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Servlets were designed to allow for extension of a server providing any service. Currently, however, only HTTP and JSP page servlets are supported. In the future, a developer may be able to extend an FTP server or an SMTP server using servlets.
A servlet extends a server's functionality by offering a specific service within a well-defined framework. It is a small piece of Java code -- often just a single class -- that provides a specific service. For example, an HTTP servlet may provide a bank customer with details of her recent deposits and withdrawals. Another HTTP servlet could allow a customer to view, and even edit, his mailing address.
To deploy a servlet usually requires configuration of the hosting server application. When the server encounters a particular type of request, it invokes the servlet, passing to it details about the request and a response object for returning the result.
All servlets implement the
javax.servlet.Servlet interface either directly -- in the case of generic servlets -- or indirectly, in the case of HTTP or JSP servlets. The
javax.servlet.Servlet interface's important methods include:
init(): defines any initialization code that should be executed when the servlet loads into memory.
service(): the main method called when the servlet receives a service request. It defines the bulk of the processing logic provided by the servlet.
destroy(): defines any clean-up code required before removing the servlet from memory.
When the servlet container first loads a servlet it invokes the servlet's
init() method to initialize the servlet. Then, as requests are made to execute the servlet, the servlet container repeatedly invokes
service() method to provide the required service. Finally, when the servlet container no longer needs the servlet, it invokes the servlet's
destroy() method and unloads it from memory. Note that during the lifetime of a single servlet instance, the
destroy() methods will be invoked only once, whereas the
service() method will be invoked many times -- once each time a request is made to execute the servlet.
JSP Page servlets are mostly of interest to implementers of JSP containers and are beyond the scope of this article. Rather, we now go on to look specifically at HTTP servlets.