Weird behaviour when calling `ApplicationContext.getBean()` with a null class reference

[stsypanov]

I've run into wierd behaviour when calling ApplicationContext::getBean for nullable class:

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DemoApplication.class, args);

        Class o = null;
        Object bean = context.getBean(o);//todo should be NPE
    }
}

Execution of this method throws exception with list of all beans in the context

Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'java.lang.Object' available: more than one 'primary' bean found among candidates: [org.springframework.context.annotation.internalConfigurationAnnotationProcessor, org.springframework.context.annotation.internalAutowiredAnnotationProcessor, org.springframework.context.annotation.internalCommonAnnotationProcessor, org.springframework.context.event.internalEventListenerProcessor, org.springframework.context.event.internalEventListenerFactory, demoApplication, org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory, org.springframework.boot.autoconfigure.AutoConfigurationPackages, org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration, org.springframework.boot.autoconfigure.condition.BeanTypeRegistry, propertySourcesPlaceholderConfigurer, org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration$TomcatWebSocketConfiguration, websocketServletWebServerCustomizer, org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryConfiguration$EmbeddedTomcat, tomcatServletWebServerFactory, org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration, servletWebServerFactoryCustomizer, tomcatServletWebServerFactoryCustomizer, server-org.springframework.boot.autoconfigure.web.ServerProperties, org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor, org.springframework.boot.context.properties.ConfigurationBeanFactoryMetadata, webServerFactoryCustomizerBeanPostProcessor, errorPageRegistrarBeanPostProcessor, org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletConfiguration, dispatcherServlet, spring.http-org.springframework.boot.autoconfigure.http.HttpProperties, spring.mvc-org.springframework.boot.autoconfigure.web.servlet.WebMvcProperties, org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration$DispatcherServletRegistrationConfiguration, dispatcherServletRegistration, org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration, org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration, taskExecutorBuilder, applicationTaskExecutor, spring.task.execution-org.springframework.boot.autoconfigure.task.TaskExecutionProperties, org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration, defaultValidator, methodValidationPostProcessor, org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$WhitelabelErrorViewConfiguration, error, beanNameViewResolver, org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration$DefaultErrorViewResolverConfiguration, conventionErrorViewResolver, org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration, errorAttributes, basicErrorController, errorPageCustomizer, preserveErrorControllerTargetClassPostProcessor, spring.resources-org.springframework.boot.autoconfigure.web.ResourceProperties, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter$FaviconConfiguration, faviconHandlerMapping, faviconRequestHandler, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$EnableWebMvcConfiguration, requestMappingHandlerAdapter, requestMappingHandlerMapping, mvcConversionService, mvcValidator, mvcContentNegotiationManager, mvcPathMatcher, mvcUrlPathHelper, viewControllerHandlerMapping, beanNameHandlerMapping, resourceHandlerMapping, mvcResourceUrlProvider, defaultServletHandlerMapping, mvcUriComponentsContributor, httpRequestHandlerAdapter, simpleControllerHandlerAdapter, handlerExceptionResolver, mvcViewResolver, mvcHandlerMappingIntrospector, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter, defaultViewResolver, viewResolver, welcomePageHandlerMapping, requestContextFilter, org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration, hiddenHttpMethodFilter, formContentFilter, org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration, mbeanExporter, objectNamingStrategy, mbeanServer, org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration, springApplicationAdminRegistrar, org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$Jackson2ObjectMapperBuilderCustomizerConfiguration, standardJacksonObjectMapperBuilderCustomizer, spring.jackson-org.springframework.boot.autoconfigure.jackson.JacksonProperties, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperBuilderConfiguration, jacksonObjectMapperBuilder, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$ParameterNamesModuleConfiguration, parameterNamesModule, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration$JacksonObjectMapperConfiguration, jacksonObjectMapper, org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration, jsonComponentModule, org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration$StringHttpMessageConverterConfiguration, stringHttpMessageConverter, org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration$MappingJackson2HttpMessageConverterConfiguration, mappingJackson2HttpMessageConverter, org.springframework.boot.autoconfigure.http.JacksonHttpMessageConvertersConfiguration, org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration, messageConverters, org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration$LoggingCodecConfiguration, loggingCodecCustomizer, org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration$JacksonCodecConfiguration, jacksonCodecCustomizer, org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration, org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration, spring.info-org.springframework.boot.autoconfigure.info.ProjectInfoProperties, org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration, taskSchedulerBuilder, spring.task.scheduling-org.springframework.boot.autoconfigure.task.TaskSchedulingProperties, org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration, restTemplateBuilder, org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration$TomcatWebServerFactoryCustomizerConfiguration, tomcatWebServerFactoryCustomizer, org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration, org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration, characterEncodingFilter, localeCharsetMappingsCustomizer, org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration, multipartConfigElement, multipartResolver, spring.servlet.multipart-org.springframework.boot.autoconfigure.web.servlet.MultipartProperties, autoConfigurationReport, org.springframework.boot.context.ContextIdApplicationContextInitializer$ContextId, springApplicationArguments, springBootBanner, springBootLoggingSystem, environment, systemProperties, systemEnvironment, org.springframework.context.annotation.ConfigurationClassPostProcessor.importRegistry, messageSource, applicationEventMulticaster, servletContext, contextParameters, contextAttributes, lifecycleProcessor]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.determinePrimaryCandidate(DefaultListableBeanFactory.java:1529)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1128)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveBean(DefaultListableBeanFactory.java:407)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:341)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:335)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1123)
	at com.example.demo.DemoApplication.main(DemoApplication.java:14)

Shouldn't we throw NPE immediately when one instantiates ResolvableType with nullable field type instead of checking all beans in the context?

[sbrannen]

Well, the Javadoc for org.springframework.core.ResolvableType.forRawClass(Class<?>) states the following.

clazz the class to introspect (null is semantically equivalent to Object.class for typical use cases here)

Though I do agree that attempting to look up beans of type java.lang.Object for this particular use case is probably not what the user intended.

I propose that we add a specific not-null assertion for the Class<T> requiredType argument. After all, it's not annotated with @Nullable. So null should not be supported.

Another option would be to add the following instead of a not-null assertion.

if (requiredType == null) {
	throw new NoSuchBeanDefinitionException(requiredType);
}

@jhoeller, what do you think about the above proposals?

[stsypanov]

I strongly like

if (requiredType == null) {
  throw new NoSuchBeanDefinitionException(requiredType);
}

it's easier to understand than plain NPE.

[sbrannen]

In 4.3.x, the corresponding code path throws an IllegalArgumentException for a null requiredType:

https://github.com/spring-projects/spring-framework/blob/4.3.x/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java#L994

Consequently, I now propose that we simply reinstate the original behavior here (treating this effectively as a minor regression) by adding an explicit Assert.notNull(...) in 5.0.x, 5.1.x, and master (5.2.x). We should also update the Javadoc accordingly.