引言： 一個問題的分析與解決過程是表與裡的過程，是一個大膽猜測與小心求證的過程，spring boot與log4j2的整合過程中，我將描述一下分析這個問題的思路和過程。 我一直強調一點： 重要的不是解決問題的結論，而是解決問題的思路和方法，即使在解決完問題之後，依然需要回過頭覆盤，在問題分析過程中的走過的彎路。

1 專案情況介紹

Spring Boot 1.5.7 , JDK 1.8, Log4j2.7
在這個專案中，希望整合Log4j2作為預設的日誌系統輸出器。
Log4j2.xml的定義如下：

"><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}.%[email protected]%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}.%[email protected]%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的檔案定義如下：

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

上述為核心的配置檔案資訊。
在pom.xml中，針對spring-boot-web中對於logging的引用進行了排除，配置資訊如下：

<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 問題描述

基於Spring Boot的嚮導建立基礎專案，整合Log4j2，在專案啟動過程中，報出如下的錯誤資訊：

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 [email protected]:17 - no applicable action for [Properties], current ElementPath is [[Configuration][Properties]]
ERROR in [email protected]:35 - no applicable action for [Property], current ElementPath is [[Configuration][Properties][Property]]
ERROR in [email protected]:16 - no applicable action for [Appenders], current ElementPath is [[Configuration][Appenders]]
ERROR in [email protected]:53 - no applicable action for [Console], current ElementPath is [[Configuration][Appenders][Console]]
ERROR in [email protected]:92 - no applicable action for [PatternLayout], current ElementPath is [[Configuration][Appenders][Console][PatternLayout]]
ERROR in [email protected]:91 - no applicable action for [RollingRandomAccessFile], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile]]
ERROR in [email protected]:106 - no applicable action for [PatternLayout], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][PatternLayout]]
ERROR in [email protected]:23 - no applicable action for [Policies], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][Policies]]
ERROR in [email protected]:45 - no applicable action for [TimeBasedTriggeringPolicy], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][Policies][TimeBasedTriggeringPolicy]]
ERROR in [email protected]:48 - no applicable action for [DefaultRolloverStrategy], current ElementPath is [[Configuration][Appenders][RollingRandomAccessFile][DefaultRolloverStrategy]]
ERROR in [email protected]:14 - no applicable action for [Loggers], current ElementPath is [[Configuration][Loggers]]
ERROR in [email protected]:59 - no applicable action for [Logger], current ElementPath is [[Configuration][Loggers][Logger]]
ERROR in [email protected]:69 - no applicable action for [Logger], current ElementPath is [[Configuration][Loggers][Logger]]
ERROR in [email protected]:42 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Logger][AppenderRef]]
ERROR in [email protected]:46 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Logger][AppenderRef]]
ERROR in [email protected]:29 - no applicable action for [Root], current ElementPath is [[Configuration][Loggers][Root]]
ERROR in [email protected]:41 - no applicable action for [AppenderRef], current ElementPath is [[Configuration][Loggers][Root][AppenderRef]]
ERROR in [email protected]: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)

以上是在系統啟動過程中的錯誤資訊，關鍵錯誤資訊是：

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

3 問題分析過程

3.1 配置檔案的格式或者定義錯誤

根據剛才看到的錯誤資訊，第一反應是log4j2.xml中定義的錯誤資訊或者格式資訊有誤。
根據這個判斷，就直接在Log4j2的官方文件上，重新查閱了一番，結合定義中的內容格式，感覺格式都是完全正確的，沒有什麼出入問題。
於是，我就使用排除法，將Log4j2.xml中的內容按照步驟，一部分一部分的進行驗證排除，如果哪個部分中存在配置檔案的配置錯誤，則可以定位出來。
在一番定位之後，發現只要配置檔案中存在內容，就會報出類似錯誤資訊。
結論是，應該不是配置檔案的配置錯誤資訊。

3.2 下載樣例專案進行分析

於是，從網上直接下來了spring-boot中自帶的spring-boot-sample-actuator-log4j2專案，進行分析：
發現其中pom.xml中的不同之處為：

<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，就是這各位置的不同造成的。
換個角度來分析，可以得出在spring-boot-web-starter中exclude掉的logging並未完全清楚乾淨，還有其他的類庫或者包存在類似的以來。

4. 問題的解決

將spring-boot-starter-logging的包在spring-boot-starter中直接排除掉，從spring boot整體上進行替換，就可以解決這個問題：

    <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 反思分析過程

在這個分析過程中，在看到日誌的錯誤之時，第一反應就是配置檔案的配置錯誤，這個方向從一開始就是錯誤的。
在Pom.xml中雖然進行了exclude操作，但是並未真正的徹底排除掉logging；在看到樣例程式之後，才感知到時logging的遺漏引用存在導致了問題。
我又重新檢視一遍專案中報出的異常日誌，發現了一個我疏忽的細節問題：

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]

在這個提示資訊裡面，明確了告知logging有多個繫結，logback、log4j2兩種實現；目前使用的是logback的繫結，這個提示資訊竟然忽略了…….

其實這個提示資訊已經明確告知了問題出現的根源，只是當時被無意或者有意的忽略了…..

6 總結

經過這個問題的分析過程，我們可以得知，異常日誌或者錯誤日誌的分析是第一現場，非常的重要，它給了我們大量而豐富的第一手資訊，為我們分析和解決問題提示了非常多的方向，需要進行慎重和小心的求證與驗證。
之後是exclude logging的依賴，雖然進行exclude排除操作，卻並未完全進行替換掉，這個是在包的依賴中遺漏掉的。