How to Programmatically Set Log File Path in Logback

This example uses SLF4J with Logback. In order to programmatically set log file path, we need to programmatically create a FileAppender and append it into our own logger (in this example it's "MY_LOGGER").

LogbackFileUtils.java

package test;

import org.slf4j.LoggerFactory;

import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.encoder.PatternLayoutEncoder;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.FileAppender;

class LogbackFileUtils {
    
    public static final String MY_LOGGER = "MY_LOGGER";
    private static FileAppender<ILoggingEvent> fileAppender;
    private static boolean initialized = false;
    
    public static void init() {
        if (!initialized) {
            LoggerContext loggerContext = (LoggerContext) LoggerFactory.getILoggerFactory();
            Logger myLogger = loggerContext.getLogger(MY_LOGGER);
            
            PatternLayoutEncoder encoder = new PatternLayoutEncoder();
            encoder.setContext(loggerContext);
            encoder.setPattern("%d{HH:mm:ss.SSS} [%-5level] %msg %n");
            encoder.start();
            
            fileAppender = new FileAppender<ILoggingEvent>();
            fileAppender.setContext(loggerContext);
            fileAppender.setName("MY_FILE_LOGGER");
            fileAppender.setAppend(false);
            fileAppender.setEncoder(encoder);
            myLogger.addAppender(fileAppender);
            
            initialized = true;
        }
    }
    
    public static void start(String filePath) {
        init();
        stop();
        fileAppender.setFile(filePath);
        fileAppender.start();
    }
    
    public static void stop() {
        if (fileAppender.isStarted()) {
            fileAppender.stop();
        }
    }
}

This example hardcodes the pattern. We can improve this example by copying the pattern from "STDOUT" logger.

logback.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%-5level] %msg %n</pattern>
    </encoder>
  </appender>

  <root level="DEBUG">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

LogbackTest.java

package test;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class LogbackTest {

    private static Logger logger = LoggerFactory.getLogger(LogbackFileUtils.MY_LOGGER);
    
    public static void main(String[] args) {
        LogbackFileUtils.start("first.log");
        logger.info("1st file - This is an info message");
        logger.debug("1st file - This is a debug message");
        logger.error("1st file - This is an error message");
        LogbackFileUtils.stop();
        
        LogbackFileUtils.start("second.log");
        logger.info("2nd file - This is an info message");
        logger.debug("2nd file - This is a debug message");
        logger.error("2nd file - This is an error message");
        LogbackFileUtils.stop();
        
        logger.info("console only - This is an info message");
        logger.debug("console only - This is a debug message");
        logger.error("console only - This is an error message");
    }
}

We need to make sure we call the right logger (MY_LOGGER) and not anything else. There will be two files generated, first.log and second.log.