Explain the JSP Implicit Objects

JSP Implicit objects are created by the web container. These implicit objects are Java objects that implement interfaces in the Servlet and JSP API. Scripting elements in a JSP page can make use of these JSP implicit objects. There are nine (9) JSP implicit objects available.

JSP Implicit Objects are as follows:

1.request

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

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

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

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

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

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

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

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

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.

Difference between forward and 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.

Explain the scope of JSP objects.

Scope of JSP objects:
The availability of a JSP object for use from a particular place of the application is defined as the scope of that JSP object. Every object created in a JSP page will have a scope. Object scope in JSP is segregated into four parts and they are page, request, session and application.

* page
‘page’ scope means, the JSP object can be accessed only from within the same page where it was created. The default scope for JSP objects created using tag is page. JSP implicit objects out, exception, response, pageContext, config and page have ‘page’ scope.
* request
A JSP object created using the ‘request’ scope can be accessed from any pages that serves that request. More than one page can serve a single request. The JSP object will be bound to the request object. Implicit object request has the ‘request’ scope.
* session
’session’ scope means, the JSP object is accessible from pages that belong to the same session from where it was created. The JSP object that is created using the session scope is bound to the session object. Implicit object session has the ’session’ scope.
* application
A JSP object created using the ‘application’ scope can be accessed from any pages across the application. The JSP object is bound to the application object. Implicit object application has the ‘application’ scope.

How to pre-compile JSP?

Add jsp_precompile as a request parameter and send a request to the JSP file. This will make the jsp pre-compile. Why it is mentioned as pre compile instead of compilation is that, the request is not served. That is, the JSP will not be executed and the request will not be serviced. Just it will be compiled and the implementation class will be generated.

The jsp_precompile parameter may have no value, or may have values true or false. It should not have any other values like ?jsp_precompile=yes – this will result in HTTP error 500.

So the example for query strings to pre compile jsp files are
?jsp_precompile
?jsp_precompile=true
?jsp_precompile=false
?foobar=foobaz&jsp_precompile=true
?foobar=foobaz&jsp_precompile=false

How to do pre compile for a bunch of JSP files? for all JSPs in a folder or application?
There are no special features available for this in the JSP specification. But the application servers (JSP containers) provide methods to do this on their own way. At the end of this tutorial we will see how to pre compile JSP files for Apache Tomcat 6.

But if you want to pre compile in server independent manner you have to use jsp_precompile request parameter and no other option available. To do it for the whole application, you can custome write a servlet or jsp file which will raise a request for all the JSPs available with jsp_precompile added as parameter.