In Spring/Spring Boot how to make Bean initialised before Tomcat?

In Spring/Spring Boot how to make Bean initialised before Tomcat?
That is before these lines in log:

o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
o.apache.catalina.core.StandardService   : Starting service [Tomcat]
org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.73]
o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 439 ms

(and not after)

Reason: There is a Bean with heavy initialization, and if it fails
(i.e. Bean creation fails), then there is no need to start Tomcat or the server app at all.

Assuming and interface like this

public interface MyInterface {
public String greeting();
}

and an implementing class like this

package com.example.demo.service;
public class MyImpl1 implements MyInterface {
@Override
public String greeting() {
return "MyImpl1 greeting";
}
}

Add a @Component that implements BeanDefinitionRegistryPostProcessor like this:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.beans.factory.support.RootBeanDefinition;
import org.springframework.core.Ordered;
import org.springframework.core.PriorityOrdered;
import org.springframework.stereotype.Component;
@Component
public class BeanFactoryPostProcessor implements BeanDefinitionRegistryPostProcessor, PriorityOrdered {
private static Logger logger = LoggerFactory.getLogger(BeanFactoryPostProcessor.class);
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
String beanName = "myInterface"; // interface name with lowercase first letter
String className = "com.example.demo.service.MyImpl1"; // full class name of the implemention class
BeanDefinition beanDefn= new RootBeanDefinition(className);
registry.registerBeanDefinition(beanName, beanDefn);
logger.info("Registering bean {}:{}", beanName, className);
// any other early beans here
}
public int getOrder() {
return Ordered.HIGHEST_PRECEDENCE;
}
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
// do nothing
}
}

Use it in the normal way:

@Autowired
MyInterface myInterface;

Remove the @Component or @Bean annotations on the class so that it is not built twice.

This can be implemented using SpringApplicationBuilder. It provides methods parent and child. If you define your "heavy" configuration bean as parent and the usual application class as child you will get what you wanted. In case of exception in parent its child will be not started.

Solution

Heavy Config class:

@Configuration
public class HeavyConfig {
@Bean
public static HeavyConfig myBean() {
throw new NullPointerException("autch!");
}
}

Main application method:

public static void main(String[] args) {
new SpringApplicationBuilder()
.parent(HeavyConfig.class)
.child(NormalApplication.class)
.run(args);

Startup logs in case of an exception:

Starting RestApiApplication using Java 20 with PID 8076 
No active profile set, falling back to 1 default profile: "default"
Exception encountered during context initialization - cancelling refresh attempt:
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'myBean' defined
in pl.edosoft.restapi.HeavyConfig: Failed to instantiate [pl.edosoft.restapi.HeavyConfig]: Factory
method 'myBean' threw exception with message: autch!
Application run failed