What is the purpose of log4j ConsoleAppender follow parameter?

Question

In the example I am following, there is a line that goes like this in the log4j configuration file:

<Console name="ConsoleAppender" target="SYSTEM_OUT" follow="true">

I understood everything in this line except for follow="true", I have read the description of this particular parameter on the official website: This Website. I also tried searching in other websites, but to no avail.

The description goes like this:

This description got me confused, What do they mean by "honors reassignments....." , what is its purpose, what happens if I change it to false.

Coma

Answer 1

The System.out property is not read-only and can be reassigned through System#setOut. With follow="true" the appender will send the message to the current value of System.out, whereas with follow="false" logs will be sent to the original value of System.out.

You can test the two behaviors with:

      // This performs automatic initialization
      final Logger logger = LogManager.getLogger(Log4j2Test.class);
      // System.out is attached to the JVM's `stdout`
      logger.warn("Sent to 'stdout'");
      // Reassignement of System.out
      System.setOut(new PrintStream(new File("file.log")));
      // System.out is attach to the file 'file.log'
      logger.warn("Sent to 'file.log'");

From a practical perspective performance with follow="true" is worse than with follow="false" and System.out is rarely reassigned.

Ferdinande

So, I tried your example, and this is the behavior that I saw: with follow="false": The logger.warn("Sent to 'file.log'"); was thrown in the console, and the log file was empty. with follow="true": The logger.warn(.....) was thrown in the file, after being created. So, if I have to make follow=false, how am I supposed to write to a file, using Rolling File Appender, or, is the purpose of setting follow="true" is to write to a file, in xml syntax. – Coma

@SpaceSloth: in the example above I redirect System.out to a file only for demonstration purposes (so you can see what happends if System.setOut is called). It is not a good practice. If you want to log to a console (terminal), use the ConsoleAppender. If you want to log to a file, use the (Rolling)FileAppender. As you can see in the documentation the latter does not have a follow property, this is characteristic to the console appender. – Ferdinande

My problem here is that most examples use console appender like this <Appenders> <Console name="ConsoleAppender" target="SYSTEM_OUT" follow="true"> <PatternLayout pattern="${LOG_PATTERN}"/> </Console> </Appenders> And I actually have Rolling file appender <RollingFile name="FileAppender" fileName="logs/log4j2-demo.log" filePattern="logs/log4j2-demo-%d{yyyy-MM-dd}-%i.log"> – Coma

With a rolling file appender there is no follow attribute. It opens a file for writing and always sends messages to that file until rollover occurs. – Ferdinande