Mastering Spring Bean Definitions: XML, Annotations, JavaConfig & Import Techniques

Introduction

Spring plays a crucial role in the Java ecosystem as a bean factory, offering multiple ways to define beans ranging from classic XML configuration to modern annotation‑based and Java‑based approaches.

1. XML bean configuration

1.1 Constructor

Define a bean using

<bean id="personService" class="com.sue.cache.service.test7.PersonService">

with optional <constructor-arg> elements to pass arguments.

<bean id="personService" class="com.sue.cache.service.test7.PersonService">
    <constructor-arg index="0" value="susan"/>
    <constructor-arg index="1" ref="baseInfo"/>
</bean>

1.2 Setter method

Use <property> to set bean properties via generated setter methods.

<bean id="person" class="com.sue.cache.service.test7.Person">
    <property name="name" value="susan"/>
    <property name="age" value="18"/>
</bean>

1.3 Static factory

Reference a static factory method with factory-method and supply arguments through <constructor-arg>.

<bean class="com.sue.cache.service.test7.SusanBeanFactory" factory-method="createPerson">
    <constructor-arg index="0" value="susan"/>
    <constructor-arg index="1" value="18"/>
</bean>

1.4 Instance factory

Define a non‑static factory bean and use factory-bean to create the target bean.

<bean id="susanBeanFactory" class="com.sue.cache.service.test7.SusanBeanFactory"/>
<bean factory-bean="susanBeanFactory" factory-method="createPerson">
    <constructor-arg index="0" value="susan"/>
    <constructor-arg index="1" value="18"/>
</bean>

1.5 FactoryBean

Implement FactoryBean to control object creation; the bean is retrieved with getBean("userFactoryBean") while getBean("&userFactoryBean") returns the factory itself.

<bean id="userFactoryBean" class="com.sue.async.service.UserFactoryBean"/>

2. Component annotations

Since Spring 2.5, @Component, @Service, @Repository and @Controller can replace XML definitions. They require component‑scan configuration via <context:component-scan>, @ComponentScan or the implicit scan of @SpringBootApplication.

@Service
public class PersonService {
    public String get() { return "data"; }
}

3. JavaConfig

Use @Configuration classes with @Bean methods to declare beans programmatically.

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

Conditional annotations such as @ConditionalOnClass, @ConditionalOnBean, @ConditionalOnProperty allow fine‑grained activation of beans.

4. Import mechanisms

4.1 @Import of ordinary classes

Import a simple class as a bean with @Import(Role.class).

4.2 @Import of configuration classes

Import another @Configuration class to bring its beans into the context.

4.3 ImportSelector

Implement ImportSelector to return an array of fully‑qualified class names based on custom logic.

4.4 ImportBeanDefinitionRegistrar

Implement ImportBeanDefinitionRegistrar to register beans with custom names, scopes or other metadata.

5. Bean post‑processing

BeanDefinitionRegistryPostProcessor

focuses on bean registration, while BeanFactoryPostProcessor modifies already‑registered bean definitions. Both can be used to add beans programmatically, but using BeanFactoryAware for registration is discouraged.