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@SpringBootApplication注解

spring boot项目一般都会写如下代码来启动项目。

@SpringBootApplication  
public class Application {  
    public static void main(String[] args) {  
        SpringApplication.run(Application.class, args);  
    }  
}

今天就主要看一下@SpringBootApplication注解的作用。

@SpringBootApplication注解代码

@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 {
}

其中的@SpringBootConfiguration就是@Configuration

@EnableAutoConfiguration注解的代码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}

会导入一个AutoConfigurationImportSelector的类。 AutoConfigurationImportSelector类中selectImports方法其实就是在查找需要导入的@Configuration注解的类。 selectImports方法会调用getCandidateConfigurations方法，

public String[] selectImports(AnnotationMetadata annotationMetadata) {
	if (!isEnabled(annotationMetadata)) {
		return NO_IMPORTS;
	}
	AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
			.loadMetadata(this.beanClassLoader);
	AnnotationAttributes attributes = getAttributes(annotationMetadata);
	List<String> configurations = getCandidateConfigurations(annotationMetadata,
			attributes);
	configurations = removeDuplicates(configurations);
	Set<String> exclusions = getExclusions(annotationMetadata, attributes);
	checkExcludedClasses(configurations, exclusions);
	configurations.removeAll(exclusions);
	configurations = filter(configurations, autoConfigurationMetadata);
	fireAutoConfigurationImportEvents(configurations, exclusions);
	return StringUtils.toStringArray(configurations);
}
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;
}

在这里又会调用SpringFactoriesLoader类的静态方法loadFactoryNames，来获取classloader的jar包中所有META-INF/spring.factories中配置了key为EnableAutoConfiguration的所有类。

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
	String factoryClassName = factoryClass.getName();
	return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
	MultiValueMap<String, String> result = cache.get(classLoader);
	if (result != null) {
		return result;
	}

	try {
		Enumeration<URL> urls = (classLoader != null ?
				classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
		result = new LinkedMultiValueMap<>();
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			UrlResource resource = new UrlResource(url);
			Properties properties = PropertiesLoaderUtils.loadProperties(resource);
			for (Map.Entry<?, ?> entry : properties.entrySet()) {
				List<String> factoryClassNames = Arrays.asList(
						StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
				result.addAll((String) entry.getKey(), factoryClassNames);
			}
		}
		cache.put(classLoader, result);
		return result;
	}
	catch (IOException ex) {
		throw new IllegalArgumentException("Unable to load factories from location [" +
				FACTORIES_RESOURCE_LOCATION + "]", ex);
	}
}

打开spring-boot-autoconfigure-x.x.x.jar包，可以看到META-INF/spring.factories中配置了很多，这些在启动的时候都会自动实例化

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
...

随便打开一个CacheAutoConfiguration类，可以看到这个类有很多的注解

@Configuration
@ConditionalOnClass(CacheManager.class)
@ConditionalOnBean(CacheAspectSupport.class)
@ConditionalOnMissingBean(value = CacheManager.class, name = "cacheResolver")
@EnableConfigurationProperties(CacheProperties.class)
@AutoConfigureAfter({ CouchbaseAutoConfiguration.class, HazelcastAutoConfiguration.class,
		HibernateJpaAutoConfiguration.class, RedisAutoConfiguration.class })
@Import(CacheConfigurationImportSelector.class)
public class CacheAutoConfiguration {
    ...
}

SpringBoot中提供了一系列的@ConditionalOnXXX的注解，称为条件注解。对这些条件注解的支持也是SpringBoot的核心思想，正是通过这些条件注解，SpringBoot做到了需要什么功能，就增加什么依赖包这种动态可插拔的机制。

---

2019-11-17 补充：

其实之前理解的org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#selectImports不太准确。之前理解的是查找需要导入的@Configuration注解的类，这两天发现并不是查找@Configuration，而是查找spring-autoconfigure-metadata.properties中带有ConditionalOnClass文字的类，参考SpringBoot的spring-autoconfigure-metadata.properties配置文件作用