空的springboot项目导入nacos 配置中心和注册中心依赖后报错，提示 org.apache.http.impl.client.HttpClientBuilder 这个类找不到 问题的解决

0. 开发环境

• SpringBoot 2.3.3.RELEASE
• SpringCloud Hoxton.SR10
• SpringCloudAlibaba 2.2.3.RELEASE
• SpringcloudAlibabaNacos 2.2.2.RELEASE

1. 报错内容

报错过长，只留下了重要的一些报错语句

java.lang.IllegalStateException: Error processing condition on org.springframework.cloud.commons.httpclient.HttpClientConfiguration$ApacheHttpClientConfiguration.apacheHttpClientBuilder
	at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:60) ~[spring-boot-autoconfigure-2.3.3.RELEASE.jar:2.3.3.RELEASE]
	......
	
Caused by: java.lang.IllegalStateException: @ConditionalOnMissingBean did not specify a bean using type, name or annotation and the attempt to deduce the bean's type failed
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$Spec.validate(OnBeanCondition.java:487) ~[spring-boot-autoconfigure-2.3.3.RELEASE.jar:2.3.3.RELEASE]
	......
	... 18 common frames omitted
Caused by: org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanTypeDeductionException: Failed to deduce bean type for org.springframework.cloud.commons.httpclient.HttpClientConfiguration$ApacheHttpClientConfiguration.apacheHttpClientBuilder
	......
	... 20 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.apache.http.impl.client.HttpClientBuilder
	......
	... 22 common frames omitted

2021-03-15 07:34:52.548  WARN 11915 --- [           main] o.s.boot.SpringApplication               : Unable to close ApplicationContext

java.lang.IllegalStateException: Failed to introspect Class [org.springframework.cloud.commons.httpclient.HttpClientConfiguration$ApacheHttpClientConfiguration] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:481) ~[spring-core-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	......
Caused by: java.lang.NoClassDefFoundError: org/apache/http/impl/client/HttpClientBuilder
	at java.lang.Class.getDeclaredMethods0(Native Method) ~[na:1.8.0_241]
	......
	... 21 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.apache.http.impl.client.HttpClientBuilder
	at java.net.URLClassLoader.findClass(URLClassLoader.java:382) ~[na:1.8.0_241]
	......
	... 25 common frames omitted


Process finished with exit code 1

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经分析发现似乎是httpclient 的报错, 但是心想，我并没有引入这个依赖啊，那么在我没有导入nacos依赖之前，springboot是可以跑起来的，说明可能是nacos 的注册中心和配置中心依赖了httpclient.

2. 报错分析

由于报错项目是springcloud 项目，所以结构比较复杂，于是又新建了一个干净的springboot 项目来引入nacos 的依赖，结构如下：

pom.xml 核心内容如下

<parent>
    <artifactId>spring-boot-dependencies</artifactId>
    <groupId>org.springframework.boot</groupId>
    <version>2.3.3.RELEASE</version>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <version>2.3.3.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <version>2.3.3.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>com.alibaba.cloud</groupId>
        <artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
        <version>2.2.3.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>com.alibaba.cloud</groupId>
        <artifactId>spring-cloud-starter-alibaba-nacos-config</artifactId>
        <version>2.2.3.RELEASE</version>
    </dependency>
</dependencies>
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发现居然没有报错！！！！ 很神奇，于是找到依赖

发现 在 nacos 的 注册中心有引入 ribbon 的 一个 httpclient 依赖，而依赖最后依赖的就是 4.5.12 版本的 httpclient


发现这个依赖下确实有这个类，类比的，来分析报错的那个springcloud 项目

这是springcloud 中的依赖，我们发现在这里有两个不同版本的httpclient， 盲猜是低版本的依赖中没有那个类所以与springboot 项目不兼容。打开目录结构果然没有发现 httpClientBuilder 这个类。 至此就破案了，原来是maven从父工程引入的时候，引入了在前边的 4.2.1 版本的依赖而出现不兼容情况(由于jar包多，所以不确定到底是哪个jar引入了这个低版本的)

3. 解决方案

我们在 父工程中显示声明 httpclient 的版本即可

<properties>
   <httpclient.version>4.5</httpclient.version>
</properties>
<dependencyManagement>
   <dependencies>
       <dependency>
           <groupId>org.apache.httpcomponents</groupId>
           <artifactId>httpclient</artifactId>
           <version>${httpclient.version}</version>
       </dependency>
   </dependencies>
</dependencyManagement>
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