JSP implicit objects

Implicit objects are provided by JSP itself, we don’t need to define it we can simply use these objects by setting values and attributes. JSP container does all work like instantiating, defining and maintenance.

JSP provide following implicit objects

1. out

out is javax.servlet.jsp.JspWriter class use for printing. Out is Output Stream use response.getWriter() to send output to client

<%
  out.write("This is use for print output stream on client");
%>

2. page

page is used by java.lang.Object class. Page is instance of JSP page’s Servlet.

<%
    Object page = this; 
%>

3. pageContext

pageContext instance that contains all data’s information, which is associated with JSP page. This helps to access attribute and other field’s object set in JSP page. pageContext is in javax.servlet.jsp.PageContext class. pageContext is to managed various scope of attribute in shared information object.
Method of pageContext are following:

Method	Return	Description
pageContext.findAttribute(String Name)	Object	Search for named attribute in JSP page Request,session, application scope
pageContext.forward(String relativeURLPath)	void	This method is used to redirect or forward the current ServletRequest or ServletResponse to another active component
pageContext.getAttribute(String Name)	Object	Returns the object associated with name in the page scope
pageContext.getAttributesScope(String Name)	int	Get the scope where a given attribute is defined
pageContext.getErrorData()	ErrorData	Provide access to error information
pageContext.getException()	Exception	Get current value of the exception Object
pageContext.getOut()	JspWriter	Get current value of out object
pageContext.getRequest()	ServletRequest	Get current value of request object
pageContext.getResponse()	ServletResponse	Get current value of response object
pageContext.getServletConfig()	ServletConfig	Get any initialization parameters and startup configuration for this servlet
pageContext.getServletContext()	ServletContext	Gets the context from the servlet's ServletConfig object
pageContext.getSession()	HttpSession	Return the current value of session object

4. request

The HttpServletRequest object that javax.servlet.http.HttpServletRequest interface is provided to access request objects in JSP. The HTTP request is send to server through request object.
More example request object reference

5. response

The HttpServletResponse object that javax.servlet.http.HttpServletResponse interface is provided to send response object in JSP. The HTTP response is send to client through response object
More example response object reference

6. session

Session object is used to track information about a user, session is interface and maintain by HttpSession javax.servlet.http.HttpSession.
More example of session object reference

7. config

config object is related to get servlet’s configuration information. It is in javax.servlet.ServletConfig interface. Servlet configuration object is used to pass information of servlet to Servlet container during initialization of servlet. This is defined in web.xml file with <init-param> and method is used to get getServletConfig()

<servlet>
    <servlet-name>ServletName</servlet-name>
    <servlet-class>com.myapp.servlet.ServletName</servlet-class>    
        <init-param>
        <param-name>dbUserName</param-name>
        <param-value>root</param-value>
    </init-param>    
</servlet>

Method use to get init param is getInitParameter() to get value inside <param-value> and getInitParameterNames() to get name of <param-name>

8. application

Application object is used to share information for all JSPs and Servlets in the application context. Application is in javax.servlet.ServletContext.
More example of application object reference

9. exception

Exceptions are condition that can be caught and recovered from it. If exception is not caught it should throw on error page. Exception when throw on error page it should be isErrorPage= true at in directive page.

<%@ page errorPage="error.jsp" %>

<%@ page isErrorPage="true" %>

Example of Exception Implicit object in JSP

exception.jsp

<%@ page  language="java" errorPage="error.jsp" %>
<html>
<head>
<title>Implicit Exception object</title>
</head>

<body>
<%
 String a=null;
 int b=Integer.parseInt(a);
%>
</body>
</html>

error.jsp

<%@ page  language="java" import="java.io.*" isErrorPage="true"  %>
<html>
<head>
<title>Implicit Exception Error page</title>
</head>

<body>
The Exception is <strong><%= exception.toString() %></strong><br>
Message : <strong><%=exception.getMessage()%></strong><br>

</body>
</html>

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