它是 子Logger 是否继承 父Logger 的 输出源（appender）的标志位。具体说，默认情况下子Logger会继承父Logger的appender，也就是说子Logger会在父Logger的appender里输出。若是additivity设为false，则子Logger只会在自己的appender里输出，而不会在父Logger的appender里输出。
使用例子：下面是一个log4j的配置文件，父logger是reyo和error，子logger是net
#
# Priority = {FATAL|ERROR|WARN|INFO|DEBUG}
#
# +---------------------------------------------------+
# | root |
# +---------------------------------------------------+
# Category - Root
# ================
log4j.rootLogger=info,reyo,error
log4j.logger.reyo=info,reyo
log4j.additivity.reyo=false
log4j.logger.net=info,net
log4j.additivity.net=false
log4j.logger.error=error,error
log4j.additivity.error=false
# Category - other
# ========================================
log4j.logger.org.springframework=ERROR
log4j.logger.com.alibaba.druid=ERROR
# Appender - stdout
# =========================================
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} %l %p: %m%n
log4j.appender.stdout.encoding=UTF-8
log4j.appender.reyo=org.apache.log4j.DailyRollingFileAppender
log4j.appender.reyo.File=/data/reyo_test/log/reyo.log
log4j.appender.reyo.layout=org.apache.log4j.PatternLayout
log4j.appender.reyo.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} %l %p: %m%n
log4j.appender.reyo.encoding=UTF-8
log4j.appender.net=org.apache.log4j.DailyRollingFileAppender
log4j.appender.net.File=/data/reyo_test/log/net.log
log4j.appender.net.layout=org.apache.log4j.PatternLayout
log4j.appender.net.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} %l %p: %m%n
log4j.appender.net.encoding=UTF-8
log4j.appender.error=org.apache.log4j.DailyRollingFileAppender
log4j.appender.error.Threshold=ERROR
log4j.appender.error.File=/data/reyo_test/log/error.log
log4j.appender.error.layout=org.apache.log4j.PatternLayout
log4j.appender.error.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} %l %p: %m%n
log4j.appender.error.encoding=UTF-8