java barcode generator source code JSP: The Complete Reference in Java

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JSP: The Complete Reference
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I HttpSessionBindingListener An interface that allows an object to know when it has been stored in a session or removed from one The interface has two callback methods, valueBound() and valueUnbound(), which the object must implement to receive the binding notifications I HttpSessionBindingEvent An event object passed to the valueBound() and valueUnbound() methods of an HttpSessionBindingListener The event has methods for returning the session and the name under which the listener was bound to the session
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Creating Sessions
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A servlet indicates it wants to use a session by calling the getSession() or getSession(boolean create) methods in HttpServletRequest, as shown here:
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HttpSession session = requestgetSession(true);
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The getSession() method with no parameters is a convenience method that simply calls getSession(true) The create parameter indicates whether the servlet engine should create a new session if one doesn t already exist If the parameter is false, the servlet can only operate on an existing session In either case, the request is examined to see if it contains a valid session ID If so, the servlet container returns a reference to the session object, which can then be used to store and retrieve session attributes In a JSP page, session creation is automatic, unless is it suppressed in the page directive At the beginning of the _jspService() method in the generated servlet, the PageContext object is created and initialized As part of the initialization, the JspFactorygetPageContext() method calls requestgetSession(true) The newly created or accessed session is returned to the generated servlet when it calls pageContextgetSession() The session is then accessible to the rest of the JSP page as the implicit variable session, as the following shows:
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public void _jspService( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PageContext pageContext = _jspFactorygetPageContext (this, request, response, null, true, 8192, true); JspWriter out = pageContextgetOut(); HttpSession session = pageContextgetSession(); }
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If a JSP page doesn t need to use a session, it should suppress the automatic creation in the page directive:
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<%@ page session= false %>
This relieves the servlet engine from having to create and maintain a session when it isn t needed The memory requirements for unnecessary sessions can be significant When the session is first created, the client (Web browser) doesn t yet know about it When the session ID has been sent to the client and the client sends it back in the next request, the client is said to join the session A servlet or JSP page can detect whether this has happened with the isNew() method:
HttpSession session = requestgetSession(); if (sessionisNew()) { // Create an empty shopping cart }
sessionisNew() is true if the session is newly created and the client hasn t yet been informed or if the client has been informed, but chooses not to participate
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Session Tracking Mechanisms
The servlet engine tries to use cookies to keep track of the session ID In the HTTP response written by a servlet that created a session, a Set-Cookie header containing the session ID is in a cookie named JSESSIONID3
Set-Cookie: JSESSIONID=rkbg6z27j1;Path=/jspcr
If the client accepts the cookie, the client returns it in subsequent requests:
Cookie: JSESSIONID=rkbg6z27j1
If this happens, the client request can be associated with the session with no special considerations on the part of the servlet If the client doesn t accept cookies, however, the session is lost To prevent this, the servlet API has a fallback mechanism It uses URL rewriting if cookies fail This is slightly more complicated for the programmer because it means all URLs written by the servlet must have the session ID appended
3 This is prescribed by the Servlet 22 API specification Some servlet engines use a different value
JSP: The Complete Reference
But, because this is unnecessary and expensive if the client accepts cookies, the URL rewriting should only be done if you definitely know the cookie method fails Fortunately, the servlet API has methods that encapsulate all this logic The HttpServletResponse class has encodeURL() and encodeRedirectURL() methods that add the session ID to a URL only if necessary:
String myURL = responseencodeURL( /servlet/nextServlet ); outprintln("Click <A HREF='" + myURL + "'>here</A>" + " to continue");
encodeRedirectURL() should be used with URLs passed to the responsesendRedirect() method and encodeURL() with all others When the encodeURL() method is used, the session ID is always embedded in the URL when sessionisNew() is true After the first response from the client, the servlet engine determines whether the session ID was returned in a cookie If not, the servlet engine continues to append the ID to URLs passed through encodeURL() Otherwise, it switches to using cookies only and encodeURL() returns unmodified URL strings This makes testing all the possibilities unnecessary for the programmer
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