How to set the log level to DEBUG during Junit tests?

You do not need to give the JVM a different log implementation.

The logging code searches for the log4j.properties file using the classpath. So all you need to do is ensure that your test log4j.properties file is in a location that it will find before the release file.

I use Maven, which lays out files in directories to make that easy. My release log4j.properties goes in the directory src/main/resources. My test version goes in src/test/resources. The Eclipse build path (classpath) is set up to search src/test/resources before src/main/resources, so your unit tests use the test file. The JAR (or WAR) build instructions use the files from src/main/resources.

Below ROOT log level change will work for junit to set logging to desired level.

import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.Logger;

@Before
public void setUp() {
    final Logger logger = (Logger)LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
    logger.setLevel(Level.ALL);
}