SpringBoot自动配置原理详解

@SpringBootApplication
public class SpringbootWorkApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootWorkApplication.class, args);
    }
}

@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 {

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
    @AliasFor(
        annotation = Configuration.class
    )
    boolean proxyBeanMethods() default true;
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage   //自动导包
@Import({AutoConfigurationImportSelector.class}) //自动配置导入选择
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

    String[] excludeName() default {};
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({Registrar.class})
public @interface AutoConfigurationPackage {
    String[] basePackages() default {};

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

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
    Registrar() {
    }

    public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
        //断点
        AutoConfigurationPackages.register(registry, (String[])(new AutoConfigurationPackages.PackageImports(metadata)).getPackageNames().toArray(new String[0]));
    }

    public Set<Object> determineImports(AnnotationMetadata metadata) {
        return Collections.singleton(new AutoConfigurationPackages.PackageImports(metadata));
    }
}

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
}

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
         // 这打个断点，看看 返回的数据
        List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        //删除重复项
        configurations = this.removeDuplicates(configurations);
        Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
        //检查
        this.checkExcludedClasses(configurations, exclusions);
        //删除需要排除的依赖
        configurations.removeAll(exclusions);
        configurations = this.getConfigurationClassFilter().filter(configurations);
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
}

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
    return EnableAutoConfiguration.class;
}

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
   ClassLoader classLoaderToUse = classLoader;
   if (classLoaderToUse == null) {
      classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
   }
   String factoryTypeName = factoryType.getName();
   return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
    //断点查看
   Map<String, List<String>> result = cache.get(classLoader);
   if (result != null) {
      return result;
   }

   result = new HashMap<>();
   try {
      //注意这里：META-INF/spring.factories
      Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
      while (urls.hasMoreElements()) {
         URL url = urls.nextElement();
         UrlResource resource = new UrlResource(url);
         Properties properties = PropertiesLoaderUtils.loadProperties(resource);
         for (Map.Entry<?, ?> entry : properties.entrySet()) {
            String factoryTypeName = ((String) entry.getKey()).trim();
            String[] factoryImplementationNames =
                  StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
            for (String factoryImplementationName : factoryImplementationNames) {
            //断点
               result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
                     .add(factoryImplementationName.trim());
            }
         }
      }

      // Replace all lists with unmodifiable lists containing unique elements
      //去重，断点查看result值
      result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
            .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
      cache.put(classLoader, result);
   }
   catch (IOException ex) {
      throw new IllegalArgumentException("Unable to load factories from location [" +
            FACTORIES_RESOURCE_LOCATION + "]", ex);
   }
   return result;
}

public final class SpringFactoriesLoader {

   /**
    * The location to look for factories.
    * <p>Can be present in multiple JAR files.
    */
   public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";