Why do you think it does not log DEBUG messages?
If you mean that your log.debug(String) logging calls do not end up in java.util.logging log files, then I guess you have to configure the logging.properties configuration file to allow log messages at FINE level.
If you do not want to mess with the global %JRE_HOME%/lib/logging.properties, then you can just pass in -Djava.util.logging.config.file=logging.properties on the command line – this will force the logging system to look for that configuration file in the current directory.
Or use some other (programmatic) way to configure java.util.logging, see below for tutorial.
This has nothing to do with configuring SLF4J; in fact, SLF4J does not have any configuration, everything is configured by simply swapping JAR files.
For your reference:
- JDK14LoggerAdapter
- Java Logging API tutorial