我有一个添加自动配置的Spring Boot库。例如，如果Spring Security在类路径上，我已经成功地使用@ConditionalOnClass来仅激活库的部分。
目前，我的库需要spring-boot-starter-validation作为依赖项，我希望将其作为可选项。我添加了@ConditionalOnClass，如下所示：
@Bean
@ConditionalOnClass(name = "javax.validation.ConstraintViolationException")
public ConstraintViolationApiExceptionHandler constraintViolationApiExceptionHandler(ErrorHandlingProperties properties,
HttpStatusMapper httpStatusMapper,
ErrorCodeMapper errorCodeMapper,
ErrorMessageMapper errorMessageMapper) {
return new ConstraintViolationApiExceptionHandler(properties, httpStatusMapper, errorCodeMapper, errorMessageMapper);
}
但当在应用程序中运行时，我会收到以下错误：
2022-03-26 11:00:14.028 ERROR 3171 --- [ main] o.s.boot.SpringApplication : Application run failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'httpHandler' defined in class path resource [org/springframework/boot/autoconfigure/web/reactive/HttpHandlerAutoConfiguration$AnnotationConfig.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.http.server.reactive.HttpHandler]: Factory method 'httpHandler' threw exception; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'globalErrorWebExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Unsatisfied dependency expressed through method 'globalErrorWebExceptionHandler' parameter 7; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constraintViolationApiExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [io.github.wimdeblauwe.errorhandlingspringbootstarter.handler.ConstraintViolationApiExceptionHandler] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7]
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:658) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:638) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1352) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1195) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:953) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918) ~[spring-context-5.3.16.jar:5.3.16]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583) ~[spring-context-5.3.16.jar:5.3.16]
at org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.refresh(ReactiveWebServerApplicationContext.java:64) ~[spring-boot-2.6.4.jar:2.6.4]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:740) ~[spring-boot-2.6.4.jar:2.6.4]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:415) ~[spring-boot-2.6.4.jar:2.6.4]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:303) ~[spring-boot-2.6.4.jar:2.6.4]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1312) ~[spring-boot-2.6.4.jar:2.6.4]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301) ~[spring-boot-2.6.4.jar:2.6.4]
at com.wimdeblauwe.examples.reactiveerrorhandling.ReactiveErrorHandlingApplication.main(ReactiveErrorHandlingApplication.java:10) ~[classes/:na]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.http.server.reactive.HttpHandler]: Factory method 'httpHandler' threw exception; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'globalErrorWebExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Unsatisfied dependency expressed through method 'globalErrorWebExceptionHandler' parameter 7; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constraintViolationApiExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [io.github.wimdeblauwe.errorhandlingspringbootstarter.handler.ConstraintViolationApiExceptionHandler] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7]
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:653) ~[spring-beans-5.3.16.jar:5.3.16]
... 19 common frames omitted
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'globalErrorWebExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Unsatisfied dependency expressed through method 'globalErrorWebExceptionHandler' parameter 7; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constraintViolationApiExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [io.github.wimdeblauwe.errorhandlingspringbootstarter.handler.ConstraintViolationApiExceptionHandler] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7]
at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:800) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:541) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1352) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1195) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory$1.orderedStream(DefaultListableBeanFactory.java:481) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.web.server.adapter.WebHttpHandlerBuilder.applicationContext(WebHttpHandlerBuilder.java:178) ~[spring-web-5.3.16.jar:5.3.16]
at org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration$AnnotationConfig.httpHandler(HttpHandlerAutoConfiguration.java:65) ~[spring-boot-autoconfigure-2.6.4.jar:2.6.4]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.3.16.jar:5.3.16]
... 20 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constraintViolationApiExceptionHandler' defined in class path resource [io/github/wimdeblauwe/errorhandlingspringbootstarter/ReactiveErrorHandlingConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [io.github.wimdeblauwe.errorhandlingspringbootstarter.handler.ConstraintViolationApiExceptionHandler] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:597) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.addCandidateEntry(DefaultListableBeanFactory.java:1607) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:1571) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveMultipleBeans(DefaultListableBeanFactory.java:1460) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1347) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1309) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791) ~[spring-beans-5.3.16.jar:5.3.16]
... 37 common frames omitted
Caused by: java.lang.IllegalStateException: Failed to introspect Class [io.github.wimdeblauwe.errorhandlingspringbootstarter.handler.ConstraintViolationApiExceptionHandler] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7]
at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:485) ~[spring-core-5.3.16.jar:5.3.16]
at org.springframework.util.ReflectionUtils.doWithLocalMethods(ReflectionUtils.java:321) ~[spring-core-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.buildLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:232) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.findLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:210) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(InitDestroyAnnotationBeanPostProcessor.java:149) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(CommonAnnotationBeanPostProcessor.java:305) ~[spring-context-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyMergedBeanDefinitionPostProcessors(AbstractAutowireCapableBeanFactory.java:1116) ~[spring-beans-5.3.16.jar:5.3.16]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594) ~[spring-beans-5.3.16.jar:5.3.16]
... 50 common frames omitted
Caused by: java.lang.NoClassDefFoundError: javax/validation/ConstraintViolation
at java.base/java.lang.Class.getDeclaredMethods0(Native Method) ~[na:na]
at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3402) ~[na:na]
at java.base/java.lang.Class.getDeclaredMethods(Class.java:2504) ~[na:na]
at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:467) ~[spring-core-5.3.16.jar:5.3.16]
... 57 common frames omitted
Caused by: java.lang.ClassNotFoundException: javax.validation.ConstraintViolation
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) ~[na:na]
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) ~[na:na]
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[na:na]
... 61 common frames omitted
我认为您必须将@ConditionalOnClass放在类级别上。否则，Spring Boot仍然需要加载配置类来分析它，如果不是所有依赖项都在类路径上，这将失败。如果将@ConditionalOnClass放在配置类上，则Spring Boot可以在不实际加载类的情况下读取此信息，如果@ConditionalOnClass中声明的类不在类路径上，则可以跳过类加载。
在这种情况下，您甚至可以引用@ConditionalOnClass中的真实类，而不是字符串形式的类名。只需确保您的依赖项是可选的，这样它就不会作为库的传递依赖项包含在内。
顺便说一句，这是简短的回答。可在此处找到长版本：https://youtu.be/N39hpGAT43s；-(我真的很喜欢这个演讲。从中学到了很多。