BeanPostProcessor

Definition

A BeanPostProcessor is a hook that Spring calls for every bean right after it is created. It can inspect or even replace the bean. This is how Spring itself adds features such as @Transactional and @Async (by wrapping beans in proxies).

Example code

A hook that sees every bean as it is created

import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

class Greeter { }
class Mailer { }

class LoggingPostProcessor implements BeanPostProcessor {
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        if (bean instanceof Greeter || bean instanceof Mailer) {
            System.out.println("created bean: " + beanName);        // inspect (or wrap) every bean
        }
        return bean;                                               // return the bean, or a proxy around it
    }
}

@Configuration
class AppConfig {
    @Bean static LoggingPostProcessor logger() { return new LoggingPostProcessor(); }   // static: it is needed early
    @Bean Greeter greeter() { return new Greeter(); }
    @Bean Mailer mailer() { return new Mailer(); }
}

class Demo {
    public static void main(String[] args) {
        try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
            System.out.println("ready");
        }
    }
}

// Output:
// created bean: greeter
// created bean: mailer
// ready