Implicit variables or well known variables

In the previous post we have seen how jsp is compiled into a servlet. But when the code is compiled by a java compiler it will generate an error indicating that we are using a variable "j" without declaring it.

According to the jsp specification we can use the following variables as part of the scriptlets without declaring the variables in the jsp page. These variables are called as "well known variables" or "implicit variables". There are 9 implicit variables:

• pageContext
• session
• request
• response
• application
• config
• out
• page
• exception

pageContext is a convenient class which provides the methods like forward, include, getSession, getServletConfig, getServletContext, getRequest, getResponse, setAttribute, getAttribute, removeAttribute etc.

request object is an instance of a java class that implements the javax.servlet.http.HttpServletRequest interface. The request object can be used to get header information, parameters, attributes etc.

response object is an instance of a java class that implements the javax.servlet.http.HttpServletResponse interface. Using the "response" object we can set the response content type, add cookie and redirect the response.

session object is an instance of a java class that implements the javax.servlet.http.HttpSession interface. Using the "session" object we can store session state for a single user.

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.

out object is an instance of the javax.servlet.jsp.JspWriter class. Using the "out" object we can send some content to the client. In other words we can write the output content.