错误描述
<html >
<div style="color:red">
RROR StatusLogger Log4j2 could not find a logging implementation. Please add log4j-core to the classpath. Using SimpleLogger to log to the console...
</div>
</html>
错误分析过程
elasticsearch依赖定义
<properties>
<elasticsearch.version>5.5.1</elasticsearch.version>
<properties>
<!--elasticsearch-->
<dependency>
<groupId>org.elasticsearch</groupId>
<artifactId>elasticsearch</artifactId>
</dependency>
<dependency>
<groupId>org.elasticsearch.client</groupId>
<artifactId>transport</artifactId>
</dependency>
log4j2-error.png
从这些信息可以看出，组件org.elasticsearch:elasticsearch:5.5.1用到了log4j
- org.apache.logging.log4j:log4j-api:2.8.2
- org.apache.logging.log4j:log4j-core:2.8.2:optional
- org.apache.logging.log4j:log4j-1.2-api:2.8.2:optional
log4j-core是可选依赖，通过mvn dependency:tree分析，发现确实没有这个依赖，所以先尝试把log4j-core加上去。加上去后，再次运行，就不会再输出这个error了。
org.apache.logging.log4j:log4j-1.2-api这个构件是Apache Logging提供的log4j到log4j2的桥接库。所以从这里可以看出elasticsearch构件用的应该是log4j，同时它又引用的可选的桥接库，将log4j桥接到log4j2。根据mvn dependency:tree的输出可以得知spring-boot-starter-logging已经引入了以下桥接库org.slf4j:jul-to-slf4j和org.slf4j:log4j-over-slf4j。
[INFO] +- org.springframework.boot:spring-boot-starter-web:jar:1.5.10.RELEASE:compile
[INFO] | +- org.springframework.boot:spring-boot-starter:jar:1.5.10.RELEASE:compile
[INFO] | | - org.springframework.boot:spring-boot-starter-logging:jar:1.5.10.RELEASE:compile
[INFO] | | +- org.slf4j:jul-to-slf4j:jar:1.7.25:compile
[INFO] | | - org.slf4j:log4j-over-slf4j:jar:1.7.25:compile
同时发现项目所有依赖中并没有发现有log4j2的身影。所以不需要相应的桥接库，即
org.apache.logging.log4j:log4j-to-slf4j
<!-- https://mvnrepository.com/artifact/org.apache.logging.log4j/log4j-to-slf4j -->
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-to-slf4j</artifactId>
<version>2.11.1</version>
</dependency>
同时还发现项目所有依赖中也没有出现commons-logging的身影，所以不需要jcl相关桥接库，即
JCL 1.2 implemented over SLF4J
<!-- https://mvnrepository.com/artifact/org.slf4j/jcl-over-slf4j -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jcl-over-slf4j</artifactId>
<version>1.7.25</version>
</dependency>
spring-boot-starter-logging已排除对commons-logging的依赖：
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<exclusions>
<exclusion>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
spring-boot-starter-logging的pom完整定义：
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starters</artifactId>
<version>1.5.16.RELEASE</version>
</parent>
<artifactId>spring-boot-starter</artifactId>
<name>Spring Boot Starter</name>
<description>Core starter, including auto-configuration support, logging and YAML</description>
<url>http://projects.spring.io/spring-boot/</url>
<organization>
<name>Pivotal Software, Inc.</name>
<url>http://www.spring.io</url>
</organization>
<properties>
<main.basedir>${basedir}/../..</main.basedir>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-logging</artifactId>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<exclusions>
<exclusion>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.yaml</groupId>
<artifactId>snakeyaml</artifactId>
<scope>runtime</scope>
</dependency>
</dependencies>
</project>
最终解决方案
在pom中增加依赖
<!-- https://mvnrepository.com/artifact/org.apache.logging.log4j/log4j-core -->
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.11.1</version>
</dependency>
总结
通过这个解决问题的过程学习了java loging框架相关的知识及解决maven依赖相关的分析思路。涉及的知识点：
- log4j2，logback相关日志框架的选择。从springboot框架的倾向来看，比较钟情于logback, 会默认排除commons-logging。
- maven可选依赖的作用，主要是打破依赖的传递性
- maven如何排除依赖
- maven中scrope的作用
- 在项目中应该定义parent模块，并在其中使用dependencyManagement统一声明各构件的版本及排除相关依赖。
- maven模块构建的问题：在写demo验证问题的过程中遇到了一个构建问题，原因是没有从根开始构建。
常用的几个命令
mvn clean package -DskipTests
mvn clean package -DskipTests -pl module-0,moduel-1 -am
mvn clean deploy -DskipTests
mvn clean deploy -DskipTests -pl module-0,moduel-1 -am
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