.
* Since this is all java code, you need to keep them
* syntactically correct as per Java Coding Standards.
*/

/**
* Clean-Up
*/
public void destroy()
{
// clean up activities before the Servlet is put to death
}
}

The above is what a Servlets skeleton would look like. Now let us take a look at some sample code as to how a properly coded Servlet would look like:


import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import java.io.PrintWriter;
import java.io.IOException;


public class OurFirstServlet extends HttpServlet
{

public void service(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("< html >");
out.println("< head >< title >Servlet Example " +
" ");
out.println("< body >");
out.println("Not Much code, but this is enough for a Servlet.");
out.println("");
out.println("");
}
}

The above is a simple Servlet. It would display an almost blank HTML page that contains the message we put in “Not Much code, but this is enough for a Servlet.”

Note: A Servlet is not a simple java class, that you can run using a main() method. You have deploy this Servlet on a web server in order to view the output. Lets not get too ahead of ourselves here. We’ll be looking at all that later in detail. For now, this is how a Servlet would look like and that wraps up our current chapter.

Previous Chapter: Web Servers & Servlet Containers

Next Chapter: A sample JSP

Chapter 1: Servlet & JSP History

Here we are, studying for the SCWCD exam. I would like to congratulate you again for this decision of yours because of which you are going to get yourself SCWCD certified. It's a big step and am gonna be with you step by step to help you get this certification.

Well, it wouldn't be much fun preparing for a certification on JSPs and Servlets without knowing their history. Would it?

This chapter is going to be a history lesson taking you to the humble beginnings of these two wonderful technologies that have made our lives so much more easier & powerful.

How it all Began – Internet

Early in the 1950’s computer scientists in USA were working their backsides off in order to compete with Soviet Unions (The late USSR) advancements in superpower computing. They formed the Advanced Research Projects Agency (ARPA) in the year 1957. In those they still had powerful computers, but they weren’t able to talk or communicate with one another. In 1966 Lawrence G. Roberts (From MIT) proposed the first computer network which was named the ARPANET. The US Department of Defense (DoD) funded the venture and it took them 3 years to implement the network. The ARPANET team rewarded the DoD by establishing the Network Control Protocol (NCP), the first host to host protocol, which made possible for the university and the research center PC’s to communicate with one another.

With the success of the NCP, telco major AT&T installed the first cross country link between UCLA and BBN. It was a humble beginning and by 1973 hundreds of computers were talking to one another.

The real big breakthrough came in the year 1982 when the TCP/IP standard was established by Vint Cerf and Bob Kahn. Based on this development, the Domain Name System was established and by 1984 there were over 1000 hosts registered.

This was the backbone of the current day Internet. It was called NSFNET originally and with multiplying hosts it was becoming difficult to manage. By 1991 there were over a hundred thousand hosts and the system was getting out of control. There was nobody incharge and there was utter chaos all around.

In 1991, Tim Berners Lee created hyperlinks. He invented the whole protocol that made links communicate with one another and the World Wide Web was born. Telnet, email and many other services started using the networks.

In 1993, Marc Anderson and his friends wanted to see what was on the Internet, so they developed a new program called the NCSA Mosaic at the University of Illinois based on Berners Lee’s ideas. (NCSA stands for National Center for Supercomputing Applications)

Mosaic was the catalyst that caused the internet to explode. Nearly 200 million hosts were in use by the end of the decade and more than 1 billion users were using it.

This was not the end of it. Mobile phones, PDAs, GPS, Cars etc started connecting to the internet and the number of users began growing beyond numbers that we can write down or calculate.

It all started with basic HTML pages and hungry scientists created more and more advanced technologies whose powers were unbelievable. JSPs and Servlets just changed the landscape catastrophically and here we are, studying them to become better J2EE web programmers!!!

History of JSP & Servlets

The Internet's original purpose was to access and copy files from one computer to another. While TCP/IP provided a highway layer, the File Transfer Protocol (FTP) specification provided a standard way to exchange those files. It defined a way of shuttling them back and forth, but said nothing about looking at the content. HyperText Markup Language (HTML) allowed us to see the documents on the Internet. FTP can transmit HTML files just as easily as HTTP can. But we use Hypertext Transfer Protocol (HTTP) to act as an FTP specifically for HTML documents because it is a stateless protocol which makes having many short connections more efficient.
HTTP is the plumbing that connects the various computers. Now it is time to discuss about the fluid that flows through it “JSP & Servlets”

Note: JSP & Servlets arent the only technologies that are used in J2EE applications. Struts, Hibernate, Springs etc are other technologies that are used in J2EE Web applications. But, don't worry about them because they arent in the exam.

Using HTTP and HTML people were able to view/browse files and contents on a remote server. This is very useful, but people wanted live data. This is where the CGI (Common Gateway Interface) specification helped us. It helped us connect to databases and display stuff on the fly. The CGI specification was a major breakthrough in Web Application Development. The CGI standards made sure that the same CGI program worked on different Web servers.

CGI became the bread and butter of web developers. It was the most common means of displaying dynamic content on the internet. Though it was good, it wasn't good enough. It was not able to handle the performance requirements of the bursting Internet users. It was literally too much for it.

If you are asking me why CGI couldn't handle the load, the answer is simple. CGI spawned a separate process for every request that it receives. This design was able to sustain during off-peak hours but ate off server resources during peak loads which was eventually too much for it.
With growing numbers of users of web applications, scalability became a key consideration which wasn't CGI’s Middle Name and hence people started exploring other options.

Many CGI derivatives came up as server-side programming solutions that implement business logic, including ASP, ColdFusion, PHP, and Perl. Java surpassed them all due to portability and its object oriented programming design.

Alas, he we are, learning JSPs and Servlets that are the children of the Java Family which make our lives all the more easier in the world of Web Development.

Java was conceptualized in 1991 but it wasn't in the internet programming world until 1997. Servlets were the alternative to CGI and were released in 1997. Unlike CGI, which starts a process for each request, Servlets just spawn a new thread. Servlets had a better or rather efficient architecture which was able to handle the loads of the internet.

Though Servlets were awesome when compared to CGI, they still had some issues when it came to displaying dynamic content on a web page. Thankfully, Sun released the JSP (Java Server Pages) specifications in 1998, which solved all our UI woes. JSPs enabled programmers to display dynamic HTML content that could also use Java features. The combination of JSPs and Servlets was just what the Doctor Prescribed and it just revolutionized the Web Programming industry.
That's it for the history lesson. Now we are all set to dive deep into the world of magical Servlets and JSPs.

Previous Chapter: Introduction to the SCWCD Exam

Next Chapter: Web Servers & Servlet Containers