Understanding Log4j2 RollingFile Appender: TriggeringPolicy, RolloverStrategy, and DeleteAction

Log4j2 provides the RollingFileAppender to automatically roll over log files when certain conditions are met, such as reaching a specified size or time interval.

The rollover process is driven by two concepts: TriggeringPolicy , which decides when a rollover should occur, and RolloverStrategy , which decides how the rollover is performed. Log4j2 ships with a default DefaultRolloverStrategy .

An example log4j2.xml configuration demonstrates a basic RollingFile setup:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn">
  <Appenders>
    <RollingFile name="RollingFile" fileName="logs/app.log" filePattern="logs/app-%d{yyyy-MM-dd HH}.log">
      <PatternLayout><Pattern>%d %p %c{1.} [%t] %m%n</Pattern></PatternLayout>
      <Policies>
        <TimeBasedTriggeringPolicy interval="1"/>
        <SizeBasedTriggeringPolicy size="250MB"/>
      </Policies>
    </RollingFile>
  </Appenders>
  <Loggers>
    <Root level="error">
      <AppenderRef ref="RollingFile"/>
    </Root>
  </Loggers>
</Configuration>

The filePattern can contain a date/time pattern (e.g., yyyy-MM-dd HH ) and/or an integer counter %i . The pattern determines the naming of archived files.

Log4j2 supports several TriggeringPolicy types:

CronTriggeringPolicy – triggers based on a cron expression.

OnStartupTriggeringPolicy – triggers when the JVM starts.

SizeBasedTriggeringPolicy – triggers when the log file reaches a configured size (e.g., 20MB ).

TimeBasedTriggeringPolicy – triggers when the current time no longer matches the date/time pattern in filePattern .

CompositeTriggeringPolicy – combines multiple policies, such as size‑and‑time based triggers.

Typical usage focuses on SizeBasedTriggeringPolicy and TimeBasedTriggeringPolicy , either alone or together via CompositeTriggeringPolicy .

The DefaultRolloverStrategy includes a max attribute that limits the number of archived files when a %i counter is present. If the filePattern contains only a date/time pattern, max has no effect; if it contains only %i , the counter increments until max is reached and older files are deleted; if both are present, the counter resets when the date/time portion rolls over.

Three common filePattern scenarios are illustrated with XML snippets:

<RollingFile fileName="logs/app.log" filePattern="logs/app-%d{yyyy-MM-dd}.log"> ... </RollingFile>

<RollingFile fileName="logs/app.log" filePattern="logs/app-%i.log"> ... </RollingFile>

<RollingFile fileName="logs/app.log" filePattern="logs/app-%d{yyyy-MM-dd HH-mm}-%i.log"> ... </RollingFile>

Since Log4j 2.5, DeleteAction allows custom cleanup of archived logs. A sample configuration shows how to delete files matching app-*.log.gz that are older than 60 days:

<DefaultRolloverStrategy>
  <Delete basePath="${baseDir}" maxDepth="2">
    <IfFileName glob="*/app-*.log.gz"/>
    <IfLastModified age="60d"/>
  </Delete>
</DefaultRolloverStrategy>

A simple Java demo program logs incremental numbers to trigger rollovers:

public class HelloWorld {
    public static void main(String[] args) {
        Logger logger = LogManager.getLogger(LogManager.ROOT_LOGGER_NAME);
        try {
            for (int i = 0; i < 50000; i++) {
                logger.info("{}", i);
                logger.info("logger.info\n");
                Thread.sleep(100);
            }
        } catch (InterruptedException e) {}
    }
}

Additional test configurations illustrate:

Time‑based rollover every 5 seconds using TimeBasedTriggeringPolicy interval="5" .

Size‑based rollover when the file reaches 5KB using SizeBasedTriggeringPolicy size="5KB" .

Combination of time‑based and size‑based triggers with a max="3" counter and %i in the pattern.

The article concludes with a brief thank‑you note and references to the original source.