j2ee: Implicit Objects and their Scope [12/20]

Objects can be manipulated in JSP.

Objects exist in different scopes:

• page scope: a single page
• request scope: pages processing the same request
• session scope: pages processing requests that are in the same session
• application scope: all pages in an application

Remember, and object created inside a method would only have scope in that method as well. In Java, we also limit scope by using the { }

An object with a given scope is accessible to pages that are in the same scope as the page where the object was created. (more on this later)

Certain objects are created implicitly for each page by the container:

All of these are predefined and can be used inside your JSP.

• request
• response
• pageContext
• session
• application
• out
• config
• page
• exception - for error pages only

request

• A reference to the protocol dependent subclass of javax.servlet.ServletRequest (i.e. javax.servlet.http.HttpServletRequest)
• You can use it just like you did with servlets
• request scope

response

• A reference to the protocol dependent subclass of javax.servlet.ServletResponse (i.e. javax.servlet.http.HttpServletResponse)
• You can use it just like you did with servlets
• page scope

pageContext

• an object that provides a context to store references to objects used by the page
• encapsulates implementation-dependent features, and provides convenience methods.:
  • getOut(), getException(), getPage() getRequest(), getResponse(), getSession(), getServletConfig() and getServletContext().
• provides uniform access to diverse objects
  • setAttribute(), getAttribute(), findAttribute(), removeAttribute(), getAttributesScope() and getAttributeNamesInScope().
• provides methods for inclusion, error handling, and forwarding
  • forward(), include(), and handlePageException().
• A mechanism for obtaining the JspWriter for output
• A single object to manage the various scoped namespaces
• page scope

session

• A reference to the HTTP session (javax.servlet.http.HttpSession)
• You can use it just like you did with servlets.
• Only available if this page is marked as participating in a session. (How we do this is later). If it isn't participating, refering to the session is illegal and results in a translation error
• session scope

application

• The servlet context returned by calling getServletConfig().getContext() in a servlet
• You can use it as you would the context in a servlet
• application scope

out

• javax.servlet.jsp.JspWriter
• An object that writes into the output stream like the PrintWriter in a servlet (it is a subclass).
• page scope

config

• The javax.servlet.ServletConfig that you would get from calling getServletConfig() in a servlet
• use for getting init parameters
• page scope

page

• a java.lang.Object
• The instance of this page' implementation class processing the current request
• When using Java as the scripting language, then page is a synonym for this
• page scope
• not typically used

exception

• a java.lang.Throwable
• only available on error pages (we'll talk about how to create these next)
• The exception that resulted in the error page being invoked