这两天搭建了一个spring cloud项目，简单写了个hellocontroller，结果项目启动失败了。
以下是控制台打印的异常：
java.lang.IllegalStateException: Error processing condition on org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration.propertySourcesPlaceholderConfigurer
at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:60) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForBeanMethod(ConfigurationClassBeanDefinitionReader.java:184) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForConfigurationClass(ConfigurationClassBeanDefinitionReader.java:144) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitions(ConfigurationClassBeanDefinitionReader.java:120) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:331) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:236) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:280) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:96) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:707) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:533) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:143) ~[spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:758) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:750) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:405) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at ***.bxbro.sun.platform.PlatFormApplication.main(PlatFormApplication.java:11) [classes/:na]
Caused by: java.lang.IllegalStateException: Failed to introspect Class [org.springframework.cloud.***flix.hystrix.HystrixCircuitBreakerConfiguration] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:481) ~[spring-core-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:358) ~[spring-core-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.util.ReflectionUtils.getUniqueDeclaredMethods(ReflectionUtils.java:414) ~[spring-core-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.lambda$getTypeForFactoryMethod$2(AbstractAutowireCapableBeanFactory.java:742) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at java.util.concurrent.ConcurrentHashMap.***puteIfAbsent(ConcurrentHashMap.java:1660) ~[na:1.8.0_321]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.getTypeForFactoryMethod(AbstractAutowireCapableBeanFactory.java:741) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.determi***argetType(AbstractAutowireCapableBeanFactory.java:680) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.predictBeanType(AbstractAutowireCapableBeanFactory.java:648) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.isFactoryBean(AbstractBeanFactory.java:1614) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doGetBeanNamesForType(DefaultListableBeanFactory.java:523) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanNamesForType(DefaultListableBeanFactory.java:495) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
at org.springframework.boot.autoconfigure.condition.OnBeanCondition.collectBeanNamesForType(OnBeanCondition.java:238) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.autoconfigure.condition.OnBeanCondition.getBeanNamesForType(OnBeanCondition.java:231) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.autoconfigure.condition.OnBeanCondition.getBeanNamesForType(OnBeanCondition.java:221) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.autoconfigure.condition.OnBeanCondition.getMatchingBeans(OnBeanCondition.java:169) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.autoconfigure.condition.OnBeanCondition.getMatchOut***e(OnBeanCondition.java:144) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47) ~[spring-boot-autoconfigure-2.3.5.RELEASE.jar:2.3.5.RELEASE]
... 18 ***mon frames omitted
Caused by: java.lang.NoClassDefFoundError: ***/***flix/hystrix/contrib/javanica/aop/aspectj/Hystrix***mandAspect
at java.lang.Class.getDeclaredMethods0(Native Method) ~[na:1.8.0_321]
at java.lang.Class.privateGetDeclaredMethods(Class.java:2701) ~[na:1.8.0_321]
at java.lang.Class.getDeclaredMethods(Class.java:1975) ~[na:1.8.0_321]
at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:463) ~[spring-core-5.2.10.RELEASE.jar:5.2.10.RELEASE]
... 34 ***mon frames omitted
Caused by: java.lang.ClassNotFoundException: ***.***flix.hystrix.contrib.javanica.aop.aspectj.Hystrix***mandAspect
at java.***.URLClassLoader.findClass(URLClassLoader.java:387) ~[na:1.8.0_321]
at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[na:1.8.0_321]
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:355) ~[na:1.8.0_321]
at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[na:1.8.0_321]
... 38 ***mon frames omitted
以上的一大堆异常中，一开始我关注的是第一个异常。也就是，
java.lang.IllegalStateException: Error processing condition on org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration.propertySourcesPlaceholderConfigurer
从字面意思理解，看到placeholder，容易让人误以为是从配置文件读取什么配置项失败的意思，但是我的配置文件只配置了端口号，没有其他任何配置项。所以就很奇怪。然后就直接求助搜索引擎了，但是搞了好久，尝试了好多种方式一直未果。因为我发现这个异常的原因可能有很多种情况。
所以也就是说，这个异常的参考意义不大。
真正应该重视的是以Caused by 开头的那一串。字面意思：因为什么造成的。那这么多 Caused by 中，第一个Caused by 是最值得关注的（最好也结合之后的几个 Caused by）。也就是下面这个：
Caused by: java.lang.IllegalStateException: Failed to introspect Class [org.springframework.cloud.***flix.hystrix.HystrixCircuitBreakerConfiguration] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
从此就豁然开朗了。
从以上这几个Cause by大概可以猜测，初始化失败的原因，要么是hystrix的版本不对（即与spring cloud的版本不兼容），要么是没有添加hystrix这个依赖。
而我是后者。
添加hystrix的依赖之后，项目就启动成功了。
org.springframework.cloud spring-cloud-starter-***flix-hystrix
最后附上项目中各个组件的版本：
【spring cloud：Hoxton.SR8 | springboot: 2.3.5】
