Spring Unable to use property placeholders in BeanPostProcessor

I am using Spring with a legacy Tomcat application (not Spring Boot) that comes with Spring Boot BOM 2.4.3.

In my BeanPostProcessor I have @Value("${spring.kafka.maximumRequestSize:15728640}").

However, it does not appear to evaluate property placeholders correctly. I have an MVCE that shows this.

The result is

        Caused by: org.springframework.beans.TypeMismatchException: Failed to convert value of type 'java.lang.String' to required type 'int'; nested exception is java.lang.NumberFormatException: For input string: "${spring.kafka.maximumRequestSize:15728640}"
                at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:79)
                at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1328)
                at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300)
                at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657)
                ... 57 more
        Caused by: java.lang.NumberFormatException: For input string: "${spring.kafka.maximumRequestSize:15728640}"
                at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
                at java.base/java.lang.Integer.parseInt(Integer.java:638)
                at java.base/java.lang.Integer.valueOf(Integer.java:983)
                at org.springframework.util.NumberUtils.parseNumber(NumberUtils.java:211)
                at org.springframework.beans.propertyeditors.CustomNumberEditor.setAsText(CustomNumberEditor.java:115)
                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)
                ... 60 more

Related Stack Overflow question https://stackoverflow.com/questions/66278338/how-do-i-get-spel-expressions-to-work-in-a-beanpostprocessor-that-is-application?noredirect=1#comment117183151_66278338

Comment From: trajano

In my MVCE if you remove implements BeanPostProcessor the app loads up.

Comment From: trajano

To work around this issue a separate class needs to be created, but it does not seem like the proper approach as without the BeanPostProcessor the MVCE works.

@Configuration
public class PlaceholderConfiguration {

    @Bean
    public static PlaceholderConfigurerSupport placeholderConfigurerSupport() {

        final PlaceholderConfigurerSupport support = new PropertySourcesPlaceholderConfigurer();
        return support;

    }

}

Comment From: quaff

It's not SPEL #{} only placeholder ${}.

Comment From: sbrannen

This is by design.

In AbstractApplicationContext#refresh(), we see that registerBeanPostProcessors() is invoked before finishBeanFactoryInitialization().

https://github.com/spring-projects/spring-framework/blob/dfc84069178cbba57425e85841c9c3257242db0b/spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java#L566-L582

In finishBeanFactoryInitialization(), we see that a default EmbeddedValueResolver is registered if no other resolver has been registered by this point.

https://github.com/spring-projects/spring-framework/blob/dfc84069178cbba57425e85841c9c3257242db0b/spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java#L897-L902

Thus, with your original setup, there is nothing registered to resolve the property placeholder supplied to your BeanPostProcessor via the @Value annotation.

The correct solution is in fact to register a PropertySourcesPlaceholderConfigurer as a static @Bean method in a @Configuration class.

Note that registration of a PropertySourcesPlaceholderConfigurer bean is unnecessary in a Spring Boot application, since Spring Boot registers one automatically.

In light of the above, I am closing this issue.