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什么是springBoot的自动装配

SpringBoot 在启动时会扫描外部引用 jar 包中的META-INF/spring.factories文件，将文件中配置的类型信息加载到 Spring 容器

springBoot如何实现自动装配

在springBoot启动类上有一个复合注解@SpringBootApplication

@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 {

@SpringBootApplication中有三个注解
1、@SpringBootConfiguration：允许在上下文注册额外的bean或者引入其他的配置

2、@EnableAutoConfiguration 启动springBoot自动装配

3、ComponentScan：扫描注解的类（@Component、@Service、@Controller），排除使用excludeFilters 

@EnableAutoConfiguration：实现自动装配        

@AutoConfigurationPackage //作用：将main包下的所欲组件注册到容器中
@Import({AutoConfigurationImportSelector.class}) //加载自动装配类 xxxAutoconfiguration
public @interface EnableAutoConfiguration {

引入AutoConfigurationImportSelector 实现自动装配

AutoConfigurationImportSelector

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
}

public interface DeferredImportSelector extends ImportSelector {}

public interface ImportSelector {
    String[] selectImports(AnnotationMetadata importingClassMetadata);
}

AutoConfigurationImportSelector实现了DefaultImportSelector，最终实现ImportSelector 执行selectImports方法        

在AutoConfigurationImportSelector 实现中

	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		//判断是否开始自动装配
        if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
        //获取所有自动装配的bean
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}

在getAutoConfigurationEntry方法中获取需要装配的bean

 protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
        //1、是否开启自动装配
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        //2、用于获取EnableAutoConfiguration注解中的 exclude 和 excludeName
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        //3、获取需要自动装配的所有配置类，读取META-INF/spring.factories
        List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        //4、筛选所有自动装配的bean 只有符合条件的才可以引入：@ConditionalOnXXX 中的所有条件都满足，该类才会生效。
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = getConfigurationClassFilter().filter(configurations);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }

getCandidateConfigurations()

获取所有候选的配置，候选的配置来源就是方法实现中的

	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;
	}

loadFactoryNames获取所有的配置类，如果配置信息是空的，就会抛出异常说在META-INF/spring.factories中没有配置信息；就剋看出在这里加载的是spring.factories下的配置信息

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