How to upper case custom field in logback

I am using logback with spring boot and I want to upper case a custom field that I get from spring property,

  <springProperty name="foo" source="app.foo"/>
  <customFields>
    {
      "foo":"${foo}"
    }
  </customFields>

the logback configuration is used by multiple clients within a library and every client have this property app.foo (or APP_FOO env variable) and some client have put it in lower case,
that's why I want to do the field transformation within the logback config

Using ClassicConverter, you can try changing the case of the property value.

Here is the sample logback-spring.xml with both examples of ch.qos.logback.core.ConsoleAppender and net.logstash.logback.encoder.LogstashEncoder to refer the property as custom field.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>

    <springProperty name="foo" source="app.foo" defaultValue="NotSet" />

    <conversionRule conversionWord="toUpperCase" converterClass="com.example.logback_custom_fields.UpperCaseConverter"/>

    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
            <provider class="net.logstash.logback.composite.loggingevent.LoggingEventPatternJsonProvider">
                <pattern>
                    {
                    "foo": "%toUpperCase{${foo}}"
                    }
                </pattern>
            </provider>
        </encoder>
    </appender>

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%-4relative [%thread] %-5level %logger{35} -%kvp- %msg %toUpperCase{${foo}}%n</pattern>
        </encoder>
    </appender>

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

Below is the UpperCaseConverter.java.

package com.example.logback_custom_fields;

import ch.qos.logback.classic.pattern.ClassicConverter;
import ch.qos.logback.classic.spi.ILoggingEvent;

public class UpperCaseConverter extends ClassicConverter {

    @Override
    public String convert(ILoggingEvent event) {
        String arg = getFirstOption();
        if (arg == null) {
            return "NOT_SET";
        }
        // Use the argument in your conversion logic
        return arg.toUpperCase();
    }
}

Example repo where you can test this is at https://github.com/imran9m/logback-custom-fields

Once you run mvn clean install, it should execute test cases where it will populate app.foo value in uppercase like below.

{"@timestamp":"2025-01-18T22:34:27.727725475-05:00","@version":"1","message":"Tomcat started on port 39167 (http) with context path '/'","logger_name":"org.springframework.boot.web.embedded.tomcat.TomcatWebServer","thread_name":"main","level":"INFO","level_value":20000,"foo":"APPFOOPROPVALUE"}
5030 [main] INFO  o.s.b.w.e.tomcat.TomcatWebServer -- Tomcat started on port 39167 (http) with context path '/' APPFOOPROPVALUE