preface

Previous events were learned, system initializers and listeners. From here on out, go to the core of the core — bean-related.

At the heart of this section is an exploration of how beans are loaded into the Spring Ioc container.

In this section, the first of the Bean parsing sections, let’s learn what types of Bean registration are available.

Configure beans in N positions

The original is, of course, the way Xml is configured, but since this series is at the heart of Spring Boot, I won’t go into how Xml is configured in the Spring era.

Annotations @ Component

This is the most common way we use it. For classes, inject this class into the Spring Ioc container.

@Component refers to any annotation that contains @Component, such as @Controller, @Service, and so on.

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {
    @AliasFor(annotation = Component.class)
    String value(a) default "";
}
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@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Service {
    @AliasFor(annotation = Component.class)
    String value(a) default "";
 
}
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As you can see, @Controller and @Service annotations include @Component.

Annotations @ Bean

That’s the way we do it. Used on methods to register the return value of the method as a Bean.

Both of these are fairly common ways, so I’m not going to do a demonstration.

Realize the FactoryBean

  1. Abstract interface MyBean

    public interface MyBean {
    String getName(a);
}
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  2. The implementation class MyOneBean

    public class MyOneBean implements MyBean{
    @Override
    public String getName(a) {
        return "MyOneBean"; }}Copy the code

  3. MyFactoryBean

    Note that the @Component blessing is also required here.

    @Component
public class MyFactoryBean implements FactoryBean<MyBean> {
    @Override
    public MyBean getObject(a) throws Exception {
        return new MyOneBean();
    }
    @Override
    publicClass<? > getObjectType() {returnMyBean.class; }}Copy the code

  4. test

    @Test
public void one(a) {
    System.out.println(myBean.getName());
}
 
// MyOneBean
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Implement BeanDefinitionRegistryPostProcessor

  1. MyTwoBean

    public class MyTwoBean implements MyBean{
    @Override
    public String getName(a) {
        return "MyTwoBean"; }}Copy the code

  2. MyBeanDefinitionRegistryPostProcessor

    @Component
public class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
        RootBeanDefinition rootBeanDefinition = new RootBeanDefinition();
        rootBeanDefinition.setBeanClass(MyTwoBean.class);
        beanDefinitionRegistry.registerBeanDefinition("myTwoBean",rootBeanDefinition);
    }
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {}}Copy the code

  3. test

    @Testpublic void one(a) { System.out.println(myBean.getName()); }// MyTwoBean
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Implement ImportBeanDefinitionRegistrar

  1. MyThreeBean

    public class MyThreeBean implements MyBean{    @Override    public String getName(a) {        return "myThreeBean";    }}
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  2. MyImportBeanDefinitionRegistrar

    Note that there is no @Component annotation

    public class MyImportBeanDefinitionRefistart implements ImportBeanDefinitionRegistrar {    @Override    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {        RootBeanDefinition rootBeanDefinition = new RootBeanDefinition();        rootBeanDefinition.setBeanClass(MyThreeBean.class);        registry.registerBeanDefinition("myThreeBean",rootBeanDefinition);    }}
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  3. Start the class add @ Import (MyImportBeanDefinitionRefistart. Class)

  4. test

    @Testpublic void one(a) { System.out.println(myBean.getName()); }// myThreeBean
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conclusion

In this section, you learned about several ways to register beans.

This is to pave the way for later Bean loading.