[logback-user] RollingFileAppender for logs with headers

[Chris Dolan]

I use an XML-based Layout for one of my appenders. This Layout makes use
of the getFileHeader() to output "<?xml...?>" and
getPresentationHeader/Footer() methods to output
"<log><header>...</header>" and "</log>.

I want a daily rolling log that additionally rolls at a specific file
size.  So:

   <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
      <!-- rollover daily, one year duration -->
      <fileNamePattern>${log.out.dir}/Foo-${log.host}-%d{yyyy-MM-dd}.%i.xml.zip</fileNamePattern>
      <maxHistory>365</maxHistory>
      <timeBasedFileNamingAndTriggeringPolicy
class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
          <!-- or whenever the file size reaches a size -->
          <maxFileSize>5MB</maxFileSize>
      </timeBasedFileNamingAndTriggeringPolicy>
   </rollingPolicy>

The problem with this configuration is that on application reboot, I get
log files like this:

  <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
  <log xmlns="[snip]">
  <header>
   <tz>America/Chicago</tz>
   <tzoffset>-21600000</tzoffset>
  </header>
  <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
  <log xmlns="[snip]">
  <header>
   <tz>America/Chicago</tz>
   <tzoffset>-21600000</tzoffset>
  </header>

That is, the new run emits another header appended into the existing log
file. I've tried to work around it by making a subclassed appender that
looks like the following, but it fails with a NullPointerException because
the rolling filename hasn't been set up yet...

public class RollingXmlFileAppender<E> extends RollingFileAppender<E> {
    @Override
    public void openFile(String fileName) throws IOException {
        File file = new File(fileName);
        if (file.exists()) {
            try {
                getRollingPolicy().rollover();
            } catch (RolloverFailure rolloverFailure) {
                // ...
            }
        }
        super.openFile(fileName);
    }
}

Surely other people use log files with headers? How do you roll those
files? It would seem that the trigger should always be checked on start().
If so, I could probably make a TriggeringPolicy that always returns true
on its first invocation.

Chris