引言：一个问题的分析与解决过程是表与里的过程，是一个大胆猜测与小心求证的过程，spring boot与log4j2的集成过程中，我将描述一下分析这个问题的思路和过程。我一直强调一点：重要的不是解决问题的结论，而是解决问题的思路和方法，即使在解决完问题之后，依然需要回过头复盘，在问题分析过程中的走过的弯路。

1 项目情况介绍

Spring Boot 1.5.7 , JDK 1.8, Log4j2.7
在这个项目中，希望集成Log4j2作为默认的日志系统输出器。
Log4j2.xml的定义如下：

"><Configuration status="off"><Properties><Property name="LOG_PATH">.</Property></Properties><Appenders><!-- Console --><Console name="Console" target="SYSTEM_OUT"><PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %logger{36}.%M@%L - %msg%n"/><!--<PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %C{5} (%F:%L) - %msg%n"/>--></Console><!-- RollingFile --><RollingRandomAccessFile name="RollingFile"fileName="./aifasion.log"filePattern="${LOG_PATH}/aifasion.log.%d{yyyy-MM-dd-HH}"><PatternLayout pattern="%date{yyyy-MM-dd HH:mm:ss.SSS} %-5level %logger{36}.%M@%L - %msg%n"/><Policies><TimeBasedTriggeringPolicy/></Policies><DefaultRolloverStrategy max="30"/></RollingRandomAccessFile></Appenders><Loggers><!-- Spring Loggers --><Logger  name="org.springframework" level="info"/><!-- App Loggers --><Logger name="org.jd.test" level="debug" additivity="false"><AppenderRef  ref="Console"/><AppenderRef  ref="RollingFile"/></Logger><!-- Root Logger --><Root  level="info"><AppenderRef ref="Console"/><AppenderRef ref="RollingFile"/></Root ></Loggers>
</Configuration>

application.properties的文件定义如下：

server.port=8080
logging.config=classpath:log4j2.xml
debug=true

上述为核心的配置文件信息。
在pom.xml中，针对spring-boot-web中对于logging的引用进行了排除，配置信息如下：

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><exclusions><exclusion><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-logging</artifactId></exclusion></exclusions></dependency>

2 问题描述

基于Spring Boot的向导创建基础项目，集成Log4j2，在项目启动过程中，报出如下的错误信息：

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/DevSpace/M2Space/ch/qos/logback/logback-classic/1.1.11/logback-classic-1.1.11.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/DevSpace/M2Space/org/apache/logging/log4j/log4j-slf4j-impl/2.7/log4j-slf4j-impl-2.7.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder]
The Class-Path manifest attribute in D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derby-10.13.1.1.jar referenced one or more files that do not exist: D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_cs.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_de_DE.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_es.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_fr.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_hu.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_it.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_ja_JP.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_ko_KR.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_pl.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_pt_BR.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_ru.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_zh_CN.jar,D:\DevSpace\M2Space\org\apache\derby\derby\10.13.1.1\derbyLocale_zh_TW.jar
10:00:46.441 [main] DEBUG org.springframework.boot.devtools.settings.DevToolsSettings - Included patterns for restart : []
10:00:46.444 [main] DEBUG org.springframework.boot.devtools.settings.DevToolsSettings - Excluded patterns for restart : [/spring-boot-starter/target/classes/, /spring-boot-autoconfigure/target/classes/, /spring-boot-starter-[\w-]+/, /spring-boot/target/classes/, /spring-boot-actuator/target/classes/, /spring-boot-devtools/target/classes/]
10:00:46.444 [main] DEBUG org.springframework.boot.devtools.restart.ChangeableUrls - Matching URLs for reloading : [file:/D:/workspace/testdemo/target/classes/]
Logging system failed to initialize using configuration from ‘classpath:log4j2.xml’
java.lang.IllegalStateException: Logback configuration error detected:
ERROR in ch.qos.logback.core.joran.spi.Interpreter@3:17 - no applicable action for [Properties], current ElementPath is [[Configuration][Properties]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@4:35 - no applicable action for [Property], current ElementPath is [[Configuration][Properties][Property]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@7:16 - no applicable action for [Appenders], current ElementPath is [[Configuration][Appenders]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@9:53 - no applicable action for [Console], current ElementPath is [[Configuration][Appenders][Console]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@10:92 - no applicable action for [PatternLayout], current ElementPath is [[Configuration][Appenders][Console][PatternLayout]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@16:91 - no applicable action for [RollingRandomAccessFile], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@17:106 - no applicable action for [PatternLayout], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][PatternLayout]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@18:23 - no applicable action for [Policies], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][Policies]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@19:45 - no applicable action for [TimeBasedTriggeringPolicy], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][Policies][TimeBasedTriggeringPolicy]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@21:48 - no applicable action for [DefaultRolloverStrategy], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][DefaultRolloverStrategy]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@25:14 - no applicable action for [Loggers], current ElementPath is [[Configuration][Loggers]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@27:59 - no applicable action for [Logger], current ElementPath is [[Configuration][Loggers][Logger]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@30:69 - no applicable action for [Logger], current ElementPath is [[Configuration][Loggers][Logger]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@31:42 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Logger][AppenderRef]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@32:46 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Logger][AppenderRef]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@36:29 - no applicable action for [Root], current ElementPath is [[Configuration][Loggers][Root]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@37:41 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Root][AppenderRef]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@38:45 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Root][AppenderRef]]
at org.springframework.boot.logging.logback.LogbackLoggingSystem.loadConfiguration(LogbackLoggingSystem.java:162)
at org.springframework.boot.logging.AbstractLoggingSystem.initializeWithSpecificConfig(AbstractLoggingSystem.java:66)
at org.springframework.boot.logging.AbstractLoggingSystem.initialize(AbstractLoggingSystem.java:56)
at org.springframework.boot.logging.logback.LogbackLoggingSystem.initialize(LogbackLoggingSystem.java:115)
at org.springframework.boot.logging.LoggingApplicationListener.initializeSystem(LoggingApplicationListener.java:308)
at org.springframework.boot.logging.LoggingApplicationListener.initialize(LoggingApplicationListener.java:276)
at org.springframework.boot.logging.LoggingApplicationListener.onApplicationEnvironmentPreparedEvent(LoggingApplicationListener.java:239)
at org.springframework.boot.logging.LoggingApplicationListener.onApplicationEvent(LoggingApplicationListener.java:212)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:122)
at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:74)
at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:54)
at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:325)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:296)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107)
at org.jd.test.TestdemoApplication.main(TestdemoApplication.java:14)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)

以上是在系统启动过程中的错误信息，关键错误信息是：

Logging system failed to initialize using configuration from ‘classpath:log4j2.xml’
java.lang.IllegalStateException: Logback configuration error detected:

3 问题分析过程

3.1 配置文件的格式或者定义错误

根据刚才看到的错误信息，第一反应是log4j2.xml中定义的错误信息或者格式信息有误。
根据这个判断，就直接在Log4j2的官方文档上，重新查阅了一番，结合定义中的内容格式，感觉格式都是完全正确的，没有什么出入问题。
于是，我就使用排除法，将Log4j2.xml中的内容按照步骤，一部分一部分的进行验证排除，如果哪个部分中存在配置文件的配置错误，则可以定位出来。
在一番定位之后，发现只要配置文件中存在内容，就会报出类似错误信息。
结论是，应该不是配置文件的配置错误信息。

3.2 下载样例项目进行分析

于是，从网上直接下来了spring-boot中自带的spring-boot-sample-actuator-log4j2项目，进行分析：
发现其中pom.xml中的不同之处为：
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter</artifactId> <exclusions> <exclusion> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-logging</artifactId> </exclusion> </exclusions> </dependency>
而之前的exclusion是在spring-boot-web-starter中排除的，就是这各位置的不同造成的。
换个角度来分析，可以得出在spring-boot-web-starter中exclude掉的logging并未完全清楚干净，还有其他的类库或者包存在类似的以来。

4. 问题的解决

将spring-boot-starter-logging的包在spring-boot-starter中直接排除掉，从spring boot整体上进行替换，就可以解决这个问题：

    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId><exclusions><exclusion><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-logging</artifactId></exclusion></exclusions></dependency>

5 反思分析过程

在这个分析过程中，在看到日志的错误之时，第一反应就是配置文件的配置错误，这个方向从一开始就是错误的。
在Pom.xml中虽然进行了exclude操作，但是并未真正的彻底排除掉logging；在看到样例程序之后，才感知到时logging的遗漏引用存在导致了问题。
我又重新查看一遍项目中报出的异常日志，发现了一个我疏忽的细节问题：

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/DevSpace/M2Space/ch/qos/logback/logback-classic/1.1.11/logback-classic-1.1.11.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/DevSpace/M2Space/org/apache/logging/log4j/log4j-slf4j-impl/2.7/log4j-slf4j-impl-2.7.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder]

在这个提示信息里面，明确了告知logging有多个绑定，logback、log4j2两种实现；目前使用的是logback的绑定，这个提示信息竟然忽略了…….

其实这个提示信息已经明确告知了问题出现的根源，只是当时被无意或者有意的忽略了…..

6 总结

经过这个问题的分析过程，我们可以得知，异常日志或者错误日志的分析是第一现场，非常的重要，它给了我们大量而丰富的第一手信息，为我们分析和解决问题提示了非常多的方向，需要进行慎重和小心的求证与验证。
之后是exclude logging的依赖，虽然进行exclude排除操作，却并未完全进行替换掉，这个是在包的依赖中遗漏掉的。

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