Changing log level for logback-classic using reflection if it is present in the classpath without depending to the JAR.

LogbackUtils.java

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

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.text.MessageFormat;

/**
 * Contains methods to access and manipulate logback framework dynamically at run-time. Here 'dynamically' means without
 * referencing the logback JAR, but using it if found in the classpath.
 *
 * @author Muhammed Demirbas
 * @since 19 Mar 2013
 */
public final class LogbackUtils
{
    	public  static final String LOGBACK_CLASSIC        = "ch.qos.logback.classic";
	public  static final String LOGBACK_CLASSIC_LOGGER = "ch.qos.logback.classic.Logger";
	public  static final String LOGBACK_CLASSIC_LEVEL  = "ch.qos.logback.classic.Level";
	private static final Logger logger                 = LoggerFactory.getLogger(LogbackUtils.class);

	private LogbackUtils()
	{
		// Prevent instance creation
	}

	/**
	 * Dynamically sets the logback log level for the given class to the specified level.
	 *
	 * @param loggerName Name of the logger to set its log level. If blank, root logger will be used.
	 * @param logLevel   One of the supported log levels: TRACE, DEBUG, INFO, WARN, ERROR, FATAL,
	 *                      OFF. {@code null} value is considered as 'OFF'.
	 */
	public static boolean setLogLevel(String loggerName, String logLevel)
	{
		String logLevelUpper = (logLevel == null) ? "OFF" : logLevel.toUpperCase();

		try
		{
			Package logbackPackage = Package.getPackage(LOGBACK_CLASSIC);
			if (logbackPackage == null)
			{
				logger.info("Logback is not in the classpath!");
				return false;
			}

			// Use ROOT logger if given logger name is blank.
			if ((loggerName == null) || loggerName.trim().isEmpty())
			{
				loggerName = (String) getFieldVaulue(LOGBACK_CLASSIC_LOGGER, "ROOT_LOGGER_NAME");
			}

			// Obtain logger by the name
			Logger loggerObtained = LoggerFactory.getLogger(loggerName);
			if (loggerObtained == null)
			{
				// I don't know if this case occurs
				logger.warn("No logger for the name: {}", loggerName);
				return false;
			}

			Object logLevelObj = getFieldVaulue(LOGBACK_CLASSIC_LEVEL, logLevelUpper);
			if (logLevelObj == null)
			{
				logger.warn("No such log level: {}", logLevelUpper);
				return false;
			}

			Class<?>[] paramTypes =  { logLevelObj.getClass() };
			Object[]   params     =  { logLevelObj };

			Class<?> clz    = Class.forName(LOGBACK_CLASSIC_LOGGER);
			Method   method = clz.getMethod("setLevel", paramTypes);
			method.invoke(loggerObtained, params);

			logger.debug("Log level set to {} for the logger '{}'", logLevelUpper, loggerName);
			return true;
		}
		catch (Exception e)
		{
			logger.warn("Couldn't set log level to {} for the logger '{}'", logLevelUpper, loggerName, e);
			return false;
		}
	}

	private static Object getFieldVaulue(String fullClassName, String fieldName)
	{
		try
		{
			Class<?> clazz = Class.forName(fullClassName);
			Field    field = clazz.getField(fieldName);
			return field.get(null);
		}
		catch (ClassNotFoundException | IllegalAccessException | IllegalArgumentException | NoSuchFieldException |
				SecurityException ignored)
		{
			return null;
		}
	}
}

LogbackUtilsTest.java

import org.junit.Test;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertSame;

public final class LogbackUtilsTest
{
	@Test
	public void testSetLogLevel() throws Throwable
	{
		assertEquals(true,  LogbackUtils.setLogLevel(null,   null));      // Set ROOT logger level to 'OFF'
		assertEquals(true,  LogbackUtils.setLogLevel(null,   "TRACE"));   // Set ROOT logger level to TRACE
		assertEquals(true,  LogbackUtils.setLogLevel(".",    "DEBUG"));
		assertEquals(true,  LogbackUtils.setLogLevel(null,   "INFO"));
		assertEquals(true,  LogbackUtils.setLogLevel(null,   "WARN"));
		assertEquals(false, LogbackUtils.setLogLevel(null,   "WARNING")); // No such log level
		assertEquals(false, LogbackUtils.setLogLevel(null,   ""));        // No such log level
		assertEquals(false, LogbackUtils.setLogLevel(null,   "-"));       // No such log level
		assertEquals(true,  LogbackUtils.setLogLevel("org.", "DEBUG"));
	}
}

Because in this case we don't want to add logback dependency but want to change its log level if it is present. İf it is absent nothing should be done. İf it is ok to depend logback directly then you are right, no point to use this snippet.

…

On Sat, Aug 3, 2019, 01:22 Kenneth A. Argo ***@***.***> wrote: Why not just use code like this: LoggerContext loggerContext = (LoggerContext) LoggerFactory.getILoggerFactory(); Logger logger = loggerContext.getLogger(ch.qos.logback.classic.Logger.ROOT_LOGGER_NAME); ((ch.qos.logback.classic.Logger) logger).setLevel(Level.WARN); — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://gist.github.com/5411395?email_source=notifications&email_token=AAJWH7FICHVMXBNXETR5TJ3QCSXS7A5CNFSM4IJBM5GKYY3PNVWWK3TUL52HS4DFVNDWS43UINXW23LFNZ2KUY3PNVWWK3TUL5UWJTQAFWM74#gistcomment-2988542>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAJWH7FTBFXPUWQQJDGJHOTQCSXS7ANCNFSM4IJBM5GA> .