How to Create Multiple Beans of the Same Class in Spring Boot 3

1. Introduction

In Spring Boot applications, you may need to create multiple instances (beans) of the same class with different configurations. While defining several @Bean methods in a @Configuration class works, it becomes redundant and hard to maintain when many instances are required.

2. Practical Examples

2.1 Using Java Configuration

The simplest way is to define multiple @Bean methods in a configuration class.

<code>public class Person {
  private String name;
  private Integer age;
  public Person(String name, Integer age) {
    this.name = name;
    this.age = age;
  }
  // getters, setters
  @Override
  public String toString() {
    return "Person [name=" + name + ", age=" + age + "]";
  }
}</code>

<code>@Configuration
public class PersonConfig {
  @Bean
  public Person person1() {
    return new Person("Pack", 22);
  }

  @Bean
  public Person person2() {
    return new Person("xxgg", 24);
  }
}</code>

The @Bean annotation registers both beans in the Spring container, allowing them to be autowired where needed.

2.2 Using @Component

By annotating subclasses with @Component, you can create beans that inherit from a common superclass.

<code>@Component
public class PersonOne extends Person {
  public PersonOne() {
    super("Pack", 22);
  }
}

@Component
public class PersonTwo extends Person {
  public PersonTwo() {
    super("xxxooo", 24);
  }
}</code>

This approach does not create multiple instances of the same class; it creates separate subclasses, which can increase code complexity.

2.3 Using BeanFactoryPostProcessor

A custom BeanFactoryPostProcessor can register multiple bean instances before any other beans are created.

<code>public class PersonFactoryPostProcessor implements BeanFactoryPostProcessor {
  @Override
  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    Map<String, Object> map = beanFactory.getBeansWithAnnotation(Qualifier.class);
    for (Map.Entry<String, Object> entry : map.entrySet()) {
      createInstances(beanFactory, entry.getKey(), entry.getValue());
    }
  }

  private void createInstances(ConfigurableListableBeanFactory beanFactory, String beanName, Object bean) {
    Qualifier qualifier = bean.getClass().getAnnotation(Qualifier.class);
    for (String name : extractNames(qualifier)) {
      Object newBean = beanFactory.getBean(beanName);
      beanFactory.registerSingleton(name.trim(), newBean);
    }
  }

  private String[] extractNames(Qualifier qualifier) {
    return qualifier.value().split(",");
  }
}</code>

The processor scans for @Qualifier annotations, extracts bean names, and registers singleton instances accordingly.

2.4 Custom FactoryBean Implementation

You can also implement FactoryBean to control bean creation.

<code>@Qualifier(value = "personOne, personTwo")
public class Person implements FactoryBean<Object> {
  private String name;
  private Integer age;

  public Person() {}

  @Override
  public Class<Person> getObjectType() {
    return Person.class;
  }

  @Override
  public Object getObject() throws Exception {
    return new Person();
  }

  @Override
  public boolean isSingleton() {
    return true;
  }
}</code>

Using @Qualifier on the class level allows multiple bean IDs to refer to the same type.

3. Putting It All Together

<code>@Configuration
public class PersonConfig {
  @Bean
  public PersonFactoryPostProcessor personFactoryPostProcessor() {
    return new PersonFactoryPostProcessor();
  }

  @Bean
  public Person person() {
    return new Person();
  }

  @Bean
  public Human human() {
    return new Human();
  }
}</code>

While the BeanFactoryPostProcessor approach is powerful, it adds complexity and is generally not recommended for simple scenarios. The @Bean method remains the most straightforward way to create multiple beans of the same class in Spring Boot 3.