面试官：你说你精通SpringBoot，你给我说一下类的自动装配吧

剖析@SpringBootApplication注解

创建一个SpringBoot工程后，SpringBoot会为用户提供一个Application类，该类负责项目的启动：

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

这是一个被@SpringBootApplication注解的类，该注解完成了SpringBoot中类的自动装配任务：

@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 {
    
}

抛却元注解不谈，@SpringBootApplication继承了三个注解：

@SpringBootConfiguration

/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an
 * alternative to the Spring's standard @Configuration 
 * annotation so that configuration can be found
 * automatically (for example in tests).
 *
 * Application should only ever include one 
 * @SpringBootConfiguration and most idiomatic Spring Boot 
 * applications will inherit it from @SpringBootApplication.
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
    ...
}

在说明中提到，@SpringBootConfiguration注解是用来替代Spring的@Configuration，方便SpringBoot自动找到配置。

@ComponentScan

/**
 * Configures component scanning directives
 * for use with Configuration classes.
 * Provides support parallel with Spring XML's
 * <context:component-scan> element.
 *
 * Either #basePackageClasses or #basePackages
 * (or its alias #value} may be specified to
 * define specific packages to scan. If specific
 * packages are not defined, scanning will occur
 * from the package of the class that declares
 * this annotation.
 *
 * Note that the <context:component-scan> element
 * has an annotation-config attribute; however,
 * this annotation does not. This is because
 * in almost all cases when using @ComponentScan,
 * default annotation config processing
 * (e.g. processing @Autowired and friends)
 * is assumed. Furthermore, when using 
 * AnnotationConfigApplicationContext,
 * annotation config processors are always
 * registered, meaning that any attempt to disable
 * them at the @ComponentScan level would be ignored.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {
    ...
}

在说明中我们可以得知：@ComponentScan只负责指定要扫描的包，并没有装配其中的类，这个真正装配这些类是@EnableAutoConfiguration。

@EnableAutoConfiguration

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

该类真正完成了SpringBoot对于类的装配工作，具体内容在后续会作出解释。

以@Enable开头的注解

以@Enable开头的注解(@EnableXxx)一般用于开启某一项功能，是为了简化代码的导入。它是一个组合注解，一般情况下@EnableXxx注解中都会组合一个@Import注解，而该@Import注解用于导入指定的类，而被导入的类一般有三种：

配置类

• 类的特征：@Import中指定的类一般以Configuration结尾
• 类的配置：该类上会注解@Configuration

• 类的案例：定时任务启动注解：SchedulingConfiguration

  @Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  @Import(SchedulingConfiguration.class)
  @Documented
  public @interface EnableScheduling {
  }
  

选择器

• 类的特征：@Import中指定的类一般以 Selector 结尾
• 类的配置：该类直接或间接实现了ImportSelector接口，表示当前类会根据条件选择导入不同的类。

• 类的案例：Redis配置类：CachingConfigurationSelector

  @Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Import(CachingConfigurationSelector.class)
  public @interface EnableCaching {
      ...
  }
  

  public class CachingConfigurationSelector extends AdviceModeImportSelector<EnableCaching> {
      ...
      @Override
      public String[] selectImports(AdviceMode adviceMode) {
          switch (adviceMode) {
              case PROXY:
                  return getProxyImports();
              case ASPECTJ:
                  return getAspectJImports();
              default:
                  return null;
          }
      }
      ...
  }
  

注册器

• 类的特征：@Import 中指定的类一般以 Registrar 结尾。
• 类的配置：该类直接或间接实现了ImportBeanDefinitionRegistrar接口，用于导入注册器，该类可以在代码运行时动态注册指定类的实例。

• 类的案例：AspectJ：AspectJAutoProxyRegistrar

  @Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  @Import(AspectJAutoProxyRegistrar.class)
  public @interface EnableAspectJAutoProxy {
      ...
  }
  

  class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
      @Override
      public void registerBeanDefinitions(
              AnnotationMetadata importingClassMetadata,
              BeanDefinitionRegistry registry) {
  
          AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);
  
          AnnotationAttributes enableAspectJAutoProxy =
                  AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
          if (enableAspectJAutoProxy != null) {
              if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
                  AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
              }
              if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
                  AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
              }
          }
      }
  }
  

解析@EnableAutoConfiguration

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

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

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

    String[] excludeName() default {};

}

该注解是一个组合注解，用于完成自动配置，它是Spring Boot的核心注解。所谓自动配置是指，将用户自定义的类及框架本身用到的类进行装配。

@AutoConfigurationPackage

/**
 * Registers packages with AutoConfigurationPackages.
 * When no #basePackages base packages or
 * #basePackageClasses base package classes are
 * specified, the package of the annotated class is 
 * registered.
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
    ...
}

从类的说明中我的得知，该注解用于导入并装配用户自定义类，即自动扫描包中的类。若该注解未通过basePackages或basePackageClasses参数指明要扫描的包路径，则默认扫描含该注解的类所在包及其子包。

@Import

用于导入并装配框架本身的类。其参数AutoConfigurationImportSelector.java类，该类用于导入自动配置的类。其装配跟踪入口：#getCandidateConfigurations

public class AutoConfigurationImportSelector implements 
        DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, 
        EnvironmentAware, Ordered {
    ...
    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;
    }
    ...
}

#getCandidateConfigurations -> SpringFactoriesLoader.loadFactoryNames

public final class SpringFactoriesLoader {
    ...
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    ...
    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        ...
        try {
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            ...
        } catch (IOException ex) {
            ...
        }
    }
}

追踪到这里，我们得知，框架本身定义的类是从META-INF/spring.factories文件中获取的。该文件目录在哪儿呢？
在创建SpringBoot Web项目时，我们在pom.xml文件中会自动导入一个依赖：

<!-- pom.xml -->
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

打开一个starter，如spring-boot-starter-web依赖，我们可以看到其中包含了一个子依赖：

<!-- spring-boot-starter-web-2.3.4.RELEASE.pom -->
<dependencies>
    ...
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>2.3.4.RELEASE</version>
        <scope>compile</scope>
    </dependency>
    ...
</dependencies>

打开spring-boot-starter依赖，可以看到这么一个子依赖：

<!-- spring-boot-starter-2.3.4.RELEASE.pom -->
<dependencies>
    ...
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
        <version>2.3.4.RELEASE</version>
        <scope>compile</scope>
    </dependency>
    ...
</dependencies>

查看该依赖的内容，打开spring.factories文件：

这些就是框架定义的，需要装配的类。

application.yml的加载

application.yml文件对于 Spring Boot 来说是核心配置文件，至关重要！那么，该文件是如何加载到内存的呢？我们需要从启动类的run()方法开始跟踪，该跟踪过程比较深，耐心差的读者慎入。

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

进入run方法：

public class SpringApplication {
    ...
    public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
        return run(new Class<?>[] { primarySource }, args);
    }

    public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
        return new SpringApplication(primarySources).run(args);
    }

    public ConfigurableApplicationContext run(String... args) {
        ...
        // 准备运行环境
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        ...
    }

    private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        ...
        // 让监听器监听环境准备过程
        listeners.environmentPrepared(environment);
        ...
    }
    ...
}

让监听器监听环境准备过程

class SpringApplicationRunListeners {
    ...
    void environmentPrepared(ConfigurableEnvironment environment) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.environmentPrepared(environment);
        }
    }
    ...
}

发布环境准备事件

public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
    ...
    @Override
    public void environmentPrepared(ConfigurableEnvironment environment) {
        this.initialMulticaster.multicastEvent(
            new ApplicationEnvironmentPreparedEvent(
                this.application,
                this.args,
                environment
            )
        );
    }

    @Override
    public void multicastEvent(ApplicationEvent event) {
        multicastEvent(event, resolveDefaultEventType(event));
    }

    @Override
    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        Executor executor = getTaskExecutor();
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                // 触发监听器
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);
            }
        }
    }
    ...
}

触发监听器

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
    ...
    protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
        ErrorHandler errorHandler = getErrorHandler();
        if (errorHandler != null) {
            try {
                doInvokeListener(listener, event);
            }
            catch (Throwable err) {
                errorHandler.handleError(err);
            }
        }
        else {
            doInvokeListener(listener, event);
        }
    }

    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
        ...
        listener.onApplicationEvent(event);
        ...
    }
    ...
}

ApplicationListener#onApplicationEvent是一个接口方法，我们主要看它的ConfigFileApplicationListener实现类的实现

public class ConfigFileApplicationListener implements ... {
    ...
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
        }
        ...
    }

    private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
        List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
        postProcessors.add(this);
        AnnotationAwareOrderComparator.sort(postProcessors);
        for (EnvironmentPostProcessor postProcessor : postProcessors) {
            postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());
        }
    }
    ...
}

EnvironmentPostProcessor#postProcessEnvironment是一个接口方法，我们主要看它的ConfigFileApplicationListener实现类的实现

public class ConfigFileApplicationListener implements ... {
    ...
    @Override
    public void postProcessEnvironment(
            ConfigurableEnvironment environment,
            SpringApplication application) {
        // 加载配置文件
        addPropertySources(environment, application.getResourceLoader());
    }

    protected void addPropertySources(
            ConfigurableEnvironment environment,
            ResourceLoader resourceLoader) {
        RandomValuePropertySource.addToEnvironment(environment);
        new Loader(environment, resourceLoader).load();
    }

    private class Loader {
        void load() {
            FilteredPropertySource.apply(
                this.environment, 
                DEFAULT_PROPERTIES, 
                LOAD_FILTERED_PROPERTY,
                (defaultProperties) -> {
                    ...
                    while (!this.profiles.isEmpty()) {
                        ...
                        load(profile, this::getPositiveProfileFilter,
                                addToLoaded(MutablePropertySources::addLast, false));
                        ...
                    }
                    ...
                });
        }

        private void load(Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
            getSearchLocations().forEach((location) -> {
                boolean isDirectory = location.endsWith("/");
                Set<String> names = isDirectory ? getSearchNames() : NO_SEARCH_NAMES;
                names.forEach((name) -> load(location, name, profile, filterFactory, consumer));
            });
        }

        private void load(String location, String name, Profile profile, DocumentFilterFactory filterFactory,
                DocumentConsumer consumer) {
            ...
            for (PropertySourceLoader loader : this.propertySourceLoaders) {
                for (String fileExtension : loader.getFileExtensions()) {
                    if (processed.add(fileExtension)) {
                        loadForFileExtension(loader, location + name, "." + fileExtension, profile, filterFactory, consumer);
                    }
                }
            }
        }

        private void loadForFileExtension(
                PropertySourceLoader loader,
                String prefix,
                String fileExtension,
                Profile profile,
                DocumentFilterFactory filterFactory,
                DocumentConsumer consumer) {
            ...
            load(loader, prefix + fileExtension, profile, profileFilter, consumer);
        }

        private void load(
                PropertySourceLoader loader,
                String location,
                Profile profile,
                DocumentFilter filter,
                DocumentConsumer consumer) {
            ...
            List<Document> documents = loadDocuments(loader, name, resource);
            ...
        }

        private List<Document> loadDocuments(
                PropertySourceLoader loader,
                String name,
                Resource resource) throws IOException {
            DocumentsCacheKey cacheKey = new DocumentsCacheKey(loader, resource);
            List<Document> documents = this.loadDocumentsCache.get(cacheKey);
            if (documents == null) {
                List<PropertySource<?>> loaded = loader.load(name, resource);
                documents = asDocuments(loaded);
                this.loadDocumentsCache.put(cacheKey, documents);
            }
            return documents;
        }
    }
    ...
}

PropertySourceLoader#getFileExtensions和PropertySourceLoader#load都是接口方法，我们主要看它的YamlPropertySourceLoader实现类的实现

public class YamlPropertySourceLoader implements PropertySourceLoader {
    @Override
    public String[] getFileExtensions() {
        return new String[] { "yml", "yaml" };
    }

    @Override
    public List<PropertySource<?>> load(
            String name,
            Resource resource) throws IOException {
        ...
        return propertySources;
    }
}

最后

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