JSP application frameworks What are application frameworks:A framework is a reusable, semi-complete application that can be specialized to produce custom applications [Johnson]。 Like people, software applications are more alike than they are different。 They run on the same computers, expect input from the same devices, output to the same displays, and save data to the same hard disks。 Developers working on conventional desktop applications are accustomed to toolkits and development environments that leverage the sameness between applications。 Application frameworks build on this common ground to provide developers with a reusable structure that can serve as the foundation for their own products。76217

Hypertext Transfer Protocol (HTTP):

Many server applications communicate using protocols other than HTTP。 Some of these maintain an ongoing connection between the computers。 The application server knows exactly who is connected at all times and can tell when a connection is dropped。 Because they know the state of each connection and the identity of each person using it, these are known as stateful protocols。

By contrast, HTTP is known as a stateless protocol。 An HTTP server will accept any request from any client and will always provide some type of response, even if the response is just to say no。 Without the overhead of negotiating and retaining a connection, stateless protocols can handle a large volume of requests。 This is one reason why the Internet has been able to scale to millions of computers。

Another reason HTTP has become the universal standard is its simplicity。 An HTTP request looks like an ordinary text document。 This has made it easy for applications to make HTTP requests。 You can even send an HTTP request by hand using a standard utility such as Telnet。 When the HTTP response comes back, it is also in plain text that developers can read。

The first line in the HTTP request contains the method, followed by the location

of the requested resource and the version of HTTP。 Zero or more HTTP request headers follow the initial line。 The HTTP headers provide additional information to the server。 This can include the browser type and version, acceptable document types, and the browser’s cookies, just to name a few。 Of the seven request methods, GET and POST are by far the most popular。

Once the server has received and serviced the request, it will issue an HTTP response。 The first line in the response is called the status line and carries the HTTP protocol version, a numeric status, and a brief description of the status。 Following the status line, the server will return a set of HTTP response headers that work in a way similar to the request headers。

As we mentioned, HTTP does not preserve state information between requests。The server logs the request, sends the response, and goes blissfully on to the next request。 While simple and efficient, a stateless protocol is problematic for dynamic applications that need to keep track of their users。 (Ignorance is not always bliss。

Cookies and URL rewriting are two common ways to keep track of users between requests。 A cookie is a special packet of information on the user’s computer。 URL rewriting stores a special reference in the page address that a Java server can use to track users。 Neither approach is seamless, and using either means extra work when developing a web application。 On its own, a standard HTTP web server does not traffic in dynamic content。 It mainly uses the request to locate a file and then returns that file in the response。 The file is typically formatted using Hypertext Markup Language (HTML) [W3C, HTML] that the web browser can format and display。 The HTML page often includes hypertext links to other web pages and may display any number of other goodies, such as images and videos。 The user clicks a link to make another request, and the process begins a new。

Standard web servers handle static content and images quite well but need a helping hand to provide users with a customized, dynamic response。