a simple trick to change log level at runtime
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dynamic log level

a simple trick to change log level at runtime


import org.apache.log4j.Logger;
import org.junit.Test;

public class Log4jLogLevelManagerTest {

  private static final Logger logger = Logger.getLogger("test");

  public void test() {
    Log4jLogLevelManager manager = new Log4jLogLevelManager();

    // original level of logger 'test' is INFO
    logger.info("info message should be outputed");
    logger.debug("debug message should not be outputed");

    // change log level to debug
    manager.changeLogLevel("test", "DEBUG");

    logger.debug("debug message should be outputed now");

    // reset level of logger 'test'

    logger.debug("debug message should not be outputed again");



This trick is for log4j 1.x, which is used widely in Java application. LogLevelManager is the interface for this manager containing two main methods as follows

public interface LogLevelManager {

  void changeLogLevel(String loggerName, String level);

  void resetLogLevel(String loggerName);


Log4jLogLevelManager uses org.apache.log4j.LogManager rather than org.apache.log4j.Logger to provide log level management since Logger#exists(String) is a deprecated method.

For recovering log level, Log4jLogLevelManager pushes the original log level to a map when changed at the first time and sets it back to logger when resetLogLevel is called. The map here is ConcurrentHashMap to prevent concurrent issues.

Feel free to use this trick or try it if you have never thinked of changing log level at runtime. It may help you a lot when you have to solve an online problem but there's very few log at such log level. :)