13 Servlets

A servlet is a small application that exists on a Web server and that can be invoked by a CGI command. A servlet is usually called a CGI script. CGI (Common Gateway Interface) is a protocol that defines how data is passed between a Web server and a servlet.

A servlet is a program that accepts an input and calculates a result. To be precise, it does the following steps:

• Get the arguments for the servlet by calling Application.getCgiArgs. A standard application would call Application.getCmdArgs for this purpose. The former is used in exactly the same way as the latter, but instead of parsing command line arguments, it parses CGI arguments.

• Calculate the result.

• Print a header on stdout that defines the content type. The content type tells the Web browser what the type of the result is, so that it knows how to display it. For example, if the servlet outputs HTML, you have to print something like:

  'Content-type: text/html\n\n'

  (without the quotes). The Open.html class has support for this (see example below).

• Output the real data. For example, text in HTML format.

The following example follows this structure. It uses a class Open.html to generate HTML code on the fly. You can test it by sending a URL that looks like this:

'http://www.you.edu/~you/small.cgi?number=10&text=Hi'

In this example, the value 10 is passed for the argument 'number' and the value "Hi+Guys" for the argument 'text' (in CGI argument syntax, the plus is used to quote a space).

functor 
import Application Open
define 
   %% Parse the arguments
   Args={Application.getCgiArgs  
      record(number(single type:int default:0)
             text(single type:string default:"none"))}
  
   %% A file that supports HTML output
   Out={New class $ 
               from Open.file Open.html 
            end 
        init(name:stdout)}
  
   %% Print MIME content header
   {Out header}
  
   %% Print HTML output
   {Out tag(html(head(title('My First CGI'))
                 body(bgcolor:'#f0f0e0' 
                      h1('My First CGI')
                      p('The number is: '#Args.number)
                      p('The text is: '#Args.text))))}
  
   %% Terminate
   {Application.exit 0}
end

In order to compile this servlet, you have to do:

ozc --execpath=OZHOME/bin/ozengine -x small.oz -o small.cgi

Where OZHOME denotes the installation directory of the Mozart system. The execpath argument is needed because the servlet needs an absolute path. Servlets are normally executed by the Web server in an extremely minimal user environment. The user is typically called nouser or www and has almost no rights. In particular you cannot expect the Mozart system to be in the path of the user! This is why you need an absolute pathname when compiling the servlet.

On a Unix system, you can more simply invoke:

ozc --execpath=`which ozengine` -x small.oz -o small.cgi

The final step is to install the servlet in your Web server. For this you should contact your local Web site administrator.