Java.Spring.ExternalBeanRegistration

// app

@SpringBootApplication
public class App {

	public static void main(String[] args) {

		ConfigurableApplicationContext configurableContext = SpringApplication.run(App.class, args);
		AppContext.getInstance().setContext(configurableContext);

		var beanFactory = configurableContext.getBeanFactory();
		beanFactory.registerSingleton("someSingletone", new SomeSingletonBean());

		var test  = new Test();
		test.execute();

	}

}

class Test{
	public void execute(){
		var someSingleTone = (SomeSingletonBean) AppContext.getInstance().getContext().getBean("someSingletone");
		someSingleTone.sayHello();
	}

}

// someSingletoneBean

public class SomeSingletonBean {
  public void sayHello(){
      System.out.println("hello");
  }
}

// helper singletone class

package com.example.main;

import org.springframework.context.ApplicationContext;

/**
 * @author Dinesh.Lomte
 *
 */
public enum AppContext {

    INSTANCE;

    public static AppContext getInstance() {
        return INSTANCE;
    }

    private ApplicationContext applicationContext;

    /**
     * Default constructor
     */
    private AppContext() {
    }

    /**
     *
     */
    public void setContext(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

    /**
     *
     * @return
     */
    public ApplicationContext getContext() {
        return applicationContext;
    }
}