一次关于log4j的jar包冲突解决：Caused by: java.lang.NoSuchFieldError: fileName

项目中log系统使用的 logback + slf4j 实现， 今天一同事新引入几个jar包，结果tomcat启动时报了以下异常：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'javaEntranceProxyTypeHolder' defined in class path resource [sentinel-tracer-entrance.xml]: Invocation of init method failed; nested exception is java.lang.NoSuchFieldError: fileName
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1514)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:521)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458)
        at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:293)
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:290)
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:191)
        at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:328)
        ... 18 more
Caused by: java.lang.NoSuchFieldError: fileName
        at org.apache.log4j.DailyRollingFileAppender.activateOptions(DailyRollingFileAppender.java:224)
        at org.apache.log4j.DailyRollingFileAppender.(DailyRollingFileAppender.java:205)
        at com.taobao.csp.courier.RecordLog.(RecordLog.java:42)
        at com.taobao.csp.sentinel.entrypoint.ProxyTypeHolder.afterPropertiesSet(ProxyTypeHolder.java:44)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1573)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1511)

很奇怪怎么还会报 log4j 的异常， 最后查了包依赖树，发现是新加的jar包把 log4j-over-slf4j 这个包给传递依赖进来了，当有这个包存在时，日志系统会回调到log4j的实现上，所以有了上面这个log4j的异常。

解决方法： 排除传递依赖的 log4j-over-slf4j 

打开pom文件的 Dependency Hierarchy模式， 找到是哪个jar引入的这个包，把它里面的log4j-over-slf4j这个包排除掉即可，加上下面配置：

 
  org.slf4j
  log4j-over-slf4j
 

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