死磕源码系列【ImportSelector接口原理解析】

骑猪看日落 2022-12-11 07:25 168阅读 0赞

> ImportSelector接口是spring中导入外部配置的核心接口，根据给定的条件（通常是一个或多个注释属性）判定要导入那个配置类，在spring自动化配置和@EnableXXX中都有它的存在；

##### 1.ImportSelector接口源码解析 #####

/** * Interface to be implemented by types that determine which @{@link Configuration} * class(es) should be imported based on a given selection criteria, usually one or * more annotation attributes. * * An {@link ImportSelector} may implement any of the following * {@link org.springframework.beans.factory.Aware Aware} interfaces, * and their respective methods will be called prior to {@link #selectImports}: * <ul> * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li> * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li> * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li> * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li> * </ul> * * Alternatively, the class may provide a single constructor with one or more of * the following supported parameter types: * <ul> * <li>{@link org.springframework.core.env.Environment Environment}</li> * <li>{@link org.springframework.beans.factory.BeanFactory BeanFactory}</li> * <li>{@link java.lang.ClassLoader ClassLoader}</li> * <li>{@link org.springframework.core.io.ResourceLoader ResourceLoader}</li> * </ul> * * {@code ImportSelector} implementations are usually processed in the same way * as regular {@code @Import} annotations, however, it is also possible to defer * selection of imports until all {@code @Configuration} classes have been processed * (see {@link DeferredImportSelector} for details). * * @author Chris Beams * @author Juergen Hoeller * @since 3.1 * @see DeferredImportSelector * @see Import * @see ImportBeanDefinitionRegistrar * @see Configuration */
 public interface ImportSelector { 
 
 	/** * Select and return the names of which class(es) should be imported based on * the {@link AnnotationMetadata} of the importing @{@link Configuration} class. * @return the class names, or an empty array if none */
 	String[] selectImports(AnnotationMetadata importingClassMetadata);
 
 	/** * Return a predicate for excluding classes from the import candidates, to be * transitively applied to all classes found through this selector's imports. * If this predicate returns {@code true} for a given fully-qualified * class name, said class will not be considered as an imported configuration * class, bypassing class file loading as well as metadata introspection. * @return the filter predicate for fully-qualified candidate class names * of transitively imported configuration classes, or {@code null} if none * @since 5.2.4 */
 	@Nullable
 	default Predicate<String> getExclusionFilter() { 
 		return null;
 	}
 
 }

接口文档已经说的很明白，其主要作用是收集需要导入的配置类，如果该接口的实现类同时实现了org.springframework.beans.factory.Aware相关接口，如：EnvironmentAware、BeanFactoryAware、BeanClassLoaderAware、ResourceLoaderAware等，那么在调用其selectImports方法之前先调用上述接口中的回调方法；如果需要在所有的@Configuration处理完再导入，可以实现DeferredImportSelector接口；

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##### 2.DeferredImportSelector接口源码解析 #####

DeferredImportSelector接口是ImportSelector接口的子接口，该接口会在所有的@Configuration配置类（不包括自动化配置类，即spring.factories文件中的配置类）处理完成后运行；当选择器和@Conditional条件注解一起使用时是特别有用的，此接口还可以和接口Ordered或者@Ordered一起使用，定义多个选择器的优先级；

/** * A variation of {@link ImportSelector} that runs after all {@code @Configuration} beans * have been processed. This type of selector can be particularly useful when the selected * imports are {@code @Conditional}. * * Implementations can also extend the {@link org.springframework.core.Ordered} * interface or use the {@link org.springframework.core.annotation.Order} annotation to * indicate a precedence against other {@link DeferredImportSelector DeferredImportSelectors}. * * Implementations may also provide an {@link #getImportGroup() import group} which * can provide additional sorting and filtering logic across different selectors. * * @author Phillip Webb * @author Stephane Nicoll * @since 4.0 */
 public interface DeferredImportSelector extends ImportSelector { 
 
 	/** * 返回指定的导入结果集 */
 	@Nullable
 	default Class<? extends Group> getImportGroup() { 
 		return null;
 	}
 
 
 	/** * 用于从不同DeferredImportSelector中获取需要导入类的结果集 */
 	interface Group { 
 
 		/** * 根据AnnotationMetadata注解元数据获取@Configuration配置的@Import注解导入的DeferredImportSelector选择器对应的bean */
 		void process(AnnotationMetadata metadata, DeferredImportSelector selector);
 
 		/** * 返回类应该导入的Entry */
 		Iterable<Entry> selectImports();
 
 
 		/** * 存放要导入类的全限定名及AnnotationMetadata注解元数据 */
 		class Entry { 
 
 			private final AnnotationMetadata metadata;
 
 			private final String importClassName;
 
 			public Entry(AnnotationMetadata metadata, String importClassName) { 
 				this.metadata = metadata;
 				this.importClassName = importClassName;
 			}
 
 			/** * 返回要引入的Configuration类的AnnotationMetadata注解元数据 */
 			public AnnotationMetadata getMetadata() { 
 				return this.metadata;
 			}
 
 			/** * 返回要导入类的全限定名 */
 			public String getImportClassName() { 
 				return this.importClassName;
 			}
 
 			@Override
 			public boolean equals(@Nullable Object other) { 
 				if (this == other) { 
 					return true;
 				}
 				if (other == null || getClass() != other.getClass()) { 
 					return false;
 				}
 				Entry entry = (Entry) other;
 				return (this.metadata.equals(entry.metadata) && this.importClassName.equals(entry.importClassName));
 			}
 
 			@Override
 			public int hashCode() { 
 				return (this.metadata.hashCode() * 31 + this.importClassName.hashCode());
 			}
 
 			@Override
 			public String toString() { 
 				return this.importClassName;
 			}
 		}
 	}
 
 }

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##### 3.示例，AutoConfigurationImportSelector是DeferredImportSelector接口的实现类，用于处理EnableAutoConfiguration自动化配置， #####

我们知道SpringFactoriesLoader类是自动化配置的核心类，用来将spring.factories配置文件中定义的类加载到内存之中，供后面的程序将其注册到IOC容器之中；AutoConfigurationImportSelector类是DeferredImportSelector接口的一个子类，它的作用就是将SpringFactoriesLoader类加载到内中的配置类获取到，交给后置处理器加载到内存中（不是本文重点）；

AutoConfigurationGroup是一个静态内部类，实现了DeferredImportSelector.Group接口，所以其作用是根据注解的AnnotationMetadata元数据获取导入的DeferredImportSelector接口实现类对应的自动化配置类；

*  AutoConfigurationImportSelector.AutoConfigurationGroup类的核心方法process用于获取自动化配置类：

//存放配置类全限定名和注解元数据类
 		private final Map<String, AnnotationMetadata> entries = new LinkedHashMap<>();
 		//存放配置类的实体对象（包含需要导入的配置类、排除的配置类）
 		private final List<AutoConfigurationEntry> autoConfigurationEntries = new ArrayList<>();		
 		@Override
 		public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) { 
 			//获取spring.factories配置文件中的配置类（包括需要导入的、不需要导入的）
 			AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
 					.getAutoConfigurationEntry(annotationMetadata);
 			this.autoConfigurationEntries.add(autoConfigurationEntry);
 			for (String importClassName : autoConfigurationEntry.getConfigurations()) { 
 				this.entries.putIfAbsent(importClassName, annotationMetadata);
 			}
 		}

*  AutoConfigurationEntry类用于存放排除掉的配置类，以及需要导入的配置类：

protected static class AutoConfigurationEntry { 
 		//需要导入的配置类
 		private final List<String> configurations;
 		//排除不用导入的配置类
 		private final Set<String> exclusions;
 }

*  AutoConfigurationImportSelector\#getAutoConfigurationEntry方法获取基于配置类注解的AnnotationMetaData元数据导入Configuration配置类

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) { 
 		if (!isEnabled(annotationMetadata)) { 
 			return EMPTY_ENTRY;
 		}
 //获取注解的属性配置（exclude和excludeName）
 		AnnotationAttributes attributes = getAttributes(annotationMetadata);
 //获取自动化配置文件spirng.factories中的配置类
 		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
 //删除List中重复的配置类（去重方法值得参考）
 		configurations = removeDuplicates(configurations);
 //获取排除导入的配置类（包括spring.autoconfigure.exclude属性配置及注解属性exclude和excludeName）
 		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
 //检验排除类
 		checkExcludedClasses(configurations, exclusions);
 //删除掉排除的类
 		configurations.removeAll(exclusions);
 //获取过滤器，并对配置类进行过滤
 		configurations = getConfigurationClassFilter().filter(configurations);
 //触发自动化配置导入事件
 		fireAutoConfigurationImportEvents(configurations, exclusions);
 		return new AutoConfigurationEntry(configurations, exclusions);
 	}

*  AutoConfigurationImportSelector\#getCandidateConfigurations方法用于获取spring.factories配置文件中的配置类（其实际获取是直接从SpringFactoriesLoader类中的cache获取的，已经在初始化器阶段加载到缓存中了）：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { 
 //获取自动化配置对应spring.factories文件中的配置类，
 		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
 				getBeanClassLoader());
 		return configurations;
 	}

返回加载配置类：

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

*  AutoConfigurationImportSelector\#removeDuplicates方法删除重复的配置类

protected final <T> List<T> removeDuplicates(List<T> list) { 
 		return new ArrayList<>(new LinkedHashSet<>(list));
 	}

很好的去重思路，以后可以参考使用；

*  AutoConfigurationImportSelector\#getExclusions获取排除导入的配置类

protected Set<String> getExclusions(AnnotationMetadata metadata, AnnotationAttributes attributes) { 
 		Set<String> excluded = new LinkedHashSet<>();
 		//获取exclude属性指定的配置类
 		excluded.addAll(asList(attributes, "exclude"));
 		//获取excludeName属性指定的配置类
 		excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName")));
 		//获取spring.autoconfigure.exclude属性指定的配置类
 		excluded.addAll(getExcludeAutoConfigurationsProperty());
 		return excluded;
 	}

AutoConfigurationImportSelector\#getExcludeAutoConfigurationsProperty获取spring.autoconfigure.exclude属性配置类

private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
 	protected List<String> getExcludeAutoConfigurationsProperty() { 
 		Environment environment = getEnvironment();
 		if (environment == null) { 
 			return Collections.emptyList();
 		}
 		if (environment instanceof ConfigurableEnvironment) { 
 			Binder binder = Binder.get(environment);
 //获取配置文件中排除导入配置类
 			return binder.bind(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class).map(Arrays::asList)
 					.orElse(Collections.emptyList());
 		}
 		String[] excludes = environment.getProperty(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class);
 		return (excludes != null) ? Arrays.asList(excludes) : Collections.emptyList();
 	}

*  AutoConfigurationImportSelector\#getConfigurationClassFilter

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;
 	}

*  AutoConfigurationImportSelector\#fireAutoConfigurationImportEvents

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);
 			}
 		}
 	}

--------------------

*  AutoConfigurationImportSelector.AutoConfigurationGroup\#selectImports获取上述process方法处理后的配置类

@Override
 		public Iterable<Entry> selectImports() { 
 			if (this.autoConfigurationEntries.isEmpty()) { 
 				return Collections.emptyList();
 			}
 //获取所有需要排除的配置类
 			Set<String> allExclusions = this.autoConfigurationEntries.stream()
 					.map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());
 //获取所有经过自动化配置过滤器的配置类
 			Set<String> processedConfigurations = this.autoConfigurationEntries.stream()
 					.map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream)
 					.collect(Collectors.toCollection(LinkedHashSet::new));
 //排除过滤后配置类中需要排除的类
 			processedConfigurations.removeAll(allExclusions);
 
 			return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
 					.map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
 					.collect(Collectors.toList());
 		}

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[https_github.com_mingyang66_spring-parent]: https://github.com/mingyang66/spring-parent