透过Spring自动配置原理看Spring的扩展点

2022-10-27 10:09:22 浏览数 (3)

EnableAutoConfiguration注解

spring的自动配置就是得益于这个EnableAutoConfiguration注解：

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@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 {};

}

该注解导入了一个类AutoConfigurationImportSelector，从该类的名字我们可以看到，它是一个selector类，那么就具有selector类的特性。

我们都知道，spring启动的时候会自动执行selector类的方法selectImports方法，从而将配置类自动注入容器中。下面就来看看是在哪里执行的这个方法

启动加载selector

spring的启动流程在之前的文章中已经讲过了，我们知道启动的时候会经过AbstractApplicationContext类的refresh方法 在该方法中有这样一句：

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// Invoke factory processors registered as beans in the context.
        invokeBeanFactoryPostProcessors(beanFactory);

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
  PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

  // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
  // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
  if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
    beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
    beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
  }
}

最终会进入到ConfigurationClassPostProcessor类的processConfigBeanDefinitions方法：

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public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
    List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
    String[] candidateNames = registry.getBeanDefinitionNames();

    

  // Parse each @Configuration class
  ConfigurationClassParser parser = new ConfigurationClassParser(
      this.metadataReaderFactory, this.problemReporter, this.environment,
      this.resourceLoader, this.componentScanBeanNameGenerator, registry);

  Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
  Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
  do {
    parser.parse(candidates);
    parser.validate();

}

这里就是在处理标有Configuration注解的类，我们进入parser.parse(candidates);方法：

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ConfigurationClassParser.java

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
    Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter,
    boolean checkForCircularImports) {

  if (importCandidates.isEmpty()) {
    return;
  }

  if (checkForCircularImports && isChainedImportOnStack(configClass)) {
    this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
  }
  else {
    this.importStack.push(configClass);
    try {
      for (SourceClass candidate : importCandidates) {
        if (candidate.isAssignable(ImportSelector.class)) {
          // Candidate class is an ImportSelector -> delegate to it to determine imports
          Class<?> candidateClass = candidate.loadClass();
          ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
              this.environment, this.resourceLoader, this.registry);
          Predicate<String> selectorFilter = selector.getExclusionFilter();
          if (selectorFilter != null) {
            exclusionFilter = exclusionFilter.or(selectorFilter);
          }
          if (selector instanceof DeferredImportSelector) {
            this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
          }
          else {
            String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
            Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
            processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, false);
          }
        }
        else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
          // Candidate class is an ImportBeanDefinitionRegistrar ->
          // delegate to it to register additional bean definitions
          Class<?> candidateClass = candidate.loadClass();
          ImportBeanDefinitionRegistrar registrar =
              ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
                  this.environment, this.resourceLoader, this.registry);
          configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
        }
        else {
          // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
          // process it as an @Configuration class
          this.importStack.registerImport(
              currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
          processConfigurationClass(candidate.asConfigClass(configClass), exclusionFilter);
        }
      }
    }
    catch (BeanDefinitionStoreException ex) {
      throw ex;
    }
    catch (Throwable ex) {
      throw new BeanDefinitionStoreException(
          "Failed to process import candidates for configuration class ["  
          configClass.getMetadata().getClassName()   "]", ex);
    }
    finally {
      this.importStack.pop();
    }
  }
}

我们看到有这样一句selector.selectImports(currentSourceClass.getMetadata())。看到这里，是不是就很清楚了，spring就是这样在启动的时候实现了自动导入配置类的。

下面我们再来看看自动配置类的内部逻辑

AutoConfigurationImportSelector

我们进入selector类，找到selectImports方法：

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

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
    return EMPTY_ENTRY;
  }
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
  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);
}

这里我们注意到这行代码List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);，这句代码就是获取所有jar中meta-inf/spring.factories文件中的类，下面具体来看看：

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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;
}

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) {
  MultiValueMap<String, String> result = cache.get(classLoader);
  if (result != null) {
    return result;
  }

  try {
    Enumeration<URL> urls = (classLoader != null ?
        classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
        ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    result = new LinkedMultiValueMap<>();
    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();
        for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
          result.add(factoryTypeName, factoryImplementationName.trim());
        }
      }
    }
    cache.put(classLoader, result);
    return result;
  }
  catch (IOException ex) {
    throw new IllegalArgumentException("Unable to load factories from location ["  
        FACTORIES_RESOURCE_LOCATION   "]", ex);
  }
}

上面的代码中，我们注意到这样一个常量public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";，意思就是获取该文件中定义的类名，并作为列表返回。我们看一下这个文件的示例：

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# AutoConfiguration
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.springframework.cloud.client.CommonsClientAutoConfiguration,
org.springframework.cloud.client.ReactiveCommonsClientAutoConfiguration,
org.springframework.cloud.client.discovery.composite.CompositeDiscoveryClientAutoConfiguration,
org.springframework.cloud.client.discovery.composite.reactive.ReactiveCompositeDiscoveryClientAutoConfiguration,
org.springframework.cloud.client.discovery.noop.NoopDiscoveryClientAutoConfiguration,
org.springframework.cloud.client.discovery.simple.SimpleDiscoveryClientAutoConfiguration,
org.springframework.cloud.client.discovery.simple.reactive.SimpleReactiveDiscoveryClientAutoConfiguration,
org.springframework.cloud.client.hypermedia.CloudHypermediaAutoConfiguration,
org.springframework.cloud.client.loadbalancer.AsyncLoadBalancerAutoConfiguration,
org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration,
org.springframework.cloud.client.loadbalancer.reactive.LoadBalancerBeanPostProcessorAutoConfiguration,
org.springframework.cloud.client.loadbalancer.reactive.ReactorLoadBalancerClientAutoConfiguration,
org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancerAutoConfiguration,
org.springframework.cloud.client.serviceregistry.ServiceRegistryAutoConfiguration,
org.springframework.cloud.commons.httpclient.HttpClientConfiguration,
org.springframework.cloud.commons.util.UtilAutoConfiguration,
org.springframework.cloud.configuration.CompatibilityVerifierAutoConfiguration,
org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationAutoConfiguration
# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=
org.springframework.cloud.client.HostInfoEnvironmentPostProcessor
# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=
org.springframework.cloud.configuration.CompatibilityNotMetFailureAnalyzer

好了，我们回到之前的getAutoConfigurationEntry方法，

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protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
    return EMPTY_ENTRY;
  }
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
  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);
}

上面的代码很清晰：

  • 1、在配置文件中拿到类名列表
  • 2、对该列表去重
  • 3、根据注解属性，去掉指定排除的类
  • 4、根据自定义AutoConfigurationImportFilter类过滤
  • 5、回调AutoConfigurationImportListener监听的导入事件

AutoConfigurationImportFilter

在上面我们知道的，加载配置类的时候会根据条件过滤某些类，那么这里就来看下具体的实现:

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configurations = getConfigurationClassFilter().filter(configurations);

private ConfigurationClassFilter getConfigurationClassFilter() {
  if (this.configurationClassFilter == null) {
    List<AutoConfigurationImportFilter> filters = getAutoConfigurationImportFilters();
    for (AutoConfigurationImportFilter filter : filters) {
      invokeAwareMethods(filter);
    }
    this.configurationClassFilter = new ConfigurationClassFilter(this.beanClassLoader, filters);
  }
  return this.configurationClassFilter;
}

从上面的代码中，我们看到，这里的listener是怎么来的呢？

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protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
  return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class, this.beanClassLoader);
}

看到这个代码里面的调用，又回到了刚刚讲解获取自动配置类的地方，只是这里传入的Class<T> factoryType变成了AutoConfigurationImportFilter，

总结一下，也就是说：filter也是从META-INF/spring.factories配置文件里面获取出来的，示例如下：

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# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=
org.springframework.boot.autoconfigure.condition.OnBeanCondition,
org.springframework.boot.autoconfigure.condition.OnClassCondition,
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition
扩展点一

看到这里我们也可以总结出spring其中的一个扩展点：

如果业务中需要根据条件动态的控制某个类的导入，则可以实现AutoConfigurationImportFilter接口，并且实现方法match，然后放入META-INF/spring.factories配置文件中Key为：org.springframework.context.ApplicationContextInitializer的value中。

spring在导入配置类的时候，会自动执行这个扩展方法。

注：spring另外封装了一个类ConfigurationClassFilter来统一处理

AutoConfigurationImportListener

在上面我们知道的，加载配置类的时候会回调AutoConfigurationImportListener监听的导入事件，那么这里就来看下具体的实现:

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private void fireAutoConfigurationImportEvents(List<String> configurations, Set<String> exclusions) {
  List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
  if (!listeners.isEmpty()) {
    AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this, configurations, exclusions);
    for (AutoConfigurationImportListener listener : listeners) {
      invokeAwareMethods(listener);
      listener.onAutoConfigurationImportEvent(event);
    }
  }
}

从上面的代码中，我们看到，这里的listener是怎么来的呢？

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protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
  return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class, this.beanClassLoader);
}

看到这个代码里面的调用，又回到了刚刚讲解获取自动配置类的地方，只是这里传入的Class<T> factoryType变成了AutoConfigurationImportListener，

总结一下，也就是说： listener也是从META-INF/spring.factories配置文件里面获取出来的，示例如下：

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# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
代码语言：javascript复制
扩展点二

看到这里我们也可以总结出spring其中的一个扩展点：

如果业务中需要监控某个类的导入，则可以实现AutoConfigurationImportListener接口，并且实现方法onAutoConfigurationImportEvent，然后放入META-INF/spring.factories配置文件中Key为：org.springframework.context.ApplicationContextInitializer的value中。

spring在导入配置类的时候，会自动执行这个扩展方法。

spring.factories

从上面我们知道了，spring就是通过spring.factories文件让外部实现自定义扩展的，其实，该文件还可以定义其他一些classtype的key，

比如在springboot的启动中，有如下代码：

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public static void main(String[] args) {
  SpringApplication.run(xxx.class, args);
}
  
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
  this.resourceLoader = resourceLoader;
  Assert.notNull(primarySources, "PrimarySources must not be null");
  this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
  this.webApplicationType = WebApplicationType.deduceFromClasspath();
  setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
  this.mainApplicationClass = deduceMainApplicationClass();
}

可以看出，springboot在启动的时候会加载spring.factories中ApplicationContextInitializer和ApplicationListener为key的类

看到这里我们也可以总结出spring的一个扩展点：

扩展点三

如果业务需要在启动的时候自定义下容器的初始化动作，则可以实现ApplicationContextInitializer接口，并且实现方法initialize，然后放入META-INF/spring.factories配置文件中Key为：org.springframework.context.ApplicationContextInitializer的value中。

看到这里，我们是不是可以想象下，我们是不是可以在spring.factories中自定义key-value，然后通过库方法自行解析，达到任意扩展的效果呢？答案是可以的。

spring auto listener meta selector

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