SpringBoot-自动配置1

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1、想了解自动配置原理的建议先了解注解原理，了解几个基本的元注解：https://www.cnblogs.com/peida/archive/2013/04/24/3036689.html

SpringBoot实现原理：通过扫描META-INF/spring.factories文件，该文件记录了springBoot实现的自动配置列表，通过加载这个列表实现所有的自动配置

2、SpringBootApplication注解：一个组合注解

@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 {@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "exclude")Class<?>[] exclude() default {};@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")String[] excludeName() default {};@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")String[] scanBasePackages() default {};@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")Class<?>[] scanBasePackageClasses() default {};}

核心功能由@EnableAutoConfiguration实现

@EnableAutoConfiguration注解：

@SuppressWarnings("deprecation")@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage@Import(EnableAutoConfigurationImportSelector.class)public @interface EnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";Class<?>[] exclude() default {};String[] excludeName() default {};}

@EnableAutoConfiguration注解通过@Import注解引入EnableAutoConfigtionImportSelectot.class实现主要功能

EnableAutoConfigtionImportSelectot.class继承AutoConfigurationImportSelector

AutoConfigurationImportSelector类通过getCandidateConfigurations加载META-INF/spring.factories信息，获取需要自动配置的类

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,AnnotationAttributes attributes) {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;}

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