自动装配原理

https://juejin.cn/post/7333937659838562356?searchId=20240228230745DB6AB2EEC09EFAA1B090

# 自动装配原理

# @SpringBootAplication

@EnableAutoConfiguration：启用 SpringBoot 的自动配置机制；

@Configuration：允许在上下文中注册额外的 bean 或导入其他配置类；

@ComponentScan：扫描被@Component (@Service,@Controller)注解的 bean；

# @EnableAutoConfiguration

核心功能的实现实际是通过 AutoConfigurationImportSelector类；

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage //作用：将main包下的所有组件注册到容器中
@Import({AutoConfigurationImportSelector.class}) //加载自动装配类 xxxAutoconfiguration
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

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# AutoConfigurationImportSelector

实现了ImportSelector接口，实现了它的selectImports方法，用于获取符合条件的类全限定名，加载到IOC容器中；

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {

}

public interface DeferredImportSelector extends ImportSelector {

}

public interface ImportSelector {
    String[] selectImports(AnnotationMetadata var1);
}

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public String[] selectImports(AnnotationMetadata annotationMetadata) {
        // <1>.判断自动装配开关是否打开
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
          //<2>.获取所有需要装配的bean
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }
}

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# getAutoConfigurationEntry()

可以从META-INF/spring.factories加载自动配置类；

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