错误之处：如下是 pom.xml 文件，目前只有这个文件部分 dependency 报红影响一些外部jar包导入和其类库方法的使用！
pom.xml文件
<?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-starter-parent</artifactId>
<version>2.0.1.RELEASE</version>
</parent>
<groupId>com.xuecheng</groupId>
<artifactId>xc-framework-parent</artifactId>
<packaging>pom</packaging>
<version>1.0-SNAPSHOT</version>
<properties>
<java.version>1.8</java.version>
<tomcat.version>8.5.28</tomcat.version>
<spring-boot.version>2.0.1.RELEASE</spring-boot.version>
<springframework.version>5.0.5.RELEASE</springframework.version>
<mybatis-spring-boot.version>1.3.1</mybatis-spring-boot.version>
<mybatis.version>3.4.5</mybatis.version>
<druid.version>1.1.6</druid.version>
<mysql-connector-java.version>5.1.45</mysql-connector-java.version>
<commons-io.version>2.6</commons-io.version>
<org.apache.commons.io.version>1.3.2</org.apache.commons.io.version>
<commons-fileupload.version>1.3.3</commons-fileupload.version>
<commons-codec.version>1.10</commons-codec.version>
<commons-lang3.version>3.6</commons-lang3.version>
<okhttp.version>3.9.1</okhttp.version>
<feign-okhttp.version>8.18.0</feign-okhttp.version>
<lombok.version>1.16.16</lombok.version>
<springfox-swagger.version>2.7.0</springfox-swagger.version>
<fastjson.version>1.2.30</fastjson.version>
<fastdfs-client-java.version>1.27.0.0</fastdfs-client-java.version>
<mysql-connector-java.version>5.1.40</mysql-connector-java.version>
<elasticsearch.version>6.2.1</elasticsearch.version>
<guava.version>24.0-jre</guava.version>
</properties>
<!--test-->
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>Finchley.SR1</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>${mysql-connector-java.version}</version>
</dependency>
<dependency>
<groupId>org.mybatis.spring.boot</groupId>
<artifactId>mybatis-spring-boot-starter</artifactId>
<version>${mybatis-spring-boot.version}</version>
</dependency>
<dependency>
<groupId>com.github.pagehelper</groupId>
<artifactId>pagehelper-spring-boot-starter</artifactId>
<version>1.2.4</version>
</dependency>
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>druid</artifactId>
<version>${druid.version}</version>
</dependency>
<dependency>
<groupId>com.squareup.okhttp3</groupId>
<artifactId>okhttp</artifactId>
<version>${okhttp.version}</version>
</dependency>
<dependency>
<groupId>com.netflix.feign</groupId>
<artifactId>feign-okhttp</artifactId>
<version>${feign-okhttp.version}</version>
</dependency>
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>${commons-io.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-io</artifactId>
<version>${org.apache.commons.io.version}</version>
</dependency>
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>${commons-fileupload.version}</version>
</dependency>
<dependency>
<groupId>commons-codec</groupId>
<artifactId>commons-codec</artifactId>
<version>${commons-codec.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>${commons-lang3.version}</version>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>${lombok.version}</version>
</dependency>
<dependency>
<groupId>io.springfox</groupId>
<artifactId>springfox-swagger2</artifactId>
<version>${springfox-swagger.version}</version>
</dependency>
<dependency>
<groupId>io.springfox</groupId>
<artifactId>springfox-swagger-ui</artifactId>
<version>${springfox-swagger.version}</version>
</dependency>
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>fastjson</artifactId>
<version>${fastjson.version}</version>
</dependency>
<dependency>
<groupId>net.oschina.zcx7878</groupId>
<artifactId>fastdfs-client-java</artifactId>
<version>${fastdfs-client-java.version}</version>
</dependency>
<dependency>
<groupId>org.elasticsearch.client</groupId>
<artifactId>elasticsearch-rest-high-level-client</artifactId>
<version>${elasticsearch.version}</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.elasticsearch/elasticsearch -->
<dependency>
<groupId>org.elasticsearch</groupId>
<artifactId>elasticsearch</artifactId>
<version>${elasticsearch.version}</version>
</dependency>
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>${guava.version}</version>
</dependency>
</dependencies>
</dependencyManagement>
<build>
<finalName>${project.artifactId}</finalName>
<plugins>
<!-- <!– 资源文件拷贝插件 –>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-resources-plugin</artifactId>
<configuration>
<encoding>UTF-8</encoding>
</configuration>
</plugin>-->
<!-- java编译插件 -->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.8</source>
<target>1.8</target>
<encoding>UTF-8</encoding>
</configuration>
</plugin>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>具体信息：IDEA 15.0.6 + Spring Boot 2.0.1——》mysql、pagehelper、druid等部分依赖包在 External Libraries 没看到 maven 自动下载的 JAR 包
自我判断：疑似 maven 根本没去自动导入缺失的部分 jar 包——》为什么导入不了部分 jar 包？——》
自我解决1：首先怀疑联网中断问题——》右侧蓝色 maven project 进行 clean + install 并且 reimport （没用）
自我解决2：怀着悲壮的心情，将原 External Libraries 的 Maven 下载的 jar 包备份——》然后找到一份已经下载了（具备了）所有需要的 jar 包的 Libraries 文件夹并且【替换】原 Libraries ——》然而 点击maven 的 reimport，发现又变为原来的 Libraries ！
自我解决3：把 lib 里面的 jar 包 + target 文件全部删除——》reimport——》结果同上
震惊！有部分写在dependency里面却得不到maven自动下载的jar包？
神奇！左侧 maven 明明没有自动导入 apache.commons.io ，右侧居然没报错？
