package org.example;

import java.io.IOException;
import java.text.MessageFormat;

import org.apache.log4j.Layout;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.PatternLayout;
import org.apache.log4j.RollingFileAppender;
import org.apache.log4j.spi.LoggingEvent;

public class L4JRFAisSynchronous {

    static RFA appender;
    static {
        System.setProperty("log4j.defaultInitOverride", "true");
//        System.setProperty("log4j.debug", "true");
        Logger rootLogger = Logger.getRootLogger();
        if (!rootLogger.getAllAppenders().hasMoreElements()) {
            rootLogger.setLevel(Level.INFO);
            try {
                appender = new RFA(new PatternLayout("%d{ISO8601} [%t] %-5p %c %x - %m%n"), "log/L4J.log", true);
                appender.setMaxFileSize("10240KB");
                appender.setMaxBackupIndex(10);
                rootLogger.addAppender(appender);
            } catch (IOException e) {
                throw new RuntimeException("Appender configuration failed", e);
            }
        }
    }
    static final Logger log = Logger.getLogger(L4JRFAisSynchronous.class);
    static final long N = 20000;

    public static void main(String[] args) {
        System.out.println("Appender: " + appender.getClass().getSuperclass().toString());
        System.out.println("# of iterations per trial = " + N);
        trial(0);
        trial(0);
        trial(0);
        trial(0);
        trial(0);
        trial(0);
        trial(1);
        trial(10);
        trial(100);
        trial(0);
        trial(1);
        trial(10);
        trial(0);
        trial(100);
    }
    
    private static void trial(int delay) {
        appender.setDelay(delay);
        long start = System.nanoTime();
        for (int i = 0; i < N; i++) {
            log.info(delay + " " + i);
        }
        long end = System.nanoTime() - start;
        long total = end/N;
        System.out.println(MessageFormat.format("{0} nS. per append with delay = {1} nS. : appender time = {2} nS. per call.",
                                                total,
                                                delay,
                                                total - delay));
    }

    public static class RFA extends RollingFileAppender {

        int delay = 0; 
        
        public int getDelay() {
            return delay;
        }

        public void setDelay(int time) {
            this.delay = time;
        }

        public RFA() {
            super();
        }

        public RFA(Layout layout, String filename, boolean append)
            throws IOException {
            super(layout, filename, append);
        }

        public RFA(Layout layout, String filename) throws IOException {
            super(layout, filename);
        }

        @Override
        protected void subAppend(LoggingEvent arg0) {
            try {
                Thread.sleep(0, delay);
            } catch (InterruptedException e) {
                // ignore 
            }
            super.subAppend(arg0);
        }
    }
}

The Log4J RollingFileAppender is actually synchronous.

A demonstration showing that Log4J RollingFileAppender is actually synchronous.

Java