版本
<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.3.4.RELEASE</version><relativePath/> <!-- lookup parent from repository --> </parent>
我们进入SpringBootApplication注解看看有什么
@SpringBootApplication
public class DemoApplication {public static void main(String[] args) {SpringApplication.run(DemoApplication.class, args);}
}我们可以发现有一个@EnableAutoConfiguration注解，这个是实现自动装配的，继续进去看看
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}
), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
***
}我们又看到了一个@Import({AutoConfigurationImportSelector.class})
他代表引入了AutoConfigurationImportSelector.class这个类，核心类，我们看看他的内容
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";Class<?>[] exclude() default {};String[] excludeName() default {};
}可以看到他继承了DeferredImportSelector类，这个类继承了ImportSelector 类。
然后前面说这个类是@Import引入的
@Import有三种使用的方法，其中一种，如果引入的类实现了ImportSelector接口，那么不会把引入的类加入容器，而是实现ImportSelector接口下的一个方法selectImports。
- 导入普通类
- 导入实现了ImportSelector接口的类
- 导入实现了ImportBeanDefinitionRegistrar接口的类
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
****
}
public interface DeferredImportSelector extends ImportSelector {
***
}这一句就是获取自动装配类的核心代码，我们进入到getAutoConfigurationEntry这个方法中去看，它是怎么获取配置的。
AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return NO_IMPORTS;} else {AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}}这个方法中this.getCandidateConfigurations(annotationMetadata, attributes)就是获取候选的配置类，接下来，我们就进入到getCandidateConfigurations这个方法中去查看SpringBoot是怎么获取这些候选的配置类。
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);configurations.removeAll(exclusions);configurations = this.getConfigurationClassFilter().filter(configurations);this.fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);}}List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());又是这个熟悉的方法SpringFactoriesLoader.loadFactoryNames（）
这个方法在springboot启动流程中用到很多次，他就是去meta-inf下加载spring.factories文件。
利用SPI机制去加载配置类
SPI流程：
- 有关组织和公式定义接口标准
- 第三方提供具体实现: 实现具体方法, 配置 META-INF/services/${interface_name} 文件
- 开发者使用
SPI与API区别：
- API是调用并用于实现目标的类、接口、方法等的描述；
- SPI是扩展和实现以实现目标的类、接口、方法等的描述；
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;}Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
跟我们上面说的一致。
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");LinkedMultiValueMap 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()) {Entry<?, ?> entry = (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);}}}参考
深入理解 Java 中 SPI 机制 - 知乎
SpringBoot自动装配原理源码分析（详细）_springboot自动装配源码解析-CSDN博客
