Springboot启动分析（四）

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本文深入剖析Spring Boot应用启动过程，详细解读@SpringBootApplication注解的工作原理，包括如何通过@EnableAutoConfiguration实现自动配置，以及AutoConfigurationImportSelector类如何选择并导入合适的配置类。

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一，程序入口

@SpringBootApplication
public class DemoApplication {
   public static void main(String[] args) {
       SpringApplication.run(DemoApplication.class, args);
   }
}

二，进入注解@SpringBootApplication

@Target(ElementType.TYPE) //只能作用类上
@Retention(RetentionPolicy.RUNTIME) //运行起作用
@Documented //说明该注解将被包含在javadoc中
@Inherited //说明子类可以继承父类中的该注解
@SpringBootConfiguration //springboot的注解包装了springConfiguration
@EnableAutoConfiguration //自动装配的注解
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

三，进入注解@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage //自动扫描包
@Import(AutoConfigurationImportSelector.class) //导入类
public @interface EnableAutoConfiguration {

String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

四，进入注解@AutoConfigurationPackage

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

五，进入AutoConfigurationPackages.Registrar.class类中

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

       @Override
       public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
           register(registry, new PackageImport(metadata).getPackageName());
       }

       @Override
       public Set<Object> determineImports(AnnotationMetadata metadata) {
           return Collections.singleton(new PackageImport(metadata));
       }

}

　　　可以看到把当前启动类所有的包注入进入spring里面，相当于扫描了（如果没有注册到可以用注解@CompontScan扫描）当前启动类所在的包。

六，返回注解@EnableAutoConfiguration点击进入

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

七，进入AutoConfigurationImportSelector.class类（断点）

   @Override
   public String[] selectImports(AnnotationMetadata annotationMetadata) {
       if (!isEnabled(annotationMetadata)) {
           return NO_IMPORTS;
       }
       AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
               .loadMetadata(this.beanClassLoader);
       AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
               annotationMetadata);
       return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
   }

   /**
   * 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;
       }
       AnnotationAttributes attributes = getAttributes(annotationMetadata);

//读取springboot里面提供的所有的118个配置类
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
configurations = removeDuplicates(configurations);

//根据pom.xml引入的依赖检查哪些没有用到的配置类
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);

//把没有用到的配置类从configurations移除
       configurations.removeAll(exclusions);
       configurations = filter(configurations, autoConfigurationMetadata);
       fireAutoConfigurationImportEvents(configurations, exclusions);
       return new AutoConfigurationEntry(configurations, exclusions);
   }