4.4  Implementation of the Application Logic 
The JSP pages are the front end of the application, while the database stores the system data. 
The application logic, programmed in Java, pulls the two pieces together. The following figure 
shows all classes and the dependencies among them for the internal part of the application. 
Figure 15 indicates that many classes are servlet classes (i.e. they are derived from the 
HTTPServlet class). The servlet classes contain only service() methods to process client 
requests. The request is almost always received from a JSP page. 
To the left of the servlet classes, the classes containing the application logic are shown. Their 
methods perform actions such as database queries, updates, and inserts according to provided 
arguments. Various get() methods are also provided, which are used by the JSP pages to 
access the dynamic data that the private member variables of the classes contain once a 
database query to retrieve the information was performed. The dependencies of the servlets to 
the application logic classes are shown in the diagram above through solid lines. 
To the right of the servlet classes, miscellaneous supplementary classes are shown. Some of 
them are required by most of the servlets (e.g. the database bean class MyDbBean), while 
others are needed less frequently. 

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