大家好，又见面了，我是你们的朋友全栈君。

问题描述

修改springboot版本后，启动springboot项目项目报错:
java.lang.NoClassDefFoundError:org/springframework/boot/Bootstrapper

java.lang.NoClassDefFoundError: org/springframework/boot/Bootstrapper
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at org.springframework.cloud.bootstrap.encrypt.DecryptEnvironmentPostProcessor.getTextEncryptor(DecryptEnvironmentPostProcessor.java:83)
	at org.springframework.cloud.bootstrap.encrypt.DecryptEnvironmentPostProcessor.postProcessEnvironment(DecryptEnvironmentPostProcessor.java:69)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.onApplicationEnvironmentPreparedEvent(ConfigFileApplicationListener.java:177)
	at org.springframework.boot.context.config.ConfigFileApplicationListener.onApplicationEvent(ConfigFileApplicationListener.java:165)
	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:127)
	at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:76)
	at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:53)
	at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:342)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:305)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1204)
	at com.atguigu.gulimail.product.GuliMailProductApplication.main(GuliMailProductApplication.java:11)
Caused by: java.lang.ClassNotFoundException: org.springframework.boot.Bootstrapper
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 27 common frames omitted

原因

版本冲突

本人是使用idea的spring Initializr初始化器去创建的项目，随后将springboot的版本改为了较旧的 springboot 2.3.2 版本，启动后报错，把版本更改回原版本问题解决。
原来是因为使用spring Initializr创建的时候，一旦你勾中了springCloud有关技术，他会同时引入springCloud依赖版本管理：

    <properties>
        <java.version>1.8</java.version>
        <spring-cloud.version>2020.0.0</spring-cloud.version>
    </properties>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${ 
   spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

而spring boot整合springcloud及spirngcloud alibaba对版本的匹配要求是很高的，具体见下图springcloud alibaba官网中的版本说明截取
springcloud alibaba版本说明

解决办法

统一控制spring boot，spring cloud，spring cloud alibaba的版本，具体版本匹配如上图
本人采用的

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.2.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

<dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>com.alibaba.cloud</groupId>
                <artifactId>spring-cloud-alibaba-dependencies</artifactId>
                <version>2.2.3.RELEASE</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>Hoxton.SR8</version>
            </dependency>
        </dependencies>
    </dependencyManagement>

结论

使用spring Initializr初始化器去创建的项目时（用到cloud或者cloud alibaba），若想变动为其他版本时，必须同时控制三者版本，否则很容易出现上述问题。

推荐

推荐在父工程中统一管理三者版本，子模块通过快速初始化后删除其中pom中的三者版本的统一管理,如下图，删除掉

    <properties>
        <java.version>1.8</java.version>
        <spring-cloud.version>2020.0.0</spring-cloud.version>
    </properties>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${ 
   spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>