How do I make log4j create log files on demand only?

Question

We have a modular application where modules have their own log4j logs (i.e. communication log and error log). The appenders and categories for these are all configured in th

Answer 1

The file appenders have no option to lazily create the log files - the setFile method automatically creates the file if it doesn't already exist: ostream = new FileOutputStream(fileName, append);

You'll have to extend the appender and overwrite the file initialisation code yourself to get the behaviour you're after.

Answer 2

Extend the standard FileAppender class was unsuccessful for me. So I have found an other solution using appenders programmatically to create log files on demand only (and with timestamp in the name file). I have written these two methods :

public void startLog() {
    SimpleDateFormat sdf_long = new SimpleDateFormat("yyyy_MM_dd_HH_mm_ss");
    FileAppender fa = new FileAppender();
    fa.setName("foo");
    fa.setFile(sdf_long.format(new Date()) + ".log");
    fa.setLayout(new PatternLayout("%d{HH:mm:ss.SSS} %m%n"));
    fa.setThreshold(Level.DEBUG);
    fa.setAppend(true);
    fa.activateOptions();
    Logger.getRootLogger().addAppender(fa);
}

public void stopLog() {
    Logger.getRootLogger().getAppender("foo").close();
    Logger.getRootLogger().removeAppender("foo");
}

My log4j.properties file only configures the console appender. When I want to start logging I call the startLog() method. When I want to log in an other file I call stopLog() first and then startLog() method.

Answer 3

In Log4j 2, both FileAppender and RollingFileAppender has the parameter "createOnDemand" which can be used to configure to create the log file only when a log event passed to the appender.

Example:

<RollingFile name="LogFile" fileName="test.log" filePattern="test-%i.log.gz" createOnDemand="true">
    <Policies>
        <SizeBasedTriggeringPolicy size="1MB"/>
    </Policies>
    <DefaultRolloverStrategy max="5"/>
</RollingFile>

More details here: https://logging.apache.org/log4j/2.x/manual/appenders.html#RollingRandomAccessFileAppender

Answer 4

I had the same problem, so I have extended the standard FileAppender class and I have created a new LazyFileAppender that is a FileAppender that lazily initialize the log file(creates it only when the first write operation happens).

The LazyFileAppender and some other additions to the standard log4j library can be found into a simple library that I have created : log4j-additions .

You can look at the source to develop your own extension or you can use it as is ...