On rare occasions, the Google App Engine dev server displays the following error:
The important part of the error is this text:
java.io.IOException: tmpFile.renameTo(classfile) failed
This exception crops up whenever an app file (in this case a JSP file) is currently being accessed by another program. If you see this exception, double check to ensure that the named file isn’t being accessed by another program.
If this error persists, close down and reopen Eclipse and the development app server – the file may have been left open from a previous run.