一文搞懂面试官常问的：SpringBoot 自动配置原理

@SpringBootApplicationpublic 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@Indexedpublic @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";