5.2. COUNTERS AND FILES

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There are various techniques for getting around this, but the general lesson

is that passwords are not helpful unless they are very carefully hidden! We will

discuss security in more detail later.

There are other ways of restricting access beside asking for a password.

Restricting access to certain IP addresses is a common way of creating an

 intranet  like interface for your servlet. For example, Brandeis University

 owns  all IP addresses have the form

129.64.*.*

where the two asterisks represent any integers between 0 and 255. One can test

whether the user's IP address is from Brandeis using the following test:

(.startsWith (.getRemoteAddr request) "129.64.")

Similarly, one can restrict access to computers running the MacIntosh Operating

System or to all IP address except a few  forbidden  ones.

5.2

Counters and Files

Next we show how to create servlets that read and write to files on the server.

Some of the simple applications this enables are servlets that contain counters,

and servlets that allow you to leave a message.

We first consider the problem of implementing a servlet that keeps track of

how often it has been visited. There are many strategies one can employ for

such counters. For this example, we will write a servlet  counter.servlet  that

stores its count in a file called counter.servlet count.

The code for the  counter.servlet  servlet is in Figure 5.2. This servlet

introduces a few new ideas. First of all, it relies on a small library  files.scm 

of scheme procedures for reading and writing to the count file. We assume that

this file is loaded when the server is first started up.

The servlet must first load the library before it can use the special counter

library procedures. This is done using the  load  procedures

(load "webapps/scheme/lib/forms.scm")

Also, since the servlet now does two things:

  update a counter and

  generate a webpage,

we must use a feature of the let* expression that we haven't mentioned so far

  the ability to do several things before computing the value to be returned by

the expression. The general form of the let* is

(let* ((V1 E1)

(V2 E2)

...