5.3. LOG FILES

69

; this is in the file "logger.servlet"

(let ((logdata (read string from file

(servlet file request "log")

""))

(current entry {[(Date.)]: [(.getRemoteAddr request)]}))

(append to file (servlet file request "log") current entry)

{

 The list of previous visitors to this site is 

[logdata]

Your entry is 
[current entry]

}

)

Figure 5.3: A simple log servlet

5.3

log files

In this next example, we show how to create a servlet that generates an entry

in a log file for each time the servlet is visited. The logger servlet in Figure 5.3

is quite similar to the counter servlet above.

We begin by appending the current date and time to the logfile  

logger.servlet_log

Instead of  writing  to the counter file, we  append  to the log file. Then we

read the logfile and store it in the  logdata  variable which we include in the

HTML file sent back to the user.

This servlet uses two new procedures for reading/writing files:

  (read string from file FILE DEFAULT VALUE)   this reads the entire

contents of the file as a string of characters, and if the file does not yet

exist it returns the DEFAULT VALUE instead.

  (append to file FILE VALUE)   this appends the specified VALUE to

the next line of the specified FILE. If the file does not already exist then

it creates it.

Also note that the current entry is created using quasi string notation. We

could just as easily have implemented it using string append as follows

(string append (Date.) ": " (.getRemoteAddr request))

which may even be easier to read in this example.