Programmatically change log level in Log4j2

The Easy Way :

EDITED according to log4j2 version 2.4 FAQ

You can set a logger’s level with the class Configurator from Log4j Core. BUT be aware that the Configurator class is not part of the public API.

// org.apache.logging.log4j.core.config.Configurator;
Configurator.setLevel("com.example.Foo", Level.DEBUG);

// You can also set the root logger:
Configurator.setRootLevel(Level.DEBUG);

Source

The Preferable Way :

EDITED to reflect changes in the API introduced in Log4j2 version 2.0.2

If you wish to change the root logger level, do something like this :

LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
Configuration config = ctx.getConfiguration();
LoggerConfig loggerConfig = config.getLoggerConfig(LogManager.ROOT_LOGGER_NAME); 
loggerConfig.setLevel(level);
ctx.updateLoggers();  // This causes all Loggers to refetch information from their LoggerConfig.

Here is the javadoc for LoggerConfig.

The accepted answer by @slaadvak did not work for me for Log4j2 2.8.2. The following did.

To change the log Level universally use:

Configurator.setAllLevels(LogManager.getRootLogger().getName(), level);

To change the log Level for only the current class, use:

Configurator.setLevel(LogManager.getLogger(CallingClass.class).getName(), level);

If you want to change a single specific logger level (not the root logger or loggers configured in the configuration file) you can do this:

public static void setLevel(Logger logger, Level level) {
    final LoggerContext ctx = (LoggerContext) LogManager.getContext(false);
    final Configuration config = ctx.getConfiguration();

    LoggerConfig loggerConfig = config.getLoggerConfig(logger.getName());
    LoggerConfig specificConfig = loggerConfig;

    // We need a specific configuration for this logger,
    // otherwise we would change the level of all other loggers
    // having the original configuration as parent as well

    if (!loggerConfig.getName().equals(logger.getName())) {
        specificConfig = new LoggerConfig(logger.getName(), level, true);
        specificConfig.setParent(loggerConfig);
        config.addLogger(logger.getName(), specificConfig);
    }
    specificConfig.setLevel(level);
    ctx.updateLoggers();
}