SpringBoot学习（五）：自动配置的源码实现（一）@EnableAutoConfiguration详解

柔光的暖阳◎ 2022-03-20 20:29 212阅读 0赞

*  这篇文章是 [SpringBoot学习（五）：自动配置的源码实现  
    ][SpringBoot_]系列的第一篇。

### @EnableAutoConfiguration注解 ###

*  @EnableAutoConfiguration为@SpringBootApplication注解的其中一个注解，作用是激活SpringBoot的自动配置功能。注解的定义如下：可以看到包含@AutoConfigurationPackage注解，通过@Import注解导入AutoConfigurationImportSelector。其中@AutoConfigurationPackage自身会导入AutoConfigurationPackages.Registrar，故@EnableAutoConfiguration注解导入了两个类AutoConfigurationImportSelector和AutoConfigurationPackages.Registrar。
 *  其中自动配置主要是由AutoConfigurationImportSelector来完成的。AutoConfigurationImportSelector是DeferredImportSelector接口的实现类，在selectImports方法中，调用SpringFactoriesLoader.loadFactoryNames从META-INF/spring.factories文件中获取EnableAutoConfiguration对应的类列表，其中该类列表为需要自动配置的类的全限定名的列表，如org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration；
    
        @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 { };
        
        }
        
        @Target(ElementType.TYPE)
        @Retention(RetentionPolicy.RUNTIME)
        @Documented
        @Inherited
        @Import(AutoConfigurationPackages.Registrar.class)
        public @interface AutoConfigurationPackage { 
        
        }
 *  所以Spring容器在处理（使用ConfigurationClassPostProcessor，BeanFactoryPostProcessor接口的实现类）包含这个@EnableAutoConfiguration注解的类时，会处理内部的@Import注解来完成以上两个类的处理。

#### AutoConfigurationImportSelector ####

*  具体实现通过SpringFactoriesLoader.loadFactoryNames来从META-INF/spring.factories文件中，获取使用@Configuration注解的类全限定名列表。Spring容器会加载每个全限定名对应的配置类，并基于条件化配置，将该每个配置类内部使用@Bean注解的方法对应的Bean，可选地注册到Spring容器。类定义如下：实现了DeferredImportSelector接口，DeferredImportSelector继承于ImportSelector接口。
    
        /** * {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration * auto-configuration}. This class can also be subclassed if a custom variant of * {@link EnableAutoConfiguration @EnableAutoConfiguration} is needed. * * @since 1.3.0 * @see EnableAutoConfiguration */
        public class AutoConfigurationImportSelector
        	implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
        	BeanFactoryAware, EnvironmentAware, Ordered { 
        	
        	...
        	
        }

#### ImportSelector ####

*  接口定义如下：
    
        /** * 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. * ... * @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. */
        	String[] selectImports(AnnotationMetadata importingClassMetadata);
        
        }
 *  ImportSelector接口与@Import功能差不多，主要用来导入使用@Configuration注解的类，不过ImportSelector可以更方便编程方式导入多个类，并支持条件化决定是否需要导入。接口定义如下：
    
        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. */
        	String[] selectImports(AnnotationMetadata importingClassMetadata);
        
        }

#### DeferredImportSelector：延迟版ImportSelector ####

*  DeferredImportSelector接口与ImportSelector接口的主要区别是：该接口实现类定义的需要导入Spring容器的类是延迟导入的。即会在所有其他@Configuration注解的类的BeanDefinition都被ConfigurationClassParser处理完之后，该接口实现类定义的类列表的导入操作，才被ConfigurationClassParser执行。ConfigurationClassParser的parse方法的源码实现如下：
    
        public void parse(Set<BeanDefinitionHolder> configCandidates) { 
        
            // 处理其他使用@Configuration注解的类
            
        	for (BeanDefinitionHolder holder : configCandidates) { 
        		BeanDefinition bd = holder.getBeanDefinition();
        		try { 
        			if (bd instanceof AnnotatedBeanDefinition) { 
        				parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
        			}
        			else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) { 
        				parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
        			}
        			else { 
        				parse(bd.getBeanClassName(), holder.getBeanName());
        			}
        		}
        		catch (BeanDefinitionStoreException ex) { 
        			throw ex;
        		}
        		catch (Throwable ex) { 
        			throw new BeanDefinitionStoreException(
        					"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
        		}
        	}
        	
        	
            // 延迟执行deferredImportSelector定义的类的处理
        	this.deferredImportSelectorHandler.process();
        }
 *  DeferredImportSelector接口，通常用来处理条件化注解@Conditional系列的类的导入，即由于应用定义的所有@Configuration注解的类都已经被处理完了，故可以检查该接口定义的需要导入的类是否已经在应用中定义过了，定义过则不导入，从而实现如@ConditionalOnMissingBean，@ConditionalOnMissingClass等条件化注解的语义。
 *  DeferredImportSelector接口定义如下：
    
        /** * 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}. */
        public interface DeferredImportSelector extends ImportSelector { 
        
        	/** * Return a specific import group or {@code null} if no grouping is required. * @return the import group class or {@code null} */
        	@Nullable
        	default Class<? extends Group> getImportGroup() { 
        		return null;
        	}
        	
        	// 同一类型的importSelector（可以是一个importSelector实例或者属于一个importSelector类型的多个importSelector实例）
        	// 所导入的多个importedClass为属于一个Group
        
        	/** * Interface used to group results from different import selectors. */
        	interface Group { 
        
        		/** * Process the {@link AnnotationMetadata} of the importing @{@link Configuration} * class using the specified {@link DeferredImportSelector}. */
        		void process(AnnotationMetadata metadata, DeferredImportSelector selector);
        
                // // 从META-INF/spring.factories文件的EnableAutoConfigure注解
        		// 对应的需要自动加载的类全限定名列表
        		// 该列表都是String类型
        		
        		/** * Return the {@link Entry entries} of which class(es) should be imported for this * group. */
        		Iterable<Entry> selectImports();
        
                // 一个Entry代表一个需要导入的类
                
        		/** * An entry that holds the {@link AnnotationMetadata} of the importing * {@link Configuration} class and the class name to import. */
        		class Entry { 
        
        			private final AnnotationMetadata metadata;
        
        			private final String importClassName;
        
        		    ...
        		    
        		}
        	}
        
        }
    
     *  Group接口：包含了一个Group内部接口，Group接口又定义了一个内部类Entry，其中Group接口为同一类型的importSelector实例集合，所导入的多个importedClass为属于一个Group，即可以是一个importSelector实例或者属于一个importSelector类型的多个importSelector实例。DeferredImportSelectorGrouping为Group接口的一个实现类，所包含的importSelector的类型都是DeferredImportSelector。
     *  Group.Entry：一个Entry代表一个需要导入的类，包含类全限定名importClassName，类注解信息metadata两个属性。每个Group可以包含多个Entry，即包含多个需要导入的类。
 *  该系列下一篇文章为：[SpringBoot学习（五）：自动配置的源码实现（二）Spring容器对自动配置的加载  
    ][SpringBoot_Spring_]

[SpringBoot_]: https://blog.csdn.net/u010013573/article/details/86737899
[SpringBoot_Spring_]: https://blog.csdn.net/u010013573/article/details/86743857