When you attempt to run a servlet you may find it produces an error and you will see this error message in the browser. You may see the cause of the error and even the line number. This is useful but may not help you in every circumstance. For example, you will not see any of the print statement outputs since the servlet never got created.
Various other messages are put into the log file on the server when you run a servlet. For example any System.out.println() or System.err.println() commands will have their output sent to the log files. This will occur even if the servlet fails in the middle.
Also, messages like
SAXParseException: Premature end of file
Exception MemberList init failed,exception massege:null
Error: URI=file:///usr/local/tomcat/users/ploni/library.xml Line=2: Document root element "Library", must match DOCTYPE root "null".
will be printed in the log file.
This file is located on horayot (ise) here:
/usr/local/tomcat/logs/catalina.out
The file is viewable by all the students. Do not attempt to write to or
overwrite the file, this will not work. All the servlets running on
horayot will write to this file so you are likely to see error messages of
other students.
If you encounter a null pointer exception and want to see the value of
certain variables before the exception occured, for example to see how far the program reached, you can use the System.err.printlin() function to print variables. I suggest you print some kind of identifying string before the
variable so that you can easilly find it in the log file. This file is big
and will require searching through it. For example, use a command like this:
System.err.printlin("Nachum Variable: a=" . a )
then open the file catalina.out with emacs or vi and search for Nachum Variable
to see what a is.
When you exit emacs or vi, you may be told that the file has changed on disk,
do you want to overwrite it? Choose no, since you in fact will not be able
to overwrite the file since you do not have permissions to do so and you will
not be able to exit emacs or vi otherwise.
Alternatively, you can grep the file for every line containing Nachum Variable like this:
grep "Nachum Variable" catalina.out
This will print every line in the log that contains the string Nachum Variable.