在应用程序关闭之前，log4j RollingFileAppender不会写入日志

Question

We're using log4j 1.2.17 via slf4j-log4j12 1.7.7 with Java 8. I'm running the application from within Eclipse 4.4.1 on Windows 7 Professional 64-bit.

Our root log4j.properties file looks like this:

# Root logger option
log4j.rootLogger=INFO, stdout

# Direct log messages to stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p (%c{1}) [%d] - %l: %m%n

In the application we set up a rolling file appender based upon the configuration:

EnhancedPatternLayout patternLayout = new EnhancedPatternLayout("%-5p (%c{1}) [%d] - %l: %m%n");
RollingFileAppender rollingFileAppender = new RollingFileAppender(patternLayout, logPath, true);
rollingFileAppender.setMaxBackupIndex(5);
rollingFileAppender.setMaxFileSize(5MB);
Logger.getRootLogger().addAppender(rollingFileAppender);

That works nicely... when the application is finally closed! While the application is running the log file sits at zero bytes, which isn't very helpful.

(In this version of our application we switched from using straight log4j to using slf4j->log4j. I don't remember offhand if the behavior was different before we switched.)

Update: I've traced through the creation of the actual writer. log4j creates a FileOutputStream , then wraps that in an OutputStreamWriter , which it then wraps in a CountingQuietWriter which is called by the appender. The bufferedIO flag is turned off. I have also traced through writing a log message, and it seems to be written normally to the CountingQuietWriter -> OutputStreamWriter -> FileOutputStream . Yet nothing appears in the log file until I close the application.

Why isn't the application writing to the log immediately, or at least every so often after the buffer gets full?

Answer 1

You can try this, forcing the appender to write each line whenever it's logged by the application :

rollingFileAppender.setImmediateFlush(true);
rollingFileAppender.setBufferedIO(false);

The weird thing is that these values are the default one, but you can try it anyway.

Answer 2

In my experience, this could very well just be your OS/filesystem/storage appliance/etc issue, or the application your using to view the file size, not retrieving/replicating/updating file metadata...and the data might actually be there. The file system may only replicate/report metadata after your write lock is released, for example.

When you open the file (read-only) while the application is running, do you see data, despite the reported file size?