[java] How do I set log4j level on the command line?

I want to add some log.debug statements to a class I'm working on, and I'd like to see that in output when running the test. I'd like to override the log4j properties on the command line, with something like this:

-Dlog4j.logger.com.mypackage.Thingie=DEBUG

I do this kind of thing frequently. I am specifically only interested in a way to pass this on the command line. I know how to do it with a config file, and that doesn't suit my workflow.

Based on Thorbjørn Ravn Andersens suggestion I wrote some code that makes this work

Add the following early in the main method and it is now possible to set the log level from the comand line. This have been tested in a project of mine but I'm new to log4j and might have made some mistake. If so please correct me.

    Logger.getRootLogger().setLevel(Level.WARN);
    HashMap<String,Level> logLevels=new HashMap<String,Level>();
    logLevels.put("ALL",Level.ALL);
    logLevels.put("TRACE",Level.TRACE);
    logLevels.put("DEBUG",Level.DEBUG);
    logLevels.put("INFO",Level.INFO);
    logLevels.put("WARN",Level.WARN);
    logLevels.put("ERROR",Level.ERROR);
    logLevels.put("FATAL",Level.FATAL);
    logLevels.put("OFF",Level.OFF);
    for(String name:System.getProperties().stringPropertyNames()){
        String logger="log4j.logger.";
        if(name.startsWith(logger)){
            String loggerName=name.substring(logger.length());
            String loggerValue=System.getProperty(name);
            if(logLevels.containsKey(loggerValue))
                Logger.getLogger(loggerName).setLevel(logLevels.get(loggerValue));
            else
                Logger.getRootLogger().warn("unknown log4j logg level on comand line: "+loggerValue);
        }
    }

With Log4j2, this can be achieved using the following utility method added to your code.

private static void setLogLevel() {
  if (Boolean.getBoolean("log4j.debug")) {
    Configurator.setLevel(System.getProperty("log4j.logger"), Level.DEBUG);
  }
}

You need these imports

import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.core.config.Configurator;

Now invoke the setLogLevel method in your main() or whereever appropriate and pass command line params -Dlog4j.logger=com.mypackage.Thingie and -Dlog4j.debug=true.

Based on @lijat, here is a simplified implementation. In my spring-based application I simply load this as a bean.

public static void configureLog4jFromSystemProperties()
{
  final String LOGGER_PREFIX = "log4j.logger.";

  for(String propertyName : System.getProperties().stringPropertyNames())
  {
    if (propertyName.startsWith(LOGGER_PREFIX)) {
      String loggerName = propertyName.substring(LOGGER_PREFIX.length());
      String levelName = System.getProperty(propertyName, "");
      Level level = Level.toLevel(levelName); // defaults to DEBUG
      if (!"".equals(levelName) && !levelName.toUpperCase().equals(level.toString())) {
        logger.error("Skipping unrecognized log4j log level " + levelName + ": -D" + propertyName + "=" + levelName);
        continue;
      }
      logger.info("Setting " + loggerName + " => " + level.toString());
      Logger.getLogger(loggerName).setLevel(level);
    }
  }
}

As part of your jvm arguments you can set -Dlog4j.configuration=file:"<FILE_PATH>". Where FILE_PATH is the path of your log4j.properties file.

Please note that as of log4j2, the new system variable to use is log4j.configurationFile and you put in the actual path to the file (i.e. without the file: prefix) and it will automatically load the factory based on the extension of the configuration file:

-Dlog4j.configurationFile=/path/to/log4jconfig.{ext}

In my pretty standard setup I've been seeing the following work well when passed in as VM Option (commandline before class in Java, or VM Option in an IDE):

-Droot.log.level=TRACE

These answers actually dissuaded me from trying the simplest possible thing! Simply specify a threshold for an appender (say, "console") in your log4j.configuration like so:

log4j.appender.console.threshold=${my.logging.threshold}

Then, on the command line, include the system property -Dlog4j.info -Dmy.logging.threshold=INFO. I assume that any other property can be parameterized in this way, but this is the easiest way to raise or lower the logging level globally.