Sample configuration to inject a TestAppender into log4j2

log4j2-test.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Properties>
        <Property name="LOG_PATTERN">
            %d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n
        </Property>
    </Properties>

    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="${LOG_PATTERN}" />
        </Console>

        <TestAppender name="TestAppender">
            <PatternLayout pattern="${LOG_PATTERN}" />
        </TestAppender>
    </Appenders>
    <Loggers>
        <Root level="info" additivity="false">
            <AppenderRef ref="Console" />
            <AppenderRef ref="TestAppender" level="error" />
        </Root>
    </Loggers>
</Configuration>