Scheme Servlets: Counters, Logs, and Email
Required Reading
Loading the library
To use the capabilities described in this section, you need to
have your sxml servlet load in the "schemelib/forms.silk" library.
This is done by writing your servlet in the following form:
| <![CDATA[
(begin
(load "schemelib/forms.silk")
(let (
(var1 (.getParameter request "var1"))
...
)
...
)
)
]]> |
Email
Our first example, is a simple servlet that asks the user a
question and then email them an acknowledgement:
| <![CDATA[
(begin
"This is the mail_test.sxml servlet"
(load "schemelib/forms.silk")
(let (
(email (.getParameter request "email"))
(opsys (.getParameter request "opsys"))
)
(if (equal? email #null)
<xml>
<html><head><title>Mail demo</title></head>
<body bgcolor=lightgreen>
<h1>Mail demo</h1>
<form method=post action="mail_test.sxml">
Enter your email address:
<input type=text name=email><br>
Select your favorite type of operating system:
<select name="opsys">
<option>windows</option>
<option>mac</option>
<option>linux</option>
</select><br>
<input type=submit value="vote">
</form>
</body></html></xml>
(begin
(send-mail request
email
"tjhickey@brandeis.edu"
"servlet mail test"
<xml>
Thanks for visiting the servlet mail test page on
<scheme>(Date.)</scheme> and voting for <scheme>opsys</scheme>
</xml>)
<xml>
<html>
<head><title>Test</title></head>
<body bgcolor=lightgreen>
<h1> Mail has been sent to <scheme>email</scheme></h1>
Thanks.
</body>
</html>
</xml>
)
)))
]]> |
Observe that the servlet first loads in the library. Then it reads the
"email" parameter. If that parameter is #null (e.g. the user did not
fill out the form), then the servlet will generate a form asking
for an email address and asking a question about operating systems.
If "email" is not #null, then the servlet does two things. First it
sends mail to the given address from "tjhickey@brandeis.edu" with
the subject "servlet mail test". The text of the email message contains
both the date and their vote.
In general the format for the send-mail command is
| <![CDATA[
(send-mail
request ;; this is mandatory
TO ;; email address of recipient
FROM ;; your email address
SUBJECT ;; subject of the message
TEXT ;; body of the message
)
]]> |
Please don't abuse this feature by sending unwanted email.
Implementing a counter
To implement a counter we need to use two of the library procedures
(read-from-file httpservlet request "NAME-OF-FILE" INITIAL-VALUE)
(write-to-file httpservlet request "NAME-OF-FILE" VALUE-TO-WRITE)
The following example shows a simple servlet that does nothing but
count hits.
| <![CDATA[
(begin
(load "schemelib/forms.silk")
(let (
(count (read-from-file httpservlet request "count" 0))
)
(write-to-file httpservlet request "count" (+ 1 count))
<xml>
<html>
<head><title>Test</title></head>
<body bgcolor=lightgreen>
<h1> This page has been visited
<scheme>count</scheme>
times
</h1>
</body>
</html>
</xml>
))
]]> |
Implementing a log
Next we show how to take information from the webpage form and
store it in a file in your folder on the server. This again uses
the "schemelib/forms.silk" library
|
<![CDATA[
(begin
(load "schemelib/forms.silk")
(let (
(data (read-string-from-file httpservlet request "mylogfile" "no log yet"))
)
(append-to-file httpservlet request "mylogfile"
(list (Date.) (.getRemoteHost request) ))
<xml>
<html>
<head><title>Test</title></head>
<body bgcolor=lightgreen>
<h1> The log of browsers visiting the page is shown below
<pre><scheme>data</scheme>
</pre>
The current local time is <scheme>(Date.)</scheme>
</h1>
</body>
</html>
</xml>
))
]]>
|
The key procedures here are
| <![CDATA[
(read-string httpservlet request LOGFILENAME INITIAL_VALUE)
]]> |
This reads in the logfile as a big string assuming it is stored in
your folder in a file named YOURSERVLET_LOGIFILENAME.
|
<![CDATA[
(append-to-file httpservlet request LOGFILENAME VALUE_TO_APPEND)
]]>
|
This adds the "VALUE_TO_APPEND" to the logfile with the specified name.