Learning Java

Talking to CGI Programs and Servlets

CGI stands for Common Gateway Interface; it is an API for writing applications (often scripts) that can be run by a web server to service a particular range of URLs. Servlets are an implementation very similar to CGI using a component-ized framework in Java. CGI programs and servlets can perform dynamic activities like automatically generating web documents. More important, they can accept data sent from the browser; they are most frequently used to process forms.

The name/value pairs of HTML form fields are encoded by the client web browser in a special format and sent to the application using one of two methods. The first method, using the HTTP command GET , encodes the user’s input into the URL and requests the corresponding document. The server recognizes that the first part of the URL refers to a program and invokes it, passing along the information encoded in the URL as a parameter. The second method uses the HTTP command POST to ask the server to accept the encoded data and pass it to the CGI program as a stream.

In Java, we can create a URL that refers to a CGI program and send it data using either the GET or POST methods. Why would we want to talk to a CGI? Well, CGI remains a widely used technique for building web applications. Other techniques such as opening sockets or talking via RMI are coming on strong, but CGI has been in widespread use for several years. Another important reason for using CGI is that many firewalls block socket ...