springboot源码分析系列（三）--@EnableAutoConfiguration自动配置加载过程

r囧r小猫 2021-11-09 04:20 448阅读 0赞

# 为什么需要自动化配置 #

在常规的spring应用程序中，充斥着大量的配置文件，我们需要手动去配置这些文件，如配置组件扫描、视图解析器、http编码等等。常规的配置让开发人员将更多的经历耗费在了配置文件上。而这些配置都是一些固定模式的配置方式，甚至很多都是模板代码。那既然是这样一种情况，有没有一种可能性，让spring自动完成这些模板配置工作呢？答案是肯定的，这就是SpringBoot AutoConfiguration产生的初衷。将开发人员从繁重的配置工作中解放出来，把这些繁琐的配置交由SpringBoot完成，如果我们需要自己配置参数，只需要覆盖自动配置的参数即可。

# SpringBoot自动化配置的核心原理 #

在之前的文章中，我们看过了SpringBoot的核心注解@SpringBootApplication注解的源码。其中有三个注解：@SpringBootConfiguration，@EnableAutoConfiguration，@ComponentScan。真正起自动化配置作用的是@EnableAutoConfiguration。以前我们需要配置的东西，SpringBoot帮我们自动配置，@EnableAutoConfiguration告诉SpringBoot开启自动配置功能，这样自动配置才能生效  
  下面我们来分析一下@EnableAutoConfiguration这个注解的加载过程

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 { };
    }

由源码可知，@EnableAutoConfiguration是一个组合注解，由@AutoConfigurationPackage，@Import(AutoConfigurationImportSelector.class)这两个注解组成。

## @AutoConfigurationPackage ##

@AutoConfigurationPackage的主要作用是自动配置包

/** * Indicates that the package containing the annotated class should be registered with * {@link AutoConfigurationPackages}. * * @author Phillip Webb * @since 1.3.0 * @see AutoConfigurationPackages */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @Import(AutoConfigurationPackages.Registrar.class)
    public @interface AutoConfigurationPackage { 
    
    }

## @Import(AutoConfigurationImportSelector.class) ##

Spring底层注解@Import，给容器中导入一个组件；导入的组件由AutoConfigurationPackages.Registrar.class  
将主配置类（@SpringBootApplication标注的类）的所在包以及下面所有子包里面的所有组件扫描到Spring容器。  
  AutoConfigurationImportSelector的作用是导入哪些组件的选择器。将所有需要导入的组件以全类名的方式返回，这些组件就会被添加到容器中；也会给容器导入非常多的自动配置类（xxxAutoConfiguration），就是给容器中导入这个场景需要的所有组件，并配置好这些组件。  
  有了自动配置类，免去了我们手动编写配置注入功能组件等的工作  
具体的工作流程如下：  
![springboot自动配置][springboot]

# @EnableAutoConfiguration加载过程 #

自动配置主要由AutoConfigurationImportSelector实现的，我们主要从这个类开始讲起。AutoConfigurationImportSelector是@EnableAutoConfiguration“@Import”的DeferredImportSelector实现类，由于DeferredImportSelector作为ImportSelector的子接口，所以组件自动配置逻辑均在selectImports(AnnotationMetadata)方法中实现

自动配置加载过程主要分为以下几个步骤：

*  1.判断是否开启自动配置
 *  2.从META-INF/spring-autoconfigure-metadata.properties文件中载入属性配置
 *  3.获取所有的配置列表

public String[] selectImports(AnnotationMetadata annotationMetadata) { 
    	//1.是否开启自动配置，默认开启
    	if (!isEnabled(annotationMetadata)) { 
    		return NO_IMPORTS;
    	}
    	//2.从META-INF/spring-autoconfigure-metadata.properties文件中载入属性配置（有一些有默认值），获取注解信息
    	AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
    			.loadMetadata(this.beanClassLoader);
    	//3.获取所有的配置列表
    	AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
    			annotationMetadata);
    	return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }

## 1.是否开启自动配置，默认开启 ##

protected boolean isEnabled(AnnotationMetadata metadata) { 
    	if (getClass() == AutoConfigurationImportSelector.class) { 
    		return getEnvironment().getProperty(EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class, true);
    	}
    	return true;
    }

## 2.从META-INF/spring-autoconfigure-metadata.properties文件中载入属性配置 ##

//文件为需要加载的配置类的类路径
    protected static final String PATH = "META-INF/" + "spring-autoconfigure-metadata.properties";
    
    public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) { 
    	return loadMetadata(classLoader, PATH);
    }
    
    static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) { 
    	try { 
    		//1.读取spring-boot-autoconfigure-2.1.6.RELEASE.jar包中pring-autoconfigure-metadata.properties的信息生成urls枚举对象
    		Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
    				: ClassLoader.getSystemResources(path);
    		Properties properties = new Properties();
    		//2.解析urls枚举对象中的信息封装成properties对象并加载
    		while (urls.hasMoreElements()) { 
    			properties.putAll(PropertiesLoaderUtils.loadProperties(new UrlResource(urls.nextElement())));
    		}
    		//3.根据封装好的properties对象生成AutoConfigurationMetadata对象返回
    		return loadMetadata(properties);
    	}
    	catch (IOException ex) { 
    		throw new IllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", ex);
    	}
    }

## 3.获取所有的配置列表 ##

/** * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata} * of the importing {@link Configuration @Configuration} class. * @param autoConfigurationMetadata the auto-configuration metadata * @param annotationMetadata the annotation metadata of the configuration class * @return the auto-configurations that should be imported */
    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
    		AnnotationMetadata annotationMetadata) { 
    	if (!isEnabled(annotationMetadata)) { 
    		return EMPTY_ENTRY;
    	}
    	//1.将注解元信息封装成注解属性对象
    	AnnotationAttributes attributes = getAttributes(annotationMetadata);
    	//2.获取到配置类的全路径字符串集合
    	List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    	configurations = removeDuplicates(configurations);
    	//需要排除的自动装配类（springboot的主类上 @SpringBootApplication(exclude = {com.demo.XXX.class})指定的排除的自动装配类）
    	Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    	checkExcludedClasses(configurations, exclusions);
    	//将需要排除的类从 configurations remove掉
    	configurations.removeAll(exclusions);
    	//过滤掉不需要装配的类。过滤的逻辑有很多，比如我们常用的@ConditionXXX注解
    	configurations = filter(configurations, autoConfigurationMetadata);
    	fireAutoConfigurationImportEvents(configurations, exclusions);
    	return new AutoConfigurationEntry(configurations, exclusions);
    }

### 3.1将注解元信息封装成注解属性对象 ###

/** * Return the appropriate {@link AnnotationAttributes} from the * {@link AnnotationMetadata}. By default this method will return attributes for * {@link #getAnnotationClass()}. * @param metadata the annotation metadata * @return annotation attributes */
    protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) { 
    	String name = getAnnotationClass().getName();
    	AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(name, true));
    	Assert.notNull(attributes, () -> "No auto-configuration attributes found. Is " + metadata.getClassName()
    			+ " annotated with " + ClassUtils.getShortName(name) + "?");
    	return attributes;
    }

### 3.2获取到配置类的全路径字符串集合 ###

getCandidateConfigurations(annotationMetadata, attributes);这个方法中有一个重要方法loadFactoryNames，这个方法是让SpringFactoryLoader去加载一些组件的名字

/** * Return the auto-configuration class names that should be considered. By default * this method will load candidates using {@link SpringFactoriesLoader} with * {@link #getSpringFactoriesLoaderFactoryClass()}. * @param metadata the source metadata * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation * attributes} * @return a list of candidate configurations */
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { 
    	/** * 这个方法需要传入两个参数getSpringFactoriesLoaderFactoryClass()和getBeanClassLoader() * getSpringFactoriesLoaderFactoryClass()这个方法返回的是EnableAutoConfiguration.class * getBeanClassLoader()这个方法返回的是beanClassLoader（类加载器） */
    	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;
    }
    
    /** * Return the class used by {@link SpringFactoriesLoader} to load configuration * candidates. * @return the factory class */
    protected Class<?> getSpringFactoriesLoaderFactoryClass() { 
    	return EnableAutoConfiguration.class;
    }
    
    protected ClassLoader getBeanClassLoader() { 
    	return this.beanClassLoader;
    }

下面来看下loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),getBeanClassLoader());这个方法

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { 
        //factoryClassName为org.springframework.boot.autoconfigure.EnableAutoConfiguration
        String factoryClassName = factoryClass.getName();
        //该方法返回的是所有spring.factories文件中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的类路径
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }
    
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { 
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if(result != null) { 
            return result;
        } else { 
            try { 
    	        //如果类加载器不为null，则加载类路径下spring.factories文件，将其中设置的配置类的全路径信息封装 为Enumeration类对象
                Enumeration<URL> urls = classLoader != null?classLoader.getResources("META-INF/spring.factories"):ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();
    
    			//循环Enumeration类对象，根据相应的节点信息生成Properties对象，通过传入的键获取值，在将值切割为一个个小的字符串转化为Array，方法result集合中
                while(urls.hasMoreElements()) { 
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    //读取文件内容，properties类似于HashMap，包含了属性的key和value
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();
    
                    //属性文件中可以用','分割多个value
                    while(var6.hasNext()) { 
                        Entry<?, ?> entry = (Entry)var6.next();
                        String factoryClassName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;
    
                        for(int var11 = 0; var11 < var10; ++var11) { 
                            String factoryName = var9[var11];
                            result.add(factoryClassName, factoryName.trim());
                        }
                    }
                }
    
                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) { 
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

# 总结 #

springboot底层实现自动配置的步骤：

*  1.springboot应用启动
 *  2.@SpringBootApplication起作用
 *  3.@EnableAutoConfiguration
 *  4.@AutoConfigurationPackage：这个组合注解主要是@Import(AutoConfigurationPackages.Registrar.class)，它通过将Registrar类导入到容器中，而Registrar类作用是扫描主配置类同级目录以及子包，并将相应的组件导入到springboot创建管理的容器中
 *  5.@Import(AutoConfigurationImportSelector.class)：它通过将AutoConfigurationImportSelector类导入到容器中，AutoConfigurationImportSelector类作用是通过selectImports方法实现将配置类信息交给SpringFactory加载器进行一系列的容器创建过程，具体实现可查看上面的源码

[springboot]: /images/20211109/c4a2ca84dc9e412dbd70442330a93ddc.png