When a servlet is asked to handle a request, it typically needs specific information about the request so that it can process the request appropriately. Most frequently, a servlet will retrieve the value of a form variable and use that value in its output. A servlet may also need access to information about the environment in which it is running. For example, a servlet may need to find out about the actual user who is accessing the servlet, for authentication purposes.
interfaces provide access to this kind of information. When a servlet
is asked to handle a request, the server passes it a request object
that implements one of these interfaces.
With this object, the
servlet can determine the actual request (e.g.,
protocol, URL, type), access parts
of the raw request (e.g., headers, input stream), and get any
client-specific request parameters (e.g., form variables, extra path
information). For instance, the
getProtocol( ) method returns the protocol used by the request, while
getRemoteHost( ) returns the name of the
client host. The interfaces also provide
methods that let a servlet get information about the server (e.g.,
getServerPort( )). As we saw earlier, the
method provides access to request parameters such as form variables.
There is also the
getParameterValues( ) method, which returns an array of strings that contains all the values for a particular parameter. This array ...