springboot配置类自动加载机制

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我是靠谱客的博主老实自行车，最近开发中收集的这篇文章主要介绍springboot配置类自动加载机制，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

简单介绍一下springboot是如何自动识别加载spring.factories里的EnableAutoConfiguration

springboot项目启动类都会使用@SpringBootApplication（或是@EnableAutoConfiguration）注解。而@SpringBootApplication注解实际上也使用了@EnableAutoConfiguration，如下

@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 {
    // 省略其他代码
}

那我们主要看下@EnableAutoConfiguration注解做了些什么事情

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    // 省略其他代码
}

从源码上可以看到，该注解通过@Import注解引入了AutoConfigurationImportSelector.class，主要逻辑也是在这个类里实现的。继续看下这个类的主要一些逻辑：

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
	if (!isEnabled(annotationMetadata)) {
		return NO_IMPORTS;
	}
	// 关注这个方法
	AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
	return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
	if (!isEnabled(annotationMetadata)) {
		return EMPTY_ENTRY;
	}
	AnnotationAttributes attributes = getAttributes(annotationMetadata);
	// 看下此方法
	List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
	configurations = removeDuplicates(configurations);
	Set<String> exclusions = getExclusions(annotationMetadata, attributes);
	checkExcludedClasses(configurations, exclusions);
	configurations.removeAll(exclusions);
	configurations = getConfigurationClassFilter().filter(configurations);
	fireAutoConfigurationImportEvents(configurations, exclusions);
	return new AutoConfigurationEntry(configurations, exclusions);
}

// 返回类名
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;
}

// 默认返回EnableAutoConfiguration.class
// spring.factories文件里配置org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxx
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
	return EnableAutoConfiguration.class;
}

selectImports方法会返回需要import的类名，然后通过ConfigurationClassParser进行解析注入到容器中。