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Tuesday, 30 July 2013

Jsp and Servlet Tutorials - Jsp lifecycle - jspInit(), _jspService() & jspDestroy() (Part 5 of 8)

JSP so called Java Server Pages is an important part of Java Development methodology. Jsp is used to generate dynamic content and show it as html on a web page. A JSP file contains a .jsp extension and can be places anywhere in a project where a normal web page can reside.

In this particular blog we will learn about complete life cycle of a jsp file and implementation of jsp lifecycle methods. A JSP file goes through a number of states during its execution, here is a brief introduction to those states :

JSP lifecycle is consists of jspInit(), _jspService() and jspDestroy(), and these methods are called life cycle methods of the JSP.




JSP page translation and compilation phase

Every JSP page is first translated to a servlet before going through other lifecycle states. The container than validates the syntax for correctness of jsp components used on the page. The translated servlet file is than compiles into a java class file.


JSP class loading and execution.

In execution phase of a compiled jsp file the container maintained the request-response instances occurred during the lifecycle. Jsp consists of three lifecycle methods listed below.

1) jspInit() method
2) jspService() method
3) jspDestroy() method


What is jspInit() method

Once a JSP file is successfully compiled into a class file, jsp engine calls jspInit() method to initialize class file. This method is called by the container only once during the lifecycle. jspInit() method can be overridden by the author to initialize resources such as database and network related connections.
 public void jspInit() {
  // initialization code or something needs to be called before JSP page
  // starts serving the requests.
 }

Now the question arises why we shouldn’t use constructor instead of init method of jsp or servlet ?. Of course ! we can use constructor instead of init() method, there is nothing to stop us. In previous versions of java it was not possible to dynamically call constructor with arguments. This does no longer applied, but servlet containers still looks for an no-argument constructor. So we  won't have access to a ServletConfig or ServletContext if we use constructor instead of init() method.


What is _jspService() method

_jspService() method is called every time a request comes to jsp during its lifecycle.  The (_) underscore sign indicates that this method is generated by Container and hence cannot be overriden. The jsp code written by us goes in _jspService() because this is implicitly implemented. In case we  tries to override it explicitly, JSP compliler will give an error saying “the method is already implemented and cannot override’.



What is jspDestroy() method()

jspDestroy() method is called when servlet(jsp) is no longer in use, with the call of this method jsp lifecycle is ended and jsp submits itself to garbage collection.
 public void jspDestroy() {
  // code related to release resources like database ot network
  // connections.
 }
We can always call jspDestroy() at any stage of its lifecycle but it does not make sense because even if you called jspDestroy() in the beginning, it doesn't stops the JSP to execute the _jspService() method and sending the response back to the client. This is one of the  lifecycle method which is invoked by the JSP Container when the jsp instance is removed.


In this particular blog we came across 'JSP life-cycle methods and their use and implementation', in upcoming blogs we will discuss more about java and other opensource technologies.







Thanks for reading !
Being Java Guys Team



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