异常解决：java.lang.NoClassDefFoundError: org/springframework/boot/Bootstrapper

异常解决：java.lang.NoClassDefFoundError: org/springframework/boot/Bootstrapper
跟着以前的教学视频学习的过程中出现了这个问题，找了好久终于找出了原因，特此记录。

问题描述

问题发生在springboot去添加spring-cloud的maven依赖包后，启动springboot程序，idea突然报了以下错误：

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

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 2.1.8 版本，启动后报错

反复寻找之后原来是因为使用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>1.8java.version>
        <spring-cloud.version>2020.0.0spring-cloud.version>
    properties>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloudgroupId>
                <artifactId>spring-cloud-dependenciesartifactId>
                <version>${spring-cloud.version}version>
                <type>pomtype>
                <scope>importscope>
            dependency>
        dependencies>
    dependencyManagement>

解决

自定义springCloud依赖版本即可

总结

当你使用idea初始化器去创建spring项目，然后你又需要改变当前版本的时候，最好注意maven依赖之间的版本冲突