Use of @ConditionalOnMissingClass and @ConditionalOnMissingBean on @Configuration classes may cause java.lang.ArrayStoreException: sun.reflect.annotation.TypeNotPresentExceptionProxy

[wilkinsona]

@Configuration classes are turned into beans and stored in the application context. Spring will sometimes query the class of every bean to see if it has been annotated with a particular annotation, for example to find @Controllers or @Aspects. This is typically done with AnnotationUtils.findAnnotation(…) which will fail with a very cryptic java.lang.ArrayStoreException: sun.reflect.annotation.TypeNotPresentExceptionProxy if any of the annotations it encounters refer to classes that cannot be loaded. This will always be the case with @ConditionalOnMissingClass: the only way for the bean that represents the configuration to be in the application context is if the class that it refers to is missing. It may be the case for @ConditionalOnMissingBean.

To avoid problems with @ConditionalOnMissingBean it should either be used on @Bean methods, or be protected with an @ConditionalOnClass annotation. Flyway's auto-configuration is a good example of the latter:

@Configuration
@ConditionalOnClass(Flyway.class)
@ConditionalOnBean(DataSource.class)
@ConditionalOnExpression("${flyway.enabled:true}")
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
public class FlywayAutoConfiguration {

    @Configuration
    @ConditionalOnMissingBean(Flyway.class)
    @EnableConfigurationProperties(FlywayProperties.class)
    @Import(FlywayJpaDependencyConfiguration.class)
    public static class FlywayConfiguration {

Note the @ConditionalOnClass(Flyway.class) on FlywayAutoConfiguration that makes it safe to use @ConditionalOnMissingBean(Flyway.class) on the nested FlywayConfiguration

To avoid potential problems with @ConditionalOnMissingClass I think it has to either be on an @Bean method or, if it's used on a class, then the class must be referred to by name, i.e. as a String, rather than as a Class. At the time of writing, SocialWebAutoConfiguration.AnonymousUserIdSourceConfig is the only class-level user of @ConditionalOnMissingClass where the class is referenced directly, rather than by name.

[philwebb]

It should be possible to use @ConditionalOnMissingClass and refer to a class by type. The annotation should be read using ASM so it doesn't matter if the class is not actually on the classpath. I'm sure that this used to work.

[wilkinsona]

It's Spring's reading of the annotation that causes the problem. It uses AnnotationUtils.findAnnotation in at least two places where it looks at every bean in the bean factory:

• AbstractAspectJAdvistorFactory
• RequestMappingHandlerMapping

[dsyer]

We deprecated the value() attribute and fixed the Social autoconfig in 1.1.0. So I don't think there's anything left to do here.

[snicoll]

While working on #1013 I had the same issue with ConditionalOnClass

You can reproduce by using the boot fork available in this branch alongside this stupid app

You can switch the @ConditionalOnClass to use the name attribute instead of the value attribute in DataSourceInfoProvidersConfiguration and the app will work as expected.

Could it be related to the way this configuration class is added? that is

@ConditionalOnBean(DataSource.class)
@AutoConfigureAfter(DataSourceAutoConfiguration.class)
@Import(DataSourceInfoProvidersConfiguration.class)
public class MetricDataSourceAutoConfiguration { ... }

[wilkinsona]

This is worse than I thought. Methods on a configuration class annotated with either @ConditionalOnClass or @ConditionalOnMissingClass are also a problem. BeanAnnotationHelper.isBeanAnnotated calls AnnotationUtils.findAnnotation(method, Bean.class). If the method has an annotation that references an unloadable class it blows up.

To be safe, if you use @ConditionalOnClass on a configuration class method you either have to use a String rather than a Class or the annotation should be moved up onto the class at which point using a Class is safe.

[philwebb]

@wilkinsona Has a Spring Framework bug been raised for this yet?

[dsyer]

I don't think it is a bug in Spring, or at least not an urgent one. It's the same as @ConditionalOnMissingClass only the rule for usage is different (and we shouldn't deprecate this one): you can use @ConditionalOnClass freely on types, but not on methods. The reason not to change Spring urgently is that if Spring is defensive, someone else will load the class and inspect all the annotations anyway (JBoss or Tomcat or Hibernate or whoever).

[wilkinsona]

I'm inclined to agree with Dave. I'll open an issue against Spring so that it can be discussed. I'm also going to open an issue against the JDK to see if they can make the exception that's thrown more helpful.

It's worth noting that we also need to be careful with @ConditionalOnBean