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To understand the advantages of servlets, you must have a basic understanding of how Web browsers and servers cooperate to provide content to a user Consider a request for a static Web page A user enters a Uniform Resource Locator (URL) to a browser The browser generates an HTTP request to the appropriate Web server The Web server maps this request to a specific file That file is returned in an HTTP response to the browser The HTTP header in the response indicates the type of the content The Multipurpose Internet Mail Extensions (MIME) are used for this purpose For example, ordinary ASCII text has a MIME type of text/plain The Hypertext Markup Language (HTML) source code of a Web page has a MIME type of text/html Now consider dynamic content Assume that an online bookstore uses a database to store information about its business, including book prices, availability, orders, and so forth It wants to make this information accessible to customers via Web pages The contents of those Web pages must be dynamically generated, to reflect the latest information in the database In the early days of the Web, a server could dynamically construct a page by creating a separate process to handle each client request The process would open connections to one or more databases in order to obtain the necessary information It communicated with the Web server via an interface known as the Common Gateway Interface (CGI) CGI allowed the separate process to read data from the HTTP request and write data to the HTTP response A variety of different languages were used to build CGI programs, including C, C++, and Perl However, CGI suffered serious performance problems Creating a separate process for each client request was expensive, in terms of processor and memory resources It was also expensive to open and close database connections for each client request In addition, the CGI programs were not platform-independent Therefore, other techniques were introduced, including servlets Servlets offer several advantages over CGI: Performance is significantly better Servlets execute within the address space of a Web server Creating a separate process to handle each client request isn't necessary Servlets are platform-independent, because they are written in Java Several Web servers, from vendors such as Sun, Netscape, and Microsoft, offer the Servlet API Programs developed for this API can be moved to any of these environments without recompilation The Java Security Manager on the server enforces a set of restrictions to protect the resources on a server machine You will see that some servlets are trusted and others are untrusted The full functionality of the Java class libraries is available to a servlet It can communicate with applets, databases, or other software via the sockets and RMI mechanisms that you have seen already
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The Life Cycle of a Servlet
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Three methods are central to the life cycle of a servlet: init( ), service( ), and destroy( ) They are implemented by every servlet and are invoked at specific times by the server Consider next a typical user scenario, to understand when these methods are called First, assume that a user enters a Uniform Resource Locator (URL) to a Web browser The browser then generates an HTTP request for this URL and sends it to the appropriate server Second, this HTTP request is received by the Web server The server maps this request to a particular servlet The servlet is dynamically retrieved and loaded into the address space of the server Third, the server invokes the init( ) method of the servlet This method is invoked only when the servlet is first loaded into memory You will see that initialization parameters can be passed to the servlet so that it may configure itself Fourth, the server invokes the servlet's service( ) method, which is called to process the HTTP request You will see that the servlet can read data that has been provided in the HTTP request, and may also formulate an HTTP response for the client The servlet remains in the server's address space and is available to process any other HTTP requests received from clients The service( ) method is called for each HTTP request Finally, the server may decide to unload the servlet from its memory The algorithms by which this determination is made are specific to each server The server calls the destroy( ) method to relinquish any resources, such as file handles that are allocated for the servlet Important data may be saved to a persistent store The memory allocated for the servlet and its objects can then be garbage-collected
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