JSP Life Cycle: Introduction, Phases, Methods

Details

What is JSP LifeCycle?

JSP Life Cycle is defined as translation of JSP Page into servlet as a JSP Page needs to be converted into servlet first in order to process the service requests. The Life Cycle starts with the creation of JSP and ends with the disintegration of that.

Let’s learn different phases of JSP Life Cycle

When the browser asks for a JSP, JSP engine first checks whether it needs to compile the page. If the JSP is last compiled or the recent modification is done in JSP, then the JSP engine compiles the page.

Compilation process of JSP page involves three steps:

JSP Lifecycle is depicted in the below diagram.

JSP Life Cycle

Following steps explain the JSP life cycle:

  1. Translation of JSP page
  2. Compilation of JSP page(Compilation of JSP page into _jsp.java)
  3. Classloading (_jsp.java is converted to class file _jsp.class)
  4. Instantiation(Object of generated servlet is created)
  5. Initialisation(_jspinit() method is invoked by container)
  6. Request Processing(_jspservice() method is invoked by the container)
  7. Destroy (_jspDestroy() method invoked by the container)

Let us have more detailed summary on the above points:

  1. Translation of the JSP Page:

A Java servlet file is generated from a JSP source file. This is the first step of JSP life cycle. In translation phase, container validates the syntactic correctness of JSP page and tag files.

Demo.jsp 

<html>
<head>
<title>Demo JSP</title>
</head>
<%
int demvar=0;%>
<body>
Count is:
<% Out.println(demovar++); %>
<body>
</html>

Code Explanation for Demo.jsp

Code Line 1: html start tag

Code Line 2: Head tag

Code Line 3 - 4: Title Tag i.e. Demo JSP and closing head tag

Code Line 5,6: Scriptlet tag wherein initializing the variable demo

Code Line 7 - 8: In body tag, a text to be printed in the output (Count is: )

Code Line 9: Scriplet tag where trying to print the variable demovar with incremented value

Code Line 10-11: Body and HTML tags closed

Demo JSP Page is converted into demo_jsp servlet in the below code.

JSP Life Cycle

Code explanation for Demo_jsp.java

Code Line 1: Servlet class demo_jsp is extending parent class HttpServlet

Code Line 2,3: Overriding the service method of jsp i.e. _jspservice which has HttpServletRequest and HttpServletResponse objects as its parameters

Code Line 4: Opening method

Code Line 5: Calling the method getWriter() of response object to get PrintWriterobject (prints formatted representation of objects to text output stream)

Code Line 6: Calling setContentType method of response object to set the content type

Code Line 7: Using write () method of PrintWriter object trying to parse html

Code Line 8: Initializing demovar variable to 0

Code Line 9: Calling write() method of PrintWriter object to parse the text

Code Line 10: Calling print() method of PrintWriter object to increment the variable demovar from 0+1=1.Hence, the output will be 1

Code Line 11: Using write() method of PrintWriter object trying to parse html

Output:

JSP Life Cycle

In the above example,

This is how a simple JSP page is translated into a servlet class.

  1. Compilation of the JSP Page
  1. Classloading
  1. Instantiation
  1. Initialization 
    public void jspInit()
{
	//initializing the code
}
  1. Request processing 
    void _jspservice(HttpServletRequest request HttpServletResponse response)
{
	//handling all request and responses
}
  1. Destroy 
    public void _jspdestroy()
{
            //all clean up code
}

 

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