springboot-自动配置

引导加载自动配置类

总注解@SpringBootApplication

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

1. @SpringBootConfiguration，本质上就是@Configuration。
2. @ComponentScan，指定包扫描路径。
3. @EnableAutoConfiguration部分源码如下：

   @AutoConfigurationPackage                      // 自动注册@SpringBootApplication类所在包的bean
   @Import(AutoConfigurationImportSelector.class) // 自动导入所有符合自动配置条件的bean并加载到容器
   public @interface EnableAutoConfiguration

   （1）注解@AutoConfigurationPackage给IOC容器中导入了一个Registrar组件：

   @Import(AutoConfigurationPackages.Registrar.class) 
   public @interface AutoConfigurationPackage

   Registrar实现了ImportBeanDefinitionRegistrar接口与，目的就是向IOC容器了批量注册@SpringBootApplication标注类所在包下的所有组件。

   static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
    @Override // 批量注册组件
    public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
        // new PackageImport(metadata).getPackageName()
        // => @SpringBootApplication标注类所在的包
        register(registry, new PackageImport(metadata).getPackageName());
        // 批量注册这个包下的组件
    }
   }

   （2）@Import(AutoConfigurationImportSelector.class)通过@Import注解向IOC容器中导入自动配置选择器组件。 ```java // [AutoConfigurationImportSelector]

// 1. 获取所有自动配置 selectImports() => getAutoConfigurationEntry()

// 2. 获取所有需要导入到容器中配置类 getAutoConfigurationEntry() => getCandidateConfigurations()

// 3. 获取配置类后，移除重复，排除一些，过滤一些，检查一下

// [SpringFactoriesLoader]

// 4. 使用SpringFactoriesLoader加载 getCandidateConfigurations() => SpringFactoriesLoader.loadFactoryNames()

// 5. 从META-INF/spring.factories位置加载文件，默认扫描当前系统里面所有META-INF/spring.factories loadFactoryNames() => loadSpringFactories()

Enumeration urls = classLoader != null ? classLoader.getResources(“META-INF/spring.factories”) : ClassLoader.getSystemResources(“META-INF/spring.factories”)

// 核心：所有spring-boot-autoconfigure包下的META-INF/spring.factories文件，所有关于org.springframework.boot.autoconfigure的配置项

用一张图总结一下：<br />![image.png](https://cdn.nlark.com/yuque/0/2022/png/493248/1641956307051-b982c356-96af-497e-ac0a-2387d86364fb.png#clientId=uca1b03bf-bafb-4&crop=0&crop=0&crop=1&crop=1&from=paste&height=580&id=ucb7f4611&margin=%5Bobject%20Object%5D&name=image.png&originHeight=580&originWidth=1440&originalType=binary&ratio=1&rotation=0&showTitle=false&size=102060&status=done&style=shadow&taskId=u1a86891c-3d20-40a2-ae21-946944b54db&title=&width=1440)
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## 按需开启自动配置项
虽然spring.factories的自动配置项默认全部加载，但是最终会按需配置。<br />拿`AopAutoConfiguration`的内部类举例，条件装配：
- `@ConditionalOnClass`：必须导入相关依赖才能生效
- `@ConditionalOnProperty`：必须配置文件中配置才能生效
```java
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(Advice.class)
static class AspectJAutoProxyingConfiguration {
    @Configuration(proxyBeanMethods = false)
    @EnableAspectJAutoProxy(proxyTargetClass = false)
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)
    static class JdkDynamicAutoProxyConfiguration { }
    @Configuration(proxyBeanMethods = false)
    @EnableAspectJAutoProxy(proxyTargetClass = true)
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true) 
    static class CglibAutoProxyConfiguration { }
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnMissingClass("org.aspectj.weaver.Advice") // 缺失这个类时生效
    @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true) // 默认开启AOP
    static class ClassProxyingConfiguration {
        ClassProxyingConfiguration(BeanFactory beanFactory) {
            if (beanFactory instanceof BeanDefinitionRegistry) {
                BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
                AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
                AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            }
        }
    }
}