springBoot启动的时候回扫描所有的包内容，使用SPI的思想，将pom文件中引入的包来放入到容器中。

注解引入顺序

@SpringBootApplication
@EnableAutoConfiguration

我们需要看一下自动配置的注解是如何运行的

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

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

上述代码可以了解到，@AutoConfiguration注解上有一个 @Import(AutoConfigurationImportSelector.class) 引入的注解，可以看一个这个注解在引入一个类。我们找到最核心的方法 getAutoConfigurationEntry，获取自动配置类的所有实体。

/**
 * {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration
 * auto-configuration}. This class can also be subclassed if a custom variant of
 * {@link EnableAutoConfiguration @EnableAutoConfiguration} is needed.
 *
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @author Stephane Nicoll
 * @author Madhura Bhave
 * @since 1.3.0
 * @see EnableAutoConfiguration
 */
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
     ...
 

    /**
     * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
     * of the importing {@link Configuration @Configuration} class.
     * @param annotationMetadata the annotation metadata of the configuration class
     * @return the auto-configurations that should be imported
     */
    protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }

//获取当注解的属性，就是 @EnableAutoConfiguration的两个属性
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
//通过上一环节@AutoConfigurationPackage的cache加载的所有spring.facties里面获取相关的自动配置类此时是所有的。
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = getConfigurationClassFilter().filter(configurations);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }
...

    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
                getBeanClassLoader());
        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
                + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

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You should only ever add one @SpringBootApplication or @EnableAutoConfiguration annotation. We generally recommend that you add one or the other to your primary @Configuration class only.

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