<configuration>
  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>foo.json</file>
    <encoder class="ch.qos.logback.classic.encoder.JsonEncoder">
       <withFformattedMessage>true</withFormattedMessage>
       <withMessage>false</withMessage>
       <withArguments>false</withArguments>       
    </encoder>
  </appender>


  <root>
    <level value="DEBUG"/>
    <appender-ref ref="FILE"/>
  </root>

</configuration>