初步理解springboot的 @EnableAutoConfiguration注解

最新推荐文章于 2025-06-17 10:54:46 发布

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最新推荐文章于 2025-06-17 10:54:46 发布

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分类专栏： springboot 文章标签： spring spring boot ioc bean

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本文解析了SpringBoot中@SpringBootApplication注解如何启动Spring容器。通过分析@EnableAutoConfiguration和AutoConfigurationImportSelector，揭示了自动配置的实现机制。

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问题：为什么我们在springboot的入口类上加上@SpringBootApplication注解就能启动一个spring容器？

1.首先看看该注解的源码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
	
	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};

	@AliasFor(annotation = EnableAutoConfiguration.class)
	String[] excludeName() default {};

	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};

	
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};
}

本文只关心@EnableAutoConfiguration的实现，对于@springBootConfiguration是@Configuration的扩展，@CompoentScan配置扫描包，暂不分析。

首先看看@EnableAutoConfiguration注解的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

}

这里比较重要的就是@Import注解，她指定的类是实现了ImportSelector接口，初始化容器时将调用其

public String[] selectImports(AnnotationMetadata importingClassMetadata)方法。

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

	private static final String[] NO_IMPORTS = {};

	private static final Log logger = LogFactory
			.getLog(AutoConfigurationImportSelector.class);

	private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";

	private ConfigurableListableBeanFactory beanFactory;

	private Environment environment;

	private ClassLoader beanClassLoader;

	private ResourceLoader resourceLoader;

	@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		try {
			AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
					.loadMetadata(this.beanClassLoader);
			AnnotationAttributes attributes = getAttributes(annotationMetadata);
			List<String> configurations = getCandidateConfigurations(annotationMetadata,
					attributes);
			configurations = removeDuplicates(configurations);
			configurations = sort(configurations, autoConfigurationMetadata);
			Set<String> exclusions = getExclusions(annotationMetadata, attributes);
			checkExcludedClasses(configurations, exclusions);
			configurations.removeAll(exclusions);
			configurations = filter(configurations, autoConfigurationMetadata);
			fireAutoConfigurationImportEvents(configurations, exclusions);
			return StringUtils.toStringArray(configurations);
		}
		catch (IOException ex) {
			throw new IllegalStateException(ex);
		}
	}

可以发现在该方法内部去实现了要额外加入到容器的Bean类的全路径名称。同理，我们可以自定义类实现该接口，并在selectImports()方法内，按逻辑注册需要的bean到容器