springboot自动装配原理简言之，原文标题：SpringBoot自动装配实现原理。

SpringBoot自动装配实现原理

首先我们要知道Spring Boot自动装配的核心注解是@springBootApplication，该注解是spring boot的启动类注解，它是一个复合注解，里面包含
1.@SpringBootConfiguration：

该注解上有一个 @Configuration注解，表示这个spring boot启动类是一个配置类，最终要被注入到spring容器中。

2.@EnableAutoConfiguration

表示开启自动配置，它也是一个复合注解，里面包含a：@AutoConfigurationPackage：该注解上有一个@Import(AutoConfigurationPackages.Registrar.class)注解， 其中 Registrar 类的作用是将启动类所在包下的所有子包的组件扫描注入到spring容器中。b：@Import(AutoConfigurationImportSelector.class)：其中AutoConfigurationImportSelector类中有一个getCandidateConfigurations()方法， 该方法通过SpringFactoriesLoader.loadFactoryNames()方法查找位于META-INF/spring.factories文件中的所有自动配置类，并加载这些类。

所以spring boot在整个的启动过程中，其实就是在类路径的META-INF/spring.factories 文件中找到EnableAutoConfiguration对应的所有的自动配置类，然后将所有自动配置类加载到spring容器中。
我们来看一下流程图

下面我们附上源码
1.@SpringBootApplication中的@EnableAutoConfiguration表示开启自动装配
2.@Import({AutoConfigurationImportSelector.class}) 该注解表示导入该 AutoConfigurationImportSelector类
3.AutoConfigurationImportSelector类下会有一个AutoConfigurationGroup内部类
该内部类下的process方法会先被执行

public void process(annotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) { Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector, () -> String.format("Only %s implementations are supported, got %s", AutoConfigurationImportSelector.class.getSimpleName(), deferredImportSelector.getClass().getName())); AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector) .getAutoConfigurationEntry(annotationMetadata); //保存到autoConfigurationEntries 这个list中 this.autoConfigurationEntries.add(autoConfigurationEntry); //把配置类包装成一个map返回 for (String importClassName : autoConfigurationEntry.getConfigurations()) { this.entries.putifAbsent(importClassName, annotationMetadata); }}

4.然后调用getAutoConfigurationEntry方法

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) { if (!this.isEnabled(annotationMetadata)) { return EMPTY_ENTRY; } else { AnnotationAttributes attributes = this.getAttributes(annotationMetadata); List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes); configurations = this.removeDuplicates(configurations); Set<String> exclusions = this.getExclusions(annotationMetadata, attributes); this.checkExcludedClasses(configurations, exclusions); //删除需要排除的类 configurations.removeAll(exclusions); configurations = this.getConfigurationClassFilter().filter(configurations); this.fireAutoConfigurationImportEvents(configurations, exclusions); //包装成AutoConfigurationEntry类 return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions); }}

5.这个方法会获取SpringBootApplication中的exclude、excludeName排除的类

protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) { String name = this.getAnnotationClass().getName(); AnnotationAttributes attributes = AnnotationAttributes.frommap(metadata.getAnnotationAttributes(name, true)); Assert.notNull(attributes, () -> { return "No auto-configuration attributes found. Is " metadata.getClassName() " annotated with " ClassUtils.getShortName(name) "?"; }); return attributes;}

6.通过spi机制 获取META-INFO下的spring.factoryies中@EnableAutoConfiguration注解
为key的value值，总共有129个

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;}

7.删除重复的

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

8.获取到需要排除的类

protected Set<String> getExclusions(AnnotationMetadata metadata, AnnotationAttributes attributes) { Set<String> excluded = new LinkedHashSet(); excluded.addAll(this.asList(attributes, "exclude")); excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName"))); excluded.addAll(this.getExcludeAutoConfigurationsProperty()); return excluded;}

9.保存到autoConfigurationEntries 这个list中
10.把配置类包装成一个map返回

注：9、10请参照第3步中的源码

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