開發Spring Boot項目時出現java.lang.ClassCastException

故障現象

在修改了代碼之後重啓服務，出現java.lang.ClassCastException

2020-05-13 15:59:53.238 ERROR 5964 --- [  restartedMain] c.w.d.d.i.controller.ProjectController   : Failed to initialize handler class: cn.wesure.da.ds.ide.service.dmp.strategy.ProjectAddHandler

java.lang.ClassCastException: cn.wesure.da.ds.ide.service.dmp.strategy.ProjectAddHandler cannot be cast to cn.wesure.da.ds.ide.service.dmp.strategy.AbstractRequestActionHandler
        at cn.wesure.da.ds.ide.controller.ProjectController.initHandler(ProjectController.java:61) ~[main/:na]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_191]
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_191]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_191]
        at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_191]
        at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(InitDestroyAnnotationBeanPostProcessor.java:389) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeInitMethods(InitDestroyAnnotationBeanPostProcessor.java:333) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization(InitDestroyAnnotationBeanPostProcessor.java:157) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:416) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1788) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:595) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) [spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:882) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878) ~[spring-context-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.2.5.RELEASE.jar:5.2.5.RELEASE]
        at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
        at cn.wesure.da.ds.ide.DSWebIdeApplication.main(DSWebIdeApplication.java:35) ~[main/:na]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_191]
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_191]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_191]
        at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_191]
        at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) ~[spring-boot-devtools-2.2.6.RELEASE.jar:2.2.6.RELEASE]

原因分析

1. java.lang.ClassCastException這個異常出現的原因最常見的就是class類型不匹配，但這裏的類明確是將子類強制轉換爲父類，不存在類型不匹配的問題。
   

2. 仔細看堆棧發現，啓動的第一個類竟然是org.springframework.boot.devtools.restart.RestartLauncher。上網搜索spring boot devtools跟ClassCastException，果然找到問題原因：

Spring Boot Devtools Classloader

因爲Spring Boot Devtools 使用不同的Classloader來加載類，在啓動多模塊項目時，有可能會導致類型轉換失敗。

解決辦法

官網提供解決辦法，指定不同的類使用不同的Classloader。
因爲對於devtools並不是很依賴，主要是感覺並沒有提升很高的效率，因此直接關閉掉devtools。