How can I configure Logback to log different levels for a logger to different destinations?

For example, given the following Logback configuration, will Logback record INFO messages to STDOUT and ERROR messages to STDERR?

(Note that this example is a variation of example logback-examples/src/main/java/chapters/configuration/sample4.xml shown in Chapter 3: Logback Configuration).

<configuration>
  <appender name="STDOUT"
   class="ch.qos.logback.core.ConsoleAppender">
   <encoder>
     <pattern>
        %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
      </pattern>
    </encoder>
  </appender>
  <appender name="STDERR"
   class="ch.qos.logback.core.ConsoleAppender">
   <encoder>
     <pattern>
        %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
      </pattern>
    </encoder>
    <target>System.err</target>
  </appender>
  <!-- What is the effective level of "chapters.configuration"? -->
  <logger name="chapters.configuration" level="INFO" additivity="false">
    <appender-ref ref="STDOUT" />
  </logger>
  <logger name="chapters.configuration" level="ERROR" additivity="false">
    <appender-ref ref="STDERR" />
  </logger>

  <!-- turn OFF all logging (children can override) -->
  <root level="OFF">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

Solution 1:

I believe this would be the simplest solution:

<configuration>
    <contextName>selenium-plugin</contextName>
    <!-- Logging configuration -->  
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <Target>System.out</Target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
            <level>INFO</level>
            <onMatch>ACCEPT</onMatch>
            <onMismatch>DENY</onMismatch>
        </filter>
        <encoder>
            <pattern>[%d{yyyy-MM-dd HH:mm:ss.SSS}] [%level] %msg%n</pattern>
        </encoder>
    </appender>
    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
            <Target>System.err</Target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
            <level>ERROR</level>
            <onMatch>ACCEPT</onMatch>
            <onMismatch>DENY</onMismatch>
        </filter>
        <encoder> 
            <pattern>[%d{yyyy-MM-dd HH:mm:ss.SSS}] [%level] [%thread] %logger{10} [%file:%line] %msg%n</pattern> 
        </encoder> 
    </appender>
    <root level="INFO">
        <appender-ref ref="STDOUT"/>
        <appender-ref ref="STDERR" />
    </root>
</configuration>

Solution 2:

Update: For an all configuration based approach using Groovy see Dean Hiller's answer.

You can do some interesting things with Logback filters. The below configuration will only print warn and error messages to stderr, and everything else to stdout.

logback.xml

<appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
  <target>System.out</target>
  <filter class="com.foo.StdOutFilter" />
   ...
</appender>

<appender name="stderr" class="ch.qos.logback.core.ConsoleAppender">
  <target>System.err</target>
  <filter class="com.foo.ErrOutFilter" />
   ...
</appender>

<logger name="mylogger" level="debug">
    <appender-ref ref="stdout" />
    <appender-ref ref="stderr" />
</logger>

com.foo.StdOutFilter

public class StdOutFilter extends ch.qos.logback.core.filter.AbstractMatcherFilter
{

    @Override
    public FilterReply decide(Object event)
    {
        if (!isStarted())
        {
            return FilterReply.NEUTRAL;
        }

        LoggingEvent loggingEvent = (LoggingEvent) event;

        List<Level> eventsToKeep = Arrays.asList(Level.TRACE, Level.DEBUG, Level.INFO);
        if (eventsToKeep.contains(loggingEvent.getLevel()))
        {
            return FilterReply.NEUTRAL;
        }
        else
        {
            return FilterReply.DENY;
        }
    }

}

com.foo.ErrOutFilter

public class ErrOutFilter extends ch.qos.logback.core.filter.AbstractMatcherFilter
{

    @Override
    public FilterReply decide(Object event)
    {
        if (!isStarted())
        {
            return FilterReply.NEUTRAL;
        }

        LoggingEvent loggingEvent = (LoggingEvent) event;

        List<Level> eventsToKeep = Arrays.asList(Level.WARN, Level.ERROR);
        if (eventsToKeep.contains(loggingEvent.getLevel()))
        {
            return FilterReply.NEUTRAL;
        }
        else
        {
            return FilterReply.DENY;
        }
    }

}

Solution 3:

Solution based on configuration only, with a ThresoldFilter and LevelFilters to keep things really simple to understand :

<configuration>
    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
        <target>System.err</target>
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
          <level>WARN</level>
        </filter>
        <encoder>
            <pattern>%date %level [%thread] %logger %msg%n</pattern>
        </encoder>
    </appender>

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <target>System.out</target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>DEBUG</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>INFO</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>TRACE</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>WARN</level>
          <onMatch>DENY</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>ERROR</level>
          <onMatch>DENY</onMatch>
        </filter>
        <encoder>
            <pattern>%date %level [%thread] %logger %msg%n</pattern>
        </encoder>
    </appender>

    <root level="INFO">
        <appender-ref ref="STDOUT" />
        <appender-ref ref="STDERR" />
    </root>
</configuration>

Solution 4:

okay, here is my favorite xml way of doing it. I do this for the eclipse version so I can

click on stuff to take me to the log statements and
see info and below in black and warn/severe in red

and for some reason SO is not showing this all properly but most seems to be there...

<configuration scan="true" scanPeriod="30 seconds">

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.core.filter.EvaluatorFilter">      
          <evaluator class="ch.qos.logback.classic.boolex.GEventEvaluator"> 
            <expression>
               e.level.toInt() &lt;= INFO.toInt()
            </expression>
          </evaluator>
          <OnMismatch>DENY</OnMismatch>
          <OnMatch>NEUTRAL</OnMatch>
        </filter>

        <encoder>
            <pattern>%date{ISO8601} %X{sessionid}-%X{user} %caller{1} %-4level: %message%n</pattern>
        </encoder>
    </appender>

    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter"> 
            <level>warn</level>
        </filter>

        <encoder>
            <pattern>%date{ISO8601} %X{sessionid}-%X{user} %caller{1} %-4level: %message%n</pattern>
        </encoder>
        <target>System.err</target>
    </appender>

    <root>
        <level value="INFO" />
        <appender-ref ref="STDOUT"/>
        <appender-ref ref="STDERR"/>
    </root>
</configuration>

Solution 5:

The simplest solution is to use ThresholdFilter on the appenders:

    <appender name="..." class="...">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>INFO</level>
        </filter>

Full example:

<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>INFO</level>
        </filter>
        <encoder>
            <pattern>%d %-5level: %msg%n</pattern>
        </encoder>
    </appender>

    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>ERROR</level>
        </filter>
        <target>System.err</target>
        <encoder>
            <pattern>%d %-5level: %msg%n</pattern>
        </encoder>
    </appender>

    <root>
        <appender-ref ref="STDOUT" />
        <appender-ref ref="STDERR" />
    </root>
</configuration>

Update: As Mike pointed out in the comment, messages with ERROR level are printed here both to STDOUT and STDERR. Not sure what was the OP's intent, though. You can try Mike's answer if this is not what you wanted.