SpringBoot自动配置简单梳理

一、自动配置的作用

1.将第三方的组件自动装载到IOC容器里面，不需要开发人员再去编写相关的配置，比如说我们在pom.xml中添加启动器spring-boot-starter-data-redis之后，我们就可以直接使用@Autowired来注入RedisTemplate，这就是自动配置的好处。
2.本文涉及到@Import注解的使用、SpringFactories机制

二、自动配置的原理

1.首先他是通过启动类上的@SpringBootApplication实现的，其中包含两个注解@SpringBootConfiguration、@EnableAutoConfiguration，而@SpringBootConfiguration可以看做@Configuration注解，主要实现自动配置的注解是@EnableAutoConfiguration

2.使用@Import并定义AutoConfigurationImportSelector来实现批量注册（不知道@Import作用的同学建议查一下该注解），然后我们点进去其中有个getAutoConfigurationEntry

public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }

3.进入该方法getCandidateConfigurations

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);
        }
    }

4.可以看到他调用了SpringFactoriesLoader.loadFactoryNames方法，最终调用该类的loadSpringFactories，该方法可以让ClassLoader读取所有的jar包下的META-INF/spring.factories下的文件信息，该文件包含了第三方jar包所提供的@Configuration配置类的全限定类名

    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;
    }

5.如在Spring-boot-autoconfigure模块的spring.factories中