destroy in Java

Drawing QR Code in Java destroy

String getServletInfo()

Returns a string containing information about the servlet In WebSphere, this string can be set at deployment time from the Administrative Console Returns a servlet configuration object, which should be the same one passed as parameter to the init method

ServletConfig getServletConfig()

WebSphere Application Server 40 supports version 22 of the servlet API The remainder of this chapter (and the book) relates to this version of the API

Servlet Initialization Initialization often occurs when you register the servlet with the server via the init method The init method is the perfect place to include all startup code that is required for the correct operation of the servlet and that might take a relatively long time such things as reading files, creating network connections, and so on Instances are often created before the actual requests arrive, and therefore this time is saved at the critical phase when the user is awaiting a response Also quite common in the real world is the model in which a pool of servlets is instantiated before requests start arriving (with the servlets in the pool often sharing the same resources) The servlet is guaranteed that the init method will return before any service requests are made Furthermore, the init method is not called again for the 109

same servlet instance This basically makes the init method thread-safe The init method can throw either the ServletException or UnavailableException exception in case any errors occur during initialization Servlet Processing After the servlet has been initialized, it is ready for processing The main method of a servlet, and the justification for its existence, is the service method Through this method, the servlet implements a request and response paradigm The service method takes an arriving request as a parameter in a ServletRequest and creates a response that is encapsulated in a ServletResponse The servlet model does not require much more than this a servlet is really a generic being that services requests and creates responses The request object contains information about the service request, including parameters provided by the client, as shown in Table 7-2 The response object is used to return information to the client, and its methods are shown in Table 7 -3 Servlet Finalization The last piece of the servlet contract is the finalization stage, implemented via the destroy method This method is called once by the server when it unloads the servlet After destroy is run, it is guaranteed that the service method will not be called again until the server reloads the servlet When the server removes a servlet, it calls destroy after all service calls have been completed or a service-specific number of seconds have passed, whichever comes first In the case of long-running operations, a service request could still be active when destroy is called It is the responsibility of the servlet to ensure that any threads still in the service stage complete One way to achieve this is to have the destroy method set a flag and have the servlet check whether the flag is on at the end of each request If the flag appears, and there are no other running requests, the servlet will clean up its resource Such tactics are seldom required, because in most cases servlets perform relatively short operations (if for no other reason, at least for providing reasonable responsiveness to users) Thread Safety Thread safety is an important consideration in the programming of servlets In addition, more than one active thread may be using the same servlet object Therefore, unless you specifically implement the SingleThreadModel interface in your servlet (thus indicating that you do not want your servlet to be multi-threaded), you must make sure your code is re-entrant and thread-safe To this end, use instance variables as little as possible When such variables are necessary, access to them must be made using synchronized methods so that you won't get errors when multiple threads start running concurrently Table 7-2: ServletRequest Methods Method getAttribute Signature Object getAttribute (String attribute) Description Returns the value of the named attribute of the request or null if the specified attribute does not exist Attributes are server-specific Returns enumeration