Spring Boot自动配置原理

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
  String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
  /**
   * 排除特定的自动配置类，使它们不会被应用
   */
  Class<?>[] exclude() default {};
  /**
   * 根据类名排除特定的自动配置类，使它们不会被应用
   */
  String[] excludeName() default {};
}

/**
 * 判断是否开启自动配置，直接返回true
 */
protected boolean isEnabled(AnnotationMetadata metadata) {
    if (getClass() == AutoConfigurationImportSelector.class) {
        return getEnvironment().getProperty(
            EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
            true);
    }
    return true;
}
/**
 * 返回应该考虑的自动配置类名. 默认情况下该方法将使用 {@link SpringFactoriesLoader} 
 */
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
                                                  AnnotationAttributes attributes) {
    // 这一步非常重要，加载Spring Boot自动配置的核心文件
    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;
}
/**
 * 检查排除的自动配置类
 */
private void checkExcludedClasses(List<String> configurations,
                                  Set<String> exclusions) {
    List<String> invalidExcludes = new ArrayList<>(exclusions.size());
    for (String exclusion : exclusions) {
        if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
            && !configurations.contains(exclusion)) {
            invalidExcludes.add(exclusion);
        }
    }
    if (!invalidExcludes.isEmpty()) {
        handleInvalidExcludes(invalidExcludes);
    }
}

public final class SpringFactoriesLoader {
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
    private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap();
}

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
......