008-Spring Boot @EnableAutoConfiguration深入分析、内部如何使用EnableAutoConfiguration

1、EnableAutoConfiguration原理

springboot程序入口使用注解@SpringBootApplication，SpringBootApplication注解的定义如下

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

头部有三个注解，

@SpringBootConfiguration：表示是一个配置类

@ComponentScan：自动扫描，

@EnableAutoConfiguration：

作用：从classpath中搜索所有META-INF/spring.factories配置文件，然后将其中org.springframework.boot.autoconfigure.EnableAutoConfiguration 的key对应的配置项加载到容器中。

只有spring.boot.enableautoconfiguration 为true【默认为true】的时候，才启用自动配置

关键点：1》、ImportSelector改接口的方法的返回值都会被纳入到spring容器管理中；2》、SpringFactoriesLoader该类可以从classpath中搜索所有的 META-INF/spring.factories配置文件，并读取配置，交由ImportSelector接口纳入spring处理

1.5版本以前

@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

查看EnableAutoConfigurationImportSelector 查看父类AutoConfigurationImportSelector

    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                    attributes);
            configurations = removeDuplicates(configurations);
            configurations = sort(configurations, autoConfigurationMetadata);
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = filter(configurations, autoConfigurationMetadata);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }

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