SpringBoot自动配置注解原理解析

1.SpringBoot启动主程序类

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {

        SpringApplication.run(DemoApplication.class, args);
    }
}

我们查看SpringBootApplication注解：

@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 {

这三个注解特别重要

2.@SpringBootConfiguration

@SpringBootConfiguration继承自@Configuration，二者功能也一致，标注当前类是配置类， 并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例纳入到spring容器中，并且实例名就是方法名。

 

3.@EnableAutoConfiguration

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

4. AutoConfigurationPackage注解

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata,
                BeanDefinitionRegistry registry) {
            register(registry, new PackageImport(metadata).getPackageName());
        }

new PackageImport(metadata).getPackageName()，它其实返回了当前主程序类的 同级以及子级的包组件

为了将所有的包扫描到，主类最好在根目录。

5. Import(AutoConfigurationImportSelector.class)注解

 

 

可以从图中看出  AutoConfigurationImportSelector 继承了 DeferredImportSelector 继承了 ImportSelector

ImportSelector有一个方法为：selectImports

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

可以看到第九行，它其实是去加载  public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";外部文件。这个外部文件，有很多自动配置的类。如下：