Resolution
If you want to change the default/package log level temporally you have to go to Manage Jenkins > System Log > Log Levels and set the level that you want. The logger with no name is the default logger. All the loggers without a configured level inherit the level of the default logger. This change will be lost when you restart the Jenkins instance.
If you want to make this change permanent, you have to create a groovy file in JENKINS_HOME/init.groovy.d to set up a Logger programmatically. Below is an example of how to set the log level INFO to the default logger and to the package org.apache.commons.httpclient.
import java.util.logging.LogManager
import java.util.logging.Logger
import jenkins.model.Jenkins
import java.util.logging.Level
Logger.getGlobal().setLevel(Level.INFO);
def logger = Logger.getLogger("org.apache.commons.httpclient")
logger.setLevel(Level.INFO)
LogManager.getLogManager().addLogger(logger)
You also can configure the Java logger using a logging.properties file by setting the Java property -Djava.util.logging.config.file=logging.properties on your startup Jenkins properties.
Example of logging.properties file
.level = INFO handlers= java.util.logging.FileHandler java.util.logging.FileHandler.level = INFO java.util.logging.FileHandler.formatter = java.util.logging.SimpleFormatter java.util.logging.FileHandler.pattern=support/jenkins-cli-%u.log java.util.logging.FileHandler.limit = 1000000 java.util.logging.FileHandler.count = 25 hudson.level = INFO org.apache.sshd.level = INFO winstone.level = INFO org.apache.commons.httpclient.level = INFO