Why do JBoss and Logrotate create log files full of NUL characters?

We had the same problem for a file being written by log4j. The solution was to set the property "Append" for the FileAppender to "true". After this change, we have not seen this problem with files having NUL when rotated by an external program like logrotate.

From my experience - and the reason that we do not use logrotate with Log4j is that the way logrotate works is that it renames the files, then instructs the program to close its logs and reopen them with the old file name (which does not exist anymore), normally using the HUP signal.

But Log4j cannot be told to reopen its log files, so I see you use copytruncate to copy the files instead - the problem is that Log4j uses buffered writers that keep track of the current position of the file that is being written and when you truncate the log file log4j keeps writing from where it stopped writing before the truncate. Depending on your file system implementation this should create "files with holes" - i.e. the NULL characters that you see there do not really exist - the file is actually only as big as the actual data and the NULL character is the way your viewer represents the hole. Some file systems on the other hand do not support holes and will indeed fill the file with NULL characters when Log4j resumes writing.

I suggest - don't use logrotate, find some way to rotate the files within Log4j by using a RollingFileAppender (that does support removing old files) or using the DailyRollingFileAppender and a cronjob that removes old files externally (as it was meant to be done).