Logging System.out.println to log file for single application

Question

We have multiple applications on a tomcat which are using System.out.println statements logging to catalina.out. There is a single application which creates a lot of log statements so I would like to log those application outpout to a separate logfile instead.

I have created a log4j.xml setup which logs only WARN level to catalina.out. But the RollingFileAppender does not yet work for the System.out statements and I am not sure, what to change.

I would like to not touch the other applications. Is that possible somehow?

<Configuration status="INFO">
    <Appenders>
        <Console name="stdout" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{yyyy-MM-dd HH:mm:ss} [%t] %-5level %logger{36}: %msg%n" />
        </Console>
    
    <RollingFile
        name="logFile"
        fileName="../logs/app/log.log"
        filePattern="../logs/app/log.%d{yyyy-MM-dd}.log.gz"
        ignoreExceptions="false">
        <PatternLayout>
            <Pattern>%d{yyyy-MM-dd HH:mm:ss} %-5p %m%n</Pattern>
        </PatternLayout>
        <Policies>
            <TimeBasedTriggeringPolicy interval="1"/>
        </Policies>
        <DefaultRolloverStrategy max="31" />
    </RollingFile>
    
    <Async name="logFile_async">
        <AppenderRef ref="logFile"/>
    </Async>
    
</Appenders>
<Loggers>
    <Root level="WARN">
        <AppenderRef ref="stdout" />
    </Root>
    
    <Logger name="my.company.logger" level="DEBUG">
        <AppenderRef ref="logFile_async"/>
    </Logger>
</Loggers>
</Configuration>

Using `System.out` is not a proper way to log messages and it's **quite difficult** to redirect it to a proper logging framework. You mixed up the directions: you configured Log4j2 to log to `System.out`, whereas what you **need** is to replace `System.out` with a `PrintStream` that sends all lines to Log4j2. It's not a trivial operation. — Piotr P. Karwasz, Mar 11 '21 at 13:01
@PiotrP.Karwasz thanks for confirming what i already thought. It was intended as a shortcut before being able to refactor all the System.out statements to log4j. Probably we need to do the refactoring directly then. — ulrich, Mar 11 '21 at 13:57

Piotr P. Karwasz · Answer 1 · 2021-05-18T06:13:18.203

As already stated in the comments, printing to System.out is not a proper way to log and it is difficult to redirect the data sent there to a proper logging framework.

You have two choices:

System.out to JULI redirection

You can set the swallowOutput attribute of a context to true (cf. documentation), which will redirect the standard output and error of the application to the JULI logging system.

The log4j2-appserver.jar has a JULI implementation, that sends all logs to Log4j2 (see documentation).

Direct System.out to Log4j redirection

You can define PrintStream to Logger adapter like this:

public class LogPrintStream extends PrintStream {

   private static class LogOnFlush extends ByteArrayOutputStream {

      // WeakHashMap so that we don't keep a reference to the classloaders.
      protected final Map<ClassLoader, Logger> classLoaderLoggers = new WeakHashMap<>();

      public LogOnFlush() {
         // NOP
      }

      @Override
      public void flush() {
         final String message = toString();
         if (message != null && message.length() > 0) {
            Logger logger = getLogger();
            logger.debug(message);
         }
         reset();
      }

      public Logger getLogger() {
         final ClassLoader cl = Thread.currentThread().getContextClassLoader();
         Logger logger = classLoaderLoggers.get(cl);
         if (logger == null) {
            final String name;
            if (cl instanceof WebappClassLoaderBase) {
               final WebappClassLoaderBase webCl = (WebappClassLoaderBase) cl;
               name = String.format("%s.[%s].[%s]", LogPrintStream.class.getName(), webCl.getHostName(), webCl.getContextName());
            } else {
               name = String.format("%s.[%08x]", LogPrintStream.class.getName(), cl.hashCode());
            }
            logger = LogManager.getLogger(name);
            classLoaderLoggers.put(cl, logger);
         }
         return logger;
      }
   }

   public LogPrintStream() {
      super(new LogOnFlush(), true);
   }
}

and use it to replace System.out through System#setOut at a very early stage of the server's startup. You can, e.g. use a LifecycleListener like this:

public class SystemOutReplaceListener implements LifecycleListener {

   @Override
   public void lifecycleEvent(LifecycleEvent event) {
      if (Lifecycle.AFTER_INIT_EVENT.equals(event.getType())) {
         System.setOut(new LogPrintStream());
      }
   }
}

Warning: in this example the output to System.out is sent to Log4j2. Care must be taken, so that Log4j2 will not forward its logs to System.out.

Logging System.out.println to log file for single application

1 Answers1

System.out to JULI redirection

Direct System.out to Log4j redirection

Linked