That's much to go on. I assume the classes that aren't found are part of your application? Do the users see error messages on the screen? Are there stack traces that tell what was happening at the time? Just to say that something wasn't found doesn't convey much useful information.
is the application low on memory when these exceptions are thrown (possibly low on permgen space)?
Recently, our customer found some ClassNotFoundException informations in log files, they sent the log files to me,
and I found that these classes are exist in jar， and they told me these exceptions only show when our program was busy.
Is anyone know the reason or how to resolve the problem?
How to resolve what problem? You haven't posted any problem that necessarily needs to be resolved.
Nor have you posted ANY information about:
1. what log files you are talking about
2. why you think those log messages indicate a problem of some kind.
3. what class files you are talking about
4. what jar files those class file are in
5. where those jar files came from.
6. what application you are even talking about
7. what that application even does
8. why your customers are digging around in the log files to begin with
9. why you are even concerned about this at all
You haven't said one word about any appilcation working incorrectly or producing incorrect results.
The log file was generated by log4j, these jars were what we developed, and these not found classes are business objects, the class not found exception made
our webservice application can not process business any more. the strange thing is ,when we regenerate jar and replace old ones, the exception no longer appeared.
Nothing strange about it then, the previous deployment was broken and now you temporarily fixed it with a clean version. It doesn't help you to want things to be strange, stick to what makes sense.
> and I found that these classes are exist in jar，
You did that by getting the jar from the production server and looking at I? If you did it any other way then your investigation methodology was incorrect.