SpringBoot Failure when binding the name of a non-existent class to a Class<?> property isn't very helpful

Via this question on Stack Overflow, the failure that you get when trying to bind the name of a class that isn't on the classpath to a Class<?> isn't very helpful. It makes it looks as if String -> Class conversion isn't possible:

***************************
APPLICATION FAILED TO START
***************************

Description:

Failed to bind properties under 'spring.kafka.producer.value-serializer' to java.lang.Class<?>:

    Property: spring.kafka.producer.value-serializer
    Value: io.confluent.kafka.serializers.KafkaAvroSerializer
    Origin: class path resource [application.properties] - 1:42
    Reason: No converter found capable of converting from type [java.lang.String] to type [java.lang.Class<?>]

Action:

Update your application's configuration

It's not the failure analysis as the exception that's logged when running with --debug isn't very helpful either:

org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'spring.kafka.producer.value-serializer' to java.lang.Class<?>
    at org.springframework.boot.context.properties.bind.Binder.handleBindError(Binder.java:384) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:344) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$4(Binder.java:469) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:95) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:83) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:59) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:473) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:587) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:573) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.access$300(Binder.java:534) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:471) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:411) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$4(Binder.java:469) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:95) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:83) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:59) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:473) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:587) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:573) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder$Context.access$300(Binder.java:534) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:471) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:411) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:329) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:259) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:246) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:96) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:89) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.postProcessBeforeInitialization(ConfigurationPropertiesBindingPostProcessor.java:78) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:422) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1778) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:602) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1380) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:229) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1354) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944) ~[spring-beans-5.3.8.jar:5.3.8]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918) ~[spring-context-5.3.8.jar:5.3.8]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583) ~[spring-context-5.3.8.jar:5.3.8]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:754) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:434) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:338) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1343) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1332) ~[spring-boot-2.5.1.jar:2.5.1]
    at com.example.demo.So68077601Application.main(So68077601Application.java:10) ~[main/:na]
Caused by: org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [java.lang.Class<?>]
    at org.springframework.core.convert.support.GenericConversionService.handleConverterNotFound(GenericConversionService.java:322) ~[spring-core-5.3.8.jar:5.3.8]
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:195) ~[spring-core-5.3.8.jar:5.3.8]
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:111) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:96) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:88) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindProperty(Binder.java:456) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:400) ~[spring-boot-2.5.1.jar:2.5.1]
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340) ~[spring-boot-2.5.1.jar:2.5.1]
    ... 59 common frames omitted

Debugging reveals the underlying cause which is a TypeMismatchException:

org.springframework.beans.TypeMismatchException: Failed to convert value of type 'java.lang.String' to required type 'java.lang.Class'; nested exception is java.lang.IllegalArgumentException: Could not find class [io.confluent.kafka.serializers.KafkaAvroSerializer]
    at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:79)
    at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:45)
    at org.springframework.boot.context.properties.bind.BindConverter$TypeConverterConverter.convert(BindConverter.java:204)
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41)
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:192)
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:105)
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:96)
    at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:88)
    at org.springframework.boot.context.properties.bind.Binder.bindProperty(Binder.java:456)
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:400)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340)
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$4(Binder.java:469)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:95)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:83)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:59)
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:473)
    at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:587)
    at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:573)
    at org.springframework.boot.context.properties.bind.Binder$Context.access$300(Binder.java:534)
    at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:471)
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:411)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340)
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$4(Binder.java:469)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:95)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:83)
    at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:59)
    at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$5(Binder.java:473)
    at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:587)
    at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:573)
    at org.springframework.boot.context.properties.bind.Binder$Context.access$300(Binder.java:534)
    at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:471)
    at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:411)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:340)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:329)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:259)
    at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:246)
    at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:96)
    at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:89)
    at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.postProcessBeforeInitialization(ConfigurationPropertiesBindingPostProcessor.java:78)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:422)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1778)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:602)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1380)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300)
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887)
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791)
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:229)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1354)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:754)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:434)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:338)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1343)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1332)
    at com.example.demo.So68077601Application.main(So68077601Application.java:10)
Caused by: java.lang.IllegalArgumentException: Could not find class [io.confluent.kafka.serializers.KafkaAvroSerializer]
    at org.springframework.util.ClassUtils.resolveClassName(ClassUtils.java:334)
    at org.springframework.beans.propertyeditors.ClassEditor.setAsText(ClassEditor.java:65)
    at org.springframework.beans.TypeConverterDelegate.doConvertTextValue(TypeConverterDelegate.java:429)
    at org.springframework.beans.TypeConverterDelegate.doConvertValue(TypeConverterDelegate.java:402)
    at org.springframework.beans.TypeConverterDelegate.convertIfNecessary(TypeConverterDelegate.java:155)
    at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:73)
    ... 69 more
Caused by: java.lang.ClassNotFoundException: io.confluent.kafka.serializers.KafkaAvroSerializer
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:398)
    at org.springframework.util.ClassUtils.forName(ClassUtils.java:284)
    at org.springframework.util.ClassUtils.resolveClassName(ClassUtils.java:324)
    ... 74 more

This is then wrapped in a ConversionFailedException, which BindConverter swallows:

https://github.com/spring-projects/spring-boot/blob/f244bec53d117018335eaac64aab927aeeea14e9/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/BindConverter.java#L100-L112

The last delegate is then called again. The previous failure has left its converter cache with a NO_MATCH entry for String -> Class<?> conversion which results in the ConverterNotFoundException being thrown and analysed.