SpringBoot面试题：（一）SpringBoot自动装配原理源码解析

源码研究
SpringBoot启动类：@SpringBootApplication注解

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;@SpringBootApplication
public class SpringBoot1Application {public static void main(String[] args) {SpringApplication.run(SpringBoot1Application.class, args);}
}

进入@SpringBootApplication注解

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}
), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class}
)}
)

可以看到主要是三个注解构成
@SpringBootConfiguration：springboot配置类，spring组件
@EnableAutoConfiguration：开启自动配置
@ComponentScan：组件扫描

重点研究@EnableAutoConfiguration：

@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})

@AutoConfigurationPackage：自动配置包
@Import({AutoConfigurationImportSelector.class})：导入自动配置选择器
进入选择器AutoConfigurationImportSelector.class

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return NO_IMPORTS;} else {AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}}

autoConfigurationEntry 就是自动配置类的信息，进入函数this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata)

    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, 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);configurations.removeAll(exclusions);configurations = this.filter(configurations, autoConfigurationMetadata);this.fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationEntry(configurations, exclusions);}}

this.getCandidateConfigurations(annotationMetadata, attributes)获取配置类信息，并对配置类去重排序

    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;}

通过SpringFactoriesLoader.loadFactoryNames获取配置类信息

    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {String factoryTypeName = factoryType.getName();return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, 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 {Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");MultiValueMap<String, String> result = new LinkedMultiValueMap();while(urls.hasMoreElements()) {URL url = (URL)urls.nextElement();UrlResource resource = new UrlResource(url);Properties properties = PropertiesLoaderUtils.loadProperties(resource);Iterator var6 = properties.entrySet().iterator();while(var6.hasNext()) {Map.Entry<?, ?> entry = (Map.Entry)var6.next();String factoryTypeName = ((String)entry.getKey()).trim();String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());int var10 = var9.length;for(int var11 = 0; var11 < var10; ++var11) {String factoryImplementationName = var9[var11];result.add(factoryTypeName, factoryImplementationName.trim());}}}cache.put(classLoader, result);return result;} catch (IOException var13) {throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);}}}

加载/META-INF/spring.factories文件，k-v的形式读入缓存。
这样，springboot框架就可以读入到所有的配置类信息，在执行run方法时，解析上面的注解，将AutoConfigurationImportSelector加载进方法区，通过反射创建对象，调用其某一个方法，从缓存读取前面存储的k-v,并经过一系列的过滤、去重等，最后将需要的配置类加载，生成BD对象，创建Bean对象，放入spring容器。完成自动装配。

自动装配：自动装配就是把别人（官方）写好的config配置类加载到spring容器，然后根据这个配置类生成一些项目需要的bean对象。
但是自动配置类不一定生效，要根据条件判断是否成立，只要导入对应的start,自动装配才能生效。

@ConditionalOnClass({Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class})
@ConditionalOnMissingBean({WebMvcConfigurationSupport.class})
@AutoConfigureOrder(-2147483638)
@AutoConfigureAfter({DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class, ValidationAutoConfiguration.class})
public class WebMvcAutoConfiguration {

我的理解：通过配置文件spring.factories批量获取配置类
SpringBoot自动装配的本质就是通过Spring去读取META-INF/spring.factories中保存的配置类文件然后加载bean定义的过程。
如果是标了@Configuration注解，就是批量加载了里面的bean定义;
如何实现 “自动”：通过配置文件获取对应的批量配置类，然后通过配置类批量加载bean定义，只要有写好的配置文件spring.factories就实现了自动。
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