【正文】 okies, 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 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 mon ways to keep track of users between requests. A cookie is a special packet of information on the user’s puter. 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 4 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. DEFINITION:Static content on the Web es directly from text or data files, like HTML or JPEG files. These files might be changed from time to time, but they are not altered automatically when requested by a web browser. Dynamic content, on the other hand, is generated on the fly, typically in response to an individualized request from a browser. Common Gateway Interface (CGI): The first widely used standard for producing dynamic content was the Common Gateway Interface (CGI). CGI uses standard operating system features, such as environment variables and standard input and output, to create a bridge, or gateway, between the web server and other applications on the host machine. The other applications can look at the request sent to them by the web server and create a customized response. When a web server receives a request that’s intended for a CGI program, it runs that program and provides the program with information from the ining request. The CGI program runs and sends its output back to the server. The web server then relays the response to the browser. CGI defines a set of conventions regarding what information it will pass as environment variables and how it expects standard input and output to be used. Like HTTP, CGI is flexible and easy to implement, and a great number of CGIaware programs have been written. The main drawback to CGI is that it must run a new copy of the CGIaware program for each request. This is a relatively expensive process that can bog down highvolume sites where thousands of requests are serviced per minute. Another drawback is that CGI programs tend to be platform dependent. A CGI program written for one operating system may not run on another. Java servlets: Sun’s Java Servlet platform directly addresses the two main drawbacks of CGI , servlets offer better performance and utilization of resources than 5 conventional CGI programs. Second, the writeonce, runanywhere nature of Java means that servlets are portable between operating systems that have a Java Virtual Machine (JVM). A servlet looks and feels like a miniature web server. It receives a request and renders a response. But, unlike conventional web servers, the servlet application programming interface (API) is specifically designed to help Java developers create dynamic applications. The servlet itself is simply a Java class that has been piled into byte code, like any other Java object. The servlet has access to a rich API of HTTPspecific services, but it is still just another Java object running in an application and can leverage all your other Java assets. To give conventional web servers access to servlets, the servlets are plugged into containers. The servlet container is attached to the web server. Each servlet can declare what URL patterns it would like to handle. When a request matching a registered pattern arrives, the web server passes the request to the container, and the container invokes the servlet. But unlike CGI programs, a new servlet is not created for each request. Once the container instantiates the servlet, it will just create a new thread for each request. Java t