slf4j+log4j2在tomcat8 下无日志输出

拼搏现实的明天。 2022-09-30 00:57 256阅读 0赞

http://blog.mugbya.cn/2014/10/25/slf4j-tomcat/

在写这个问题之前,先声明,这个问题肯定不是大家都能遇到,只是特性条件会出现这个情况,因了解还不是太深入,目前只记录我了解的所有.

*  [参见segmentfault上的问题描述][segmentfault]

在问题的版本差异比较时,慢慢发现一些问题:

// tomcat7
    SLF4J: Found binding in [jar:file:~/project/out/artifacts/cjtrade_Web_exploded/WEB-INF/lib/log4j-slf4j-impl-2.0.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
    SLF4J: Found binding in [jar:file:~/project/out/artifacts/cjtrade_Web_exploded/WEB-INF/lib/slf4j-log4j12-1.7.6.jar!/org/slf4j/impl/StaticLoggerBinder.class]
    
    // tomcat8
    SLF4J: Found binding in [jar:file:~/project/out/artifacts/cjtrade_Web_exploded/WEB-INF/lib/slf4j-log4j12-1.7.6.jar!/org/slf4j/impl/StaticLoggerBinder.class]
    SLF4J: Found binding in [jar:file:~/project/out/artifacts/cjtrade_Web_exploded/WEB-INF/lib/log4j-slf4j-impl-2.0.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]

如果认真对比,你会发现slf4j找到了两个实现的类,但是在不同的版本它们的顺序却不一样. 结合slf4j实际绑定的类型

// tomcat 7
    SLF4J: Actual binding is of type [org.apache.logging.slf4j.Log4jLoggerFactory]
    log4j:WARN No appenders could be found for logger (com.mchange.v2.log.MLog).
    
    // tomcat 8
    SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
    log4j:WARN No appenders could be found for logger (org.springframework.web.context.ContextLoader).

找到的绑定类的都是一样,但在不同的包下,这两个都实现了,来分别看下这两个包:

[![log4j-slf4j-impl][]][log4j-slf4j-impl] log4j-slf4j-impl  
[![slf4j-log4j12.png][]][slf4j-log4j12.png] slf4j-log4j12.png

针对于slf4j跟log4j2搭配,[log4j2][]官网有明确指出:

Use of the Log4j 2 SLF4J Binding (log4j-slf4j-impl-2.0.jar) together with the SLF4J adapter (log4j-to-slf4j-2.0.jar) should never be attempted, as it will cause events to endlessly be routed between SLF4J and Log4j 2.

所以我们应该确信用log4j-slf4j-impl这一个jar,但是slf4j-log4j12这一个是怎么回事呢,在[slf4j][]官网上有这样的叙述:

slf4j-log4j12-1.7.7.jar
    Binding for log4j version 1.2, a widely used logging framework. You also need to place log4j.jar on your class path.

从这来看,不是这个版本才对.当我们在maven中删除这个依赖,但是好像还是会有默认的依赖(1.7.6)存在.所以我们只能去lib移除了它了.这样只让slf4j绑定log4j-slf4j-impl包下的org.apache.logging.slf4j.Log4jLoggerFactory就解决了

至于为什么tomcat8 跟 tomcat7 会加载顺序不一样,这点我还是没懂.了解的太少了,只知道7跟8差异很大,你去比较conf/catalina.properties这个文件你就知道了

[segmentfault]: http://segmentfault.com/q/1010000000737900
[log4j-slf4j-impl]: http://blog.mugbya.cn/assets/java_web/log4j-slf4j-impl.png
[slf4j-log4j12.png]: http://blog.mugbya.cn/assets/java_web/slf4j-log4j12.png
[log4j2]: http://logging.apache.org/log4j/2.x/log4j-slf4j-impl/index.html
[slf4j]: http://www.slf4j.org/manual.html