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{ private Connection con; public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { try { ClassforName("sunjdbcodbcJdbcOdbcDriver"); con = DriverManagergetConnection("jdbc:odbc:usda"); // run some lengthy database operation here } catch (Exception e) { throw new ServletException(egetMessage()); } } }
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Consider what happens when two requests arrive in separate threads a few hundred milliseconds apart The first one opens the database connection and stores a reference to it in the con instance variable It then uses the connection to perform a table update or some other database operation Meanwhile, the second request arrives and opens another connection and stores a reference to it in the same con instance variable If the first operation finishes and tries to do another database operation, it no longer has its original connection object it only knows about the second one Bad things then happen when it tries to use the second connection The same type of problem with instance variables can occur in servlets that call other methods from within their service method If these other methods try to access the servlet request, response, or any object created in the service method that has been saved in an instance variable, there s no way to guarantee a request in another thread won t corrupt the variables by storing references to its own objects in them4 The safest approach is simply not to use instance variables, only local variables defined inside the service method
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4 This particular problem can be solved by using a private class (essentially a data structure) to hold all objects of interest, and then passing this class as a parameter to the subsidiary methods
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Although the single instance multiple thread model is the default, a servlet can change this behavior by implementing SingleThreadModel This interface, which has no methods, informs the servlet engine that it should create a pool of instances and allocate each incoming request to its own instance and thread This guarantees no two requests handled by the same instance will overlap in their execution of the service method Thus, instance variables can only be affected by one request at a time, making them thread safe Note, because multiple instances may exist, however, there s nothing to prevent them from executing concurrently in different threads If they access external resources like files or database connections, therefore, they can still come into conflict There are few situations in which SingleThreadModel solves a problem that couldn t be handled better by other means
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Although navigating through a Web page may seem like a conversation between client and server, in most cases, it isn t Typically, a Web client requests an HTML document, which is located by the server and transmitted back to the client If image links are in the HTML, the client (if it s a Web browser) will make additional requests to the server for each image If the user clicks a hyperlink in the page, the client issues a new HTTP request for it, but all this happens one request at a time Between each request, the server moves on to handle other requests, forgetting all about the first client No back-and-forth exchange of commands and data occurs, only a request followed by a response and a disconnect5 For basic downloading of static documents, this is adequate However, applications like shopping carts or iterative search engines need to maintain active objects on the server that are associated with particular clients It may take several requests to build these objects In this case, a need exists to keep track of to which client the objects are bound Several approaches can be used to solve this problem Most of them involve maintaining the object itself on the server, assigning it a unique key the client is asked to remember In each subsequent related request, the client passes back the key, which enables the server to reestablish the context This is similar to booking an airline ticket over the telephone The ticket agent asks the customer for her name, address, and flight information, entering all this into a data entry application that assigns a confirmation number, which is reported back
5 HTTP 11 does provide a means for connections to persist for a few seconds, so that, for example, HTML and associated images can be downloaded efficiently This requires both the client and server to know about the capability and request it explicitly The request/response protocol itself, however, is the same
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