JSP - Implicit Objects and Client Request

JSP Objects are the Java objects that the JSP Container makes available to the developers in each page and the developer can call them directly without being explicitly declared. JSP Implicit Objects are also called pre-defined variables.

Following table lists out the nine Implicit Objects that JSP supports −

S.No.	Object & Description
1	request This is the HttpServletRequest object associated with the request.
2	response This is the HttpServletResponse object associated with the response to the client.
3	out This is the PrintWriter object used to send output to the client.
4	session This is the HttpSession object associated with the request.
5	application This is the ServletContext object associated with the application context.
6	config This is the ServletConfig object associated with the page.
7	pageContext This encapsulates use of server-specific features like higher performance JspWriters.
8	page This is simply a synonym for this, and is used to call the methods defined by the translated servlet class.
9	Exception The Exception object allows the exception data to be accessed by designated JSP.

The request Object

The request object is an instance of a javax.servlet.http.HttpServletRequest object. Each time a client requests a page the JSP engine creates a new object to represent that request.

The request object provides methods to get the HTTP header information including form data, cookies, HTTP methods etc.

We can cover a complete set of methods associated with the request object in a subsequent chapter − JSP - Client Request.

The response Object

The response object is an instance of a javax.servlet.http.HttpServletResponse object. Just as the server creates the request object, it also creates an object to represent the response to the client.

The response object also defines the interfaces that deal with creating new HTTP headers. Through this object the JSP programmer can add new cookies or date stamps, HTTP status codes, etc.

We will cover a complete set of methods associated with the response object in a subsequent chapter − JSP - Server Response.

The out Object

The out implicit object is an instance of a javax.servlet.jsp.JspWriter object and is used to send content in a response.

The initial JspWriter object is instantiated differently depending on whether the page is buffered or not. Buffering can be easily turned off by using the buffered = 'false' attribute of the page directive.

The JspWriter object contains most of the same methods as the java.io.PrintWriter class. However, JspWriter has some additional methods designed to deal with buffering. Unlike the PrintWriter object, JspWriter throws IOExceptions.

Following table lists out the important methods that we will use to write boolean char, int, double, object, String, etc.

S.No.	Method & Description
1	out.print(dataType dt) Print a data type value
2	out.println(dataType dt) Print a data type value then terminate the line with new line character.
3	out.flush() Flush the stream.

The session Object

The session object is an instance of javax.servlet.http.HttpSession and behaves exactly the same way that session objects behave under Java Servlets.

The session object is used to track client session between client requests. We will cover the complete usage of session object in a subsequent chapter − JSP - Session Tracking.

The application Object

The application object is direct wrapper around the ServletContext object for the generated Servlet and in reality an instance of a javax.servlet.ServletContext object.

This object is a representation of the JSP page through its entire lifecycle. This object is created when the JSP page is initialized and will be removed when the JSP page is removed by the jspDestroy() method.

By adding an attribute to application, you can ensure that all JSP files that make up your web application have access to it.

We will check the use of Application Object in JSP - Hits Counter chapter.

The config Object

The config object is an instantiation of javax.servlet.ServletConfig and is a direct wrapper around the ServletConfig object for the generated servlet.

This object allows the JSP programmer access to the Servlet or JSP engine initialization parameters such as the paths or file locations etc.

The following config method is the only one you might ever use, and its usage is trivial −

config.getServletName();

This returns the servlet name, which is the string contained in the <servlet-name> element defined in the WEB-INF\web.xml file.

The pageContext Object

The pageContext object is an instance of a javax.servlet.jsp.PageContext object. The pageContext object is used to represent the entire JSP page.

This object is intended as a means to access information about the page while avoiding most of the implementation details.

This object stores references to the request and response objects for each request. The application, config, session, and out objects are derived by accessing attributes of this object.

The pageContext object also contains information about the directives issued to the JSP page, including the buffering information, the errorPageURL, and page scope.

The PageContext class defines several fields, including PAGE_SCOPE, REQUEST_SCOPE, SESSION_SCOPE, and APPLICATION_SCOPE, which identify the four scopes. It also supports more than 40 methods, about half of which are inherited from the javax.servlet.jsp.JspContext class.

One of the important methods is removeAttribute. This method accepts either one or two arguments. For example, pageContext.removeAttribute ("attrName") removes the attribute from all scopes, while the following code only removes it from the page scope −

pageContext.removeAttribute("attrName", PAGE_SCOPE);

The use of pageContext can be checked in JSP - File Uploading chapter.

The page Object

This object is an actual reference to the instance of the page. It can be thought of as an object that represents the entire JSP page.

The page object is really a direct synonym for the this object.

The exception Object

The exception object is a wrapper containing the exception thrown from the previous page. It is typically used to generate an appropriate response to the error condition.

JSP - Client Request

When a browser requests for a Webpage, it sends a lot of information to the web server. This information cannot be read directly because this information travels as a part of header of HTTP request. You can check HTTP Protocol for more information on this.

Following table lists out the important header information which comes from the browser. This information is frequently used in web programming −

S.No.	Header & Description
1	Accept This header specifies the MIME types that the browser or other clients can handle. Values of image/png or image/jpeg are the two most common possibilities.
2	Accept-Charset This header specifies the character sets that the browser can use to display the information. For example, ISO-8859-1.
3	Accept-Encoding This header specifies the types of encodings that the browser knows how to handle. Values of gzip or compress are the two most common possibilities.
4	Accept-Language This header specifies the client's preferred languages in case the servlet can produce results in more than one language. For example en, en-us, ru, etc.
5	Authorization This header is used by clients to identify themselves when accessing password-protected webpages.
6	Connection This header indicates whether the client can handle persistent HTTP connections. Persistent connections permit the client or other browser to retrieve multiple files with a single request. A value of Keep-Alive means that persistent connections should be used.
7	Content-Length This header is applicable only to POST requests and gives the size of the POST data in bytes.
8	Cookie This header returns cookies to servers that previously sent them to the browser.
9	Host This header specifies the host and port as given in the original URL.
10	If-Modified-Since This header indicates that the client wants the page only if it has been changed after the specified date. The server sends a code, 304 which means Not Modified header if no newer result is available.
11	If-Unmodified-Since This header is the reverse of If-Modified-Since; it specifies that the operation should succeed only if the document is older than the specified date.
12	Referer This header indicates the URL of the referring webpages. For example, if you are at Webpage 1 and click on a link to Webpage 2, the URL of Webpage 1 is included in the Referer header when the browser requests Webpage 2.
13	User-Agent This header identifies the browser or other client making the request and can be used to return different content to different types of browsers.

The HttpServletRequest Object

The request object is an instance of a javax.servlet.http.HttpServletRequest object. Each time a client requests a page, the JSP engine creates a new object to represent that request.

The request object provides methods to get HTTP header information including form data, cookies, HTTP methods, etc.

Following table lists out the important methods that can be used to read HTTP header in your JSP program. These methods are available with HttpServletRequest object which represents client request to webserver.

S.No.	Method & Description
1	Cookie[] getCookies() Returns an array containing all of the Cookie objects the client sent with this request.
2	Enumeration getAttributeNames() Returns an Enumeration containing the names of the attributes available to this request.
3	Enumeration getHeaderNames() Returns an enumeration of all the header names this request contains.
4	Enumeration getParameterNames() Returns an enumeration of String objects containing the names of the parameters contained in this request.
5	HttpSession getSession() Returns the current session associated with the this request, or if the request does not have a session, creates one.
6	HttpSession getSession(boolean create) Returns the current HttpSession associated with the this request or, if if there is no current session and create is true, returns a new session.
7	Locale getLocale() Returns the preferred Locale that the client will accept content in, based on the Accept-Language header.
8	Object getAttribute(String name) Returns the value of the named attribute as an Object, or null if no attribute of the given name exists.
9	ServletInputStream getInputStream() Retrieves the body of the request as binary data using a ServletInputStream.
10	String getAuthType() Returns the name of the authentication scheme used to protect the servlet, for example, "BASIC" or "SSL," or null if the JSP was not protected.
11	String getCharacterEncoding() Returns the name of the character encoding used in the body of this request.
12	String getContentType() Returns the MIME type of the body of the request, or null if the type is not known.
13	String getContextPath() Returns the portion of the request URI that indicates the context of the request.
14	String getHeader(String name) Returns the value of the specified request header as a String.
15	String getMethod() Returns the name of the HTTP method with which this request was made, for example, GET, POST, or PUT.
16	String getParameter(String name) Returns the value of a request parameter as a String, or null if the parameter does not exist.
17	String getPathInfo() Returns any extra path information associated with the URL the client sent when it made this request.
18	String getProtocol() Returns the name and version of the protocol the request uses.
19	String getQueryString() Returns the query string that is contained in the request URL after the path.
20	String getRemoteAddr() Returns the Internet Protocol (IP) address of the client that sent the request.
21	String getRemoteHost() Returns the fully qualified name of the client that sent the request.
22	String getRemoteUser() Returns the login of the user making this request, if the user has been authenticated, or null if the user has not been authenticated.
23	String getRequestURI() Returns the part of this request's URL from the protocol name up to the query string in the first line of the HTTP request.
24	String getRequestedSessionId() Returns the session ID specified by the client.
25	String getServletPath() Returns the part of this request's URL that calls the JSP.
26	String[] getParameterValues(String name) Returns an array of String objects containing all of the values the given request parameter has, or null if the parameter does not exist.
27	boolean isSecure() Returns a boolean indicating whether this request was made using a secure channel, such as HTTPS.
28	int getContentLength() Returns the length, in bytes, of the request body and made available by the input stream, or -1 if the length is not known.
29	int getIntHeader(String name) Returns the value of the specified request header as an int.
30	int getServerPort() Returns the port number on which this request was received.

HTTP Header Request Example

Following is the example which uses getHeaderNames() method of HttpServletRequest to read the HTTP header information. This method returns an Enumeration that contains the header information associated with the current HTTP request.

Once we have an Enumeration, we can loop down the Enumeration in the standard manner. We will use the hasMoreElements() method to determine when to stop and the nextElement() method to get the name of each parameter name.

<%@ page import = "java.io.*,java.util.*" %>

<html>
   <head>
      <title>HTTP Header Request Example</title>
   </head>

   <body>
      <center>
         <h2>HTTP Header Request Example</h2>
         
         <table width = "100%" border = "1" align = "center">
            <tr bgcolor = "#949494">
               <th>Header Name</th>
               <th>Header Value(s)</th>
            </tr>
            <%
               Enumeration headerNames = request.getHeaderNames();
               while(headerNames.hasMoreElements()) {
                  String paramName = (String)headerNames.nextElement();
                  out.print("<tr><td>" + paramName + "</td>\n");
                  String paramValue = request.getHeader(paramName);
                  out.println("<td> " + paramValue + "</td></tr>\n");
               }
            %>
         </table>
      </center>
   
   </body>
</html>

Let us now put the above code in main.jsp and try to access it.

HTTP Header Request Example

Header Name	Header Value(s)
accept	*/*
accept-language	en-us
user-agent	Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; InfoPath.2; MS-RTC LM 8)
accept-encoding	gzip, deflate
host	localhost:8080
connection	Keep-Alive
cache-control	no-cache

You can try working on all the methods in a similar way.

Anurag Rana