How to enable debug in slf4j Logger?

How to globally enable debug for all the slf4j.Logger objects?

Programmatically, with logback:

setLoggingLevel(ch.qos.logback.classic.Level.DEBUG);

where

public static void setLoggingLevel(ch.qos.logback.classic.Level level) {
    ch.qos.logback.classic.Logger root = (ch.qos.logback.classic.Logger) org.slf4j.LoggerFactory.getLogger(ch.qos.logback.classic.Logger.ROOT_LOGGER_NAME);
    root.setLevel(level);
}

exists various capability to switch debug log on:
this article have good explanation all of those. to me good fit is:

Using slf4j with Log4j logger
create file src/main/resources/log4j.properties

log4j.rootLogger=DEBUG, STDOUT
log4j.logger.deng=INFO
log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
log4j.appender.STDOUT.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

Pass the System Property -Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG at your Java startup for the SLF4J Simple api

Use logback as the slf4j binding.

The default behaviour without an configuration file is to log all events at level DEBUG and above to System.out. See http://logback.qos.ch/manual/configuration.html#automaticConf for details.

depends on what binding you are using... if e.g. it's log4j have a look at http://logging.apache.org/log4j/1.2/manual.html and its Configuration chapter

If you use log4j as the binding of slf4j you can crete a log4j.xml (or log4j.properties) file and add it to the classpath. An example could be:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">
  <appender name="CONSOLE" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out" />
    <param name="Threshold" value="DEBUG" />
    <layout class="org.apache.log4j.PatternLayout">
      <param name="ConversionPattern" value="%d{HH:mm:ss,SSS} %-5p [%c{1}] %m%n" />
    </layout>
  </appender>
  <root>
    <appender-ref ref="CONSOLE" />
  </root>
</log4j:configuration>

For log4j

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="true">

<!-- ============================== -->
<!-- Append messages to the console -->
<!-- ============================== -->

<appender name="CONSOLE" class="org.apache.log4j.ConsoleAppender">
    <param name="Target" value="System.out"/>
    <param name="Threshold" value="INFO"/>

    <layout class="org.apache.log4j.PatternLayout">
        <!-- The default pattern: Date Priority [Category] Message\n -->
        <param name="ConversionPattern" value="%d [%t] %-5p %c - %m%n"/>
    </layout>
</appender>

<appender name="web" class="org.apache.log4j.DailyRollingFileAppender">
    <param name="Threshold" value="DEBUG"/>
    <param name="Append" value="true"/>
    <param name="File" value="${catalina.home}/logs/web.log"/>
    <layout class="org.apache.log4j.PatternLayout">
        <param name="ConversionPattern" value="%d [%t] %-5p %c - %m%n"/>
    </layout>
</appender>

<category name="com.idc.scd" additivity="false">
    <priority value="${log4j.category.com.mypackage}"/>
    <appender-ref ref="web"/>
</category>

</log4j:configuration>

by this cofiguration your all "com.mypackage" logs will be written in "web.log" file under catalina.home.

Here's a sample configuration I usually use to setup logging with logback.
Gradle dependencies (the same apply to Maven)

compile group: 'org.slf4j', name: 'slf4j-api', version: '1.7.25'
compile group: 'ch.qos.logback', name: 'logback-classic', version: '1.2.3'

logback.xml configuration to be placed in the project's classpath (src/main/resources)

<configuration>

	<statusListener class="ch.qos.logback.core.status.NopStatusListener" />

	<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
		<layout class="ch.qos.logback.classic.PatternLayout">
			<Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36}.%M\(%line\) - %msg%n</Pattern>
		</layout>
	</appender>
	
	<!-- enable debug only on org.hibernate.SQL package -->
	<logger name="org.hibernate.SQL" level="debug" additivity="false">
		<appender-ref ref="STDOUT" />
	</logger>

	<root level="info">
		<appender-ref ref="STDOUT" />
	</root>

</configuration>

Just add the following to the application.properties

logging.level.org.springframework.web=DEBUG
logging.level.org.springframework.context=DEBUG