Log4j2: Automatic reconfiguration (reflect at runtime)

By setting the attribute ‘monitorInterval’ in log4j configuration file, you can reflect the changes at runtime.

<?xml version="1.0" encoding="UTF-8"?>

<Configuration monitorInterval="30">

...

</Configuration>

In above example, monitorInterval is set to 30 seconds, so for every 30 seconds the configuration file is checked for modifications. Minimum interval time to set is 5 seconds.

For example
log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="DEBUG" monitorInterval="30">
 <Appenders>
  <Console name="my_console_appender" target="SYSTEM_OUT">
   <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n" />
  </Console>
 </Appenders>
 <Loggers>
  <Root level="trace">
   <AppenderRef ref="my_console_appender" />
  </Root>
 </Loggers>
</Configuration>

HelloWorld.java

package log4j_tutorial;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class HelloWorld {
 private static final Logger log = LogManager.getLogger();

 public static void printData() {
  log.trace("Trace Message!");
  log.debug("Debug Message!");
  log.info("Info Message!");
  log.warn("Warn Message!");
  log.error("Error Message!");
  log.fatal("Fatal Message!");
 }

 public static void main(String args[]) throws InterruptedException {
  printData();
  for(int i=0; i<50; i++){
   log.info("Information message " + i);
   Thread.sleep(1000);
  }
  printData();
 }
}

Run above application and change the log level from ‘trace’ to ‘warn’. After 30 seconds the changes will reflect and you will see following output.

15:42:01.671 [main] TRACE log4j_tutorial.HelloWorld - Trace Message!
15:42:01.672 [main] DEBUG log4j_tutorial.HelloWorld - Debug Message!
15:42:01.672 [main] INFO  log4j_tutorial.HelloWorld - Info Message!
15:42:01.672 [main] WARN  log4j_tutorial.HelloWorld - Warn Message!
15:42:01.672 [main] ERROR log4j_tutorial.HelloWorld - Error Message!
15:42:01.672 [main] FATAL log4j_tutorial.HelloWorld - Fatal Message!
15:42:01.672 [main] INFO  log4j_tutorial.HelloWorld - Information message 0
15:42:02.673 [main] INFO  log4j_tutorial.HelloWorld - Information message 1
15:42:03.674 [main] INFO  log4j_tutorial.HelloWorld - Information message 2
15:42:04.674 [main] INFO  log4j_tutorial.HelloWorld - Information message 3
15:42:05.676 [main] INFO  log4j_tutorial.HelloWorld - Information message 4
15:42:06.677 [main] INFO  log4j_tutorial.HelloWorld - Information message 5
15:42:07.679 [main] INFO  log4j_tutorial.HelloWorld - Information message 6
15:42:08.679 [main] INFO  log4j_tutorial.HelloWorld - Information message 7
15:42:09.680 [main] INFO  log4j_tutorial.HelloWorld - Information message 8
15:42:10.681 [main] INFO  log4j_tutorial.HelloWorld - Information message 9
15:42:11.682 [main] INFO  log4j_tutorial.HelloWorld - Information message 10
15:42:12.682 [main] INFO  log4j_tutorial.HelloWorld - Information message 11
15:42:13.682 [main] INFO  log4j_tutorial.HelloWorld - Information message 12
15:42:14.683 [main] INFO  log4j_tutorial.HelloWorld - Information message 13
15:42:15.684 [main] INFO  log4j_tutorial.HelloWorld - Information message 14
15:42:16.686 [main] INFO  log4j_tutorial.HelloWorld - Information message 15
15:42:17.686 [main] INFO  log4j_tutorial.HelloWorld - Information message 16
15:42:18.687 [main] INFO  log4j_tutorial.HelloWorld - Information message 17
15:42:19.689 [main] INFO  log4j_tutorial.HelloWorld - Information message 18
15:42:20.690 [main] INFO  log4j_tutorial.HelloWorld - Information message 19
15:42:21.691 [main] INFO  log4j_tutorial.HelloWorld - Information message 20
15:42:22.692 [main] INFO  log4j_tutorial.HelloWorld - Information message 21
15:42:23.692 [main] INFO  log4j_tutorial.HelloWorld - Information message 22
15:42:24.693 [main] INFO  log4j_tutorial.HelloWorld - Information message 23
15:42:25.694 [main] INFO  log4j_tutorial.HelloWorld - Information message 24
15:42:26.695 [main] INFO  log4j_tutorial.HelloWorld - Information message 25
15:42:27.697 [main] INFO  log4j_tutorial.HelloWorld - Information message 26
15:42:28.698 [main] INFO  log4j_tutorial.HelloWorld - Information message 27
15:42:29.699 [main] INFO  log4j_tutorial.HelloWorld - Information message 28
15:42:30.700 [main] INFO  log4j_tutorial.HelloWorld - Information message 29
15:42:31.700 [main] INFO  log4j_tutorial.HelloWorld - Information message 30
15:42:32.701 [main] INFO  log4j_tutorial.HelloWorld - Information message 31
15:42:33.702 [main] INFO  log4j_tutorial.HelloWorld - Information message 32
15:42:34.703 [main] INFO  log4j_tutorial.HelloWorld - Information message 33
2015-10-02 15:42:34,703 Thread-1 DEBUG Reconfiguration started for context 1163157884 (org.apache.logging.log4j.core.LoggerContext@8bd1b6a)
15:42:51.719 [main] WARN  log4j_tutorial.HelloWorld - Warn Message!
15:42:51.719 [main] ERROR log4j_tutorial.HelloWorld - Error Message!
15:42:51.719 [main] FATAL log4j_tutorial.HelloWorld - Fatal Message!

As you observe the output, configuration changes are reflected at run time.

Note:

If you are running your application in eclipse, you can’t see the changes, unless you update the log4j2.xml file in ${project}/bin, or ${project}/target/classes.

It is because, Eclipse will copy your log4j2.xml file from ${project}/src to ${project}/bin. Log4j will load the log4j2.xml file in the classpath, which is ${project}/bin. If you now modify the other config file in your src/ folder that may not be copied to the bin/ folder until Eclipse detects that a rebuild is required.

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