barcode generator visual basic 6 source code Storing and Retrieving Objects from Sessions in Java

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Storing and Retrieving Objects from Sessions
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Objects are bound to a session with the setAttribute() method:
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sessionsetAttribute("jspcrsessionsmyappuser", userID);
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The name under which an object is bound can be any unique string Because sessions are shared between all servlets and JSP pages in the current HTTP session, however, it makes sense to use a name that won t conflict with other applications Most common is to choose names with a prefix that s the package name or fully qualified class name of the servlet or JSP page Any kind of object can be stored in a session, but because sessions may be serialized, a good idea is to have session objects implement javaioSerializable Note, too, only objects can be stored, not primitives like int, char or double To store these primitives, you must use their object wrappers Integer, Character, or Double Objects can be retrieved from a session with the getAttribute() method:
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String userID = (String) sessiongetAttribute( "jspcrsessionsmyappuser");
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Like a Map or Hashtable, a session stores only objects, so when they re retrieved, they must be cast into the appropriate type Primitives contained in wrapper classes must be extracted by the methods provided in the wrapper class:
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Session and Thread Management
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Integer countObject = (Integer) getAttribute("count"); int count = countObjectintValue();
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Usually, if you stored an attribute in a session, you know its name and type, and you can request it directly in this manner You can also get a list of attribute names, however, from the getAttributeName() method:
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outprintln("Objects in this session:"); outprintln("<PRE>"); Enumeration enames = sessiongetAttributeNames(); while (enameshasMoreElements()) { String name = (String) enamesnextElement(); Object value = sessiongetAttribute(name); outprintln(name + " = " + value); } outprintln("</PRE>");
When an object is no longer needed, it can be removed from the session with removeAttribute():
JSP IN ACTION sessionremoveAttribute("jspcrsessionsmyappuser");
This happens automatically when the session is closed, but situations may occur when an attribute needs to be removed earlier than this
Destroying Sessions
Once created, a session ordinarily persists until it times out or is shut down Timeout refers to the maximum length of time between requests that the session will remain valid This is an important consideration because the server has no way of knowing whether a client has finished working with a session, other than by being told explicitly or by waiting a fixed length of time The default timeout interval can be set in the deployment descriptor webxml:
<web-app> <session-config> <session-timeout> 30 </session-timeout> </session-config> </web-app>
The interval is specified as a number of minutes, 30 being the default The value entered here applies to all sessions in the application unless they individually override it
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Some applications that use scarce resources like database connections may choose to time out sooner These applications can use the setMaxInactiveInterval() method to select a shorter time period:
sessionsetMaxInactiveInterval(180);
The argument supplied to setMaxInactiveInterval() is a number of seconds4 The previous example uses 180 seconds or three minutes The current value can be obtained with getMaxInactiveInterval() If a negative value is specified, the session never times out In some cases, a definite end to the session can be provided In these cases, the invalidate() method can be used:
sessioninvalidate();
This method marks the session as being inactive and unbinds all objects bound to it For example, in a shopping cart application that uses a session to store items being ordered, after the checkout logic writes the order to a database, the session should be destroyed so, if the user purchases more items, the old session contents won t still be there
Examples Revisited
The session API can handle all the session tracking tasks described earlier in this chapter In this section, you learn how the hidden fields, URL rewriting, and cookies examples can be done using the same session API approach
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