Derek Mahar Derek Mahar - 4 months ago 57
Java Question

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

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>

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;
        }
    }

}
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