Prevent JUL loggers from being GCed once their level has been set

Fixes gh-17217

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/java/JavaLoggingSystem.java

@@ -21,6 +21,7 @@ import java.io.InputStreamReader;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Enumeration;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 import java.util.logging.Level;
@@ -51,6 +52,8 @@ public class JavaLoggingSystem extends AbstractLoggingSystem {
 
 	private static final LogLevels<Level> LEVELS = new LogLevels<>();
 
+	private final Set<Logger> configuredLoggers = Collections.synchronizedSet(new HashSet<>());
+
 	static {
 		LEVELS.map(LogLevel.TRACE, Level.FINEST);
 		LEVELS.map(LogLevel.DEBUG, Level.FINE);
@@ -119,6 +122,7 @@ public class JavaLoggingSystem extends AbstractLoggingSystem {
 		}
 		Logger logger = Logger.getLogger(loggerName);
 		if (logger != null) {
+			this.configuredLoggers.add(logger);
 			logger.setLevel(LEVELS.convertSystemToNative(level));
 		}
 	}
@@ -159,6 +163,11 @@ public class JavaLoggingSystem extends AbstractLoggingSystem {
 		return new ShutdownHandler();
 	}
 
+	@Override
+	public void cleanUp() {
+		this.configuredLoggers.clear();
+	}
+
 	private final class ShutdownHandler implements Runnable {
 
 		@Override