Spring boot 自动装配原理

SpringBoot最核心的还是自动配置，为什么它能做到开箱即用，不再需要我们手动使用@EnableXXX等注解来开启？这一切的答案就在@SpringBootApplication注解中：

@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 {}

这里重要的注解有三个：@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan。@ComponentScan就不用再说了，@SpringBootConfiguration等同于@Configuration，而@EnableAutoConfiguration就是开启自动配置：

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

}

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

}

@AutoConfigurationPackage注解的作用就是将该注解所标记类所在的包作为自动配置的包，简单看看就行，主要看
AutoConfigurationImportSelector，这个就是实现自动配置的核心类，注意这个类是实现的DeferredImportSelector接口。

在这个类中有一个selectImports方法。这个方法在我之前的文章这一次搞懂Spring事务注解的解析也有分析过，只是实现类不同，它同样会被
ConfigurationClassPostProcessor类调用，先来看这个方法做了些什么：

	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		// 获取所有的自动配置类
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
				annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}

	protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		// SPI获取EnableAutoConfiguration为key的所有实现类
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		// 把某些自动配置类过滤掉
		configurations = filter(configurations, autoConfigurationMetadata);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		// 包装成自动配置实体类
		return new AutoConfigurationEntry(configurations, exclusions);
	}

	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
		// SPI获取EnableAutoConfiguration为key的所有实现类
		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;
	}