JSP Implicit Objects

JSP Implicit Objects are as follows:

1. request implicit object

   The JSP implicit request object is an instance of a java class that implements the javax.servlet.http.HttpServletRequest interface. It represents the request made by the client. The request implicit object is generally used to get request parameters, request attributes, header information and query string values.

2. response implicit object

   The JSP implicit response object is an instance of a java class that implements the javax.servlet.http.HttpServletResponse interface. It represents the response to be given to the client. The response implicit object is generally used to set the response content type, add cookie and redirect the response.

3. out implicit object

   The JSP implicit out object is an instance of the javax.servlet.jsp.JspWriter class. It represents the output content to be sent to the client. The out implicit object is used to write the output content.

4. session implicit object

   The JSP implicit session object is an instance of a java class that implements the javax.servlet.http.HttpSession interface. It represents a client specific conversation. The session implicit object is used to store session state for a single user.

5. application implicit object

   The JSP implicit application object is an instance of a java class that implements the javax.servlet.ServletContext interface. It gives facility for a JSP page to obtain and set information about the web application in which it is running.

6. exception implicit object

   The JSP implicit exception object is an instance of the java.lang.Throwable class. It is available in JSP error pages only. It represents the occured exception that caused the control to pass to the JSP error page.

7. config implicit object

   The JSP implicit config object is an instance of the java class that implements javax.servlet.ServletConfig interface. It gives facility for a JSP page to obtain the initialization parameters available.

8. page implicit object

   The JSP implicit page object is an instance of the java.lang.Object class. It represents the current JSP page. That is, it serves as a reference to the java servlet object that implements the JSP page on which it is accessed. It is not advisable to use this page implict object often as it consumes large memory.

9. pageContext implicit object

   The JSP implicit pageContext object is an instance of the javax.servlet.jsp.PageContext abstract class. It provides useful context information. That is it provides methods to get and set attributes in different scopes and for transfering requests to other resources. Also it contains the reference to to implicit objects.

JSP Life Cycle

JSP’s life cycle can be grouped into following phases.

1. JSP Page Translation:

A java servlet file is generated from the JSP source file. This is the first step in its tedious multiple phase life cycle. In the translation phase, the container validates the syntactic correctness of the JSP pages and tag files. The container interprets the standard directives and actions, and the custom actions referencing tag libraries used in the page.

2. JSP Page Compilation:

The generated java servlet file is compiled into a java servlet class.

Note: The translation of a JSP source page into its implementation class can happen at any time between initial deployment of the JSP page into the JSP container and the receipt and processing of a client request for the target JSP page.

3. Class Loading:

The java servlet class that was compiled from the JSP source is loaded into the container.

4. Execution phase:

In the execution phase the container manages one or more instances of this class in response to requests and other events.
The interface JspPage contains jspInit() and jspDestroy(). The JSP specification has provided a special interface HttpJspPage for JSP pages serving HTTP requests and this interface contains _jspService().

5. Initialization:

jspInit() method is called immediately after the instance was created. It is called only once during JSP life cycle.

6. _jspService() execution:

This method is called for every request of this JSP during its life cycle. This is where it serves the purpose of creation. Oops! it has to pass through all the above steps to reach this phase. It passes the request and the response objects. _jspService() cannot be overridden.

7. jspDestroy() execution:

This method is called when this JSP is destroyed. With this call the servlet serves its purpose and submits itself to heaven (garbage collection). This is the end of jsp life cycle.

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

forward Vs sendRedirect

forward

Control can be forward to resources available within the server from where the call is made. This transfer of control is done by the container internally and browser / client is not involved. This is the major difference between forward and sendRedirect. When the forward is done, the original request and response objects are transfered along with additional parameters if needed.

redirect

Control can be redirect to resources to different servers or domains. This transfer of control task is delegated to the browser by the container. That is, the redirect sends a header back to the browser / client. This header contains the resource url to be redirected by the browser. Then the browser initiates a new request to the given url. Since it is a new request, the old request and response object is lost.

For example, sendRedirect can transfer control from http://javapapers.com to http://anydomain.com but forward cannot do this.

‘session’ is not lost in both forward and redirect.

To feel the difference between forward and sendRedirect visually see the address bar of your browser,
in forward, you will not see the forwarded address (since the browser is not involved)
in redirect, you can see the redirected address.

JSP include directive Vs JSP include action

<%@ include file=”filename” %> is the JSP include directive.
At JSP page translation time, the content of the file given in the include directive is ‘pasted’ as it is, in the place where the JSP include directive is used. Then the source JSP page is converted into a java servlet class. The included file can be a static resource or a JSP page. Generally JSP include directive is used to include header banners and footers. The JSP compilation procedure is that, the source JSP page gets compiled only if that page has changed. If there is a change in the included JSP file, the source JSP file will not be compiled and therefore the modification will not get reflected in the output.


<jsp:include page=”relativeURL” /> is the JSP include action element.The jsp:include action element is like a function call. At runtime, the included file will be ‘executed’ and the result content will be included with the soure JSP page. When the included JSP page is called, both the request and response objects are passed as parameters.If there is a need to pass additional parameters, then jsp:param element can be used. If the resource is static, its content is inserted into the calling JSP file, since there is no processing needed.

Include Directive	Include Action
include directive is processed at the translation time	Include action is processed at the run time.
include directive can use relative or absolute path	Include action always use relative path
Include directive can only include contents of resource it will not process the dynamic resource	Include action process the dynamic resource and result will be added to calling JSP
We can not pass any other parameter	Here we can pass other parameter also using JSP:param
We cannot  pass any request or response object to calling jsp to included file or JSP or vice versa	In this case it’s possible.