系统变量
System Properties Lookup
${sys:catalina.home}/logs

fileName="${sys:logPath:-/var/logs}/app.log"
when the logPath system property is undefined, the default value /var/logs is used



环境变量
Environment Lookup
$${env:LOG4J2_APP_PROJECT_NAME:-mall}
第二个冒号后的为默认值，注意 - 开头


${env:USER:-jdoe} 
when the USER environment variable is undefined, the default value jdoe is used



Spring Boot Lookup
The Spring Boot Lookup retrieves the values of Spring properties from the Spring configuration as well as values of the active and default profiles. 

fileName="application-${spring:profiles.active[0]}.log"
${spring:spring.application.name}



Log4j2 Lookups  
http://logging.apache.org/log4j/2.x/manual/lookups.html