springboot情操陶冶-@SpringBootApplication注解解析

承接前文springboot情操陶冶-@Configuration注解解析，本文將在前文的基礎上對@SpringBootApplication注解作下簡單的分析

@SpringBootApplication

該注解是springboot最集中的一個注解，也是應用最廣泛的注解。官方也多用此注解以啟動spring服務，我們看下其中的源碼

@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 {


	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};


	@AliasFor(annotation = EnableAutoConfiguration.class)
	String[] excludeName() default {};


	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};


	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};

}

通過上述代碼可知，其整合了@EnableAutoConfiguration和@ComponentScan兩個主要的注解，並以此作了默認的指定。

1. 屬性exclude和excludeName，其默認為空，指定的話最終由EnableAutoConfiguration.class來完成解析

2. 屬性scanBasePackages和scanBasePackageClasses，其默認為空，指定的話最終由ComponentScan.class來完成解析

3. 默認情況下，以@SpringBootApplication注解的所在類的包名作為beanDefinition掃描路徑。
   掃描過程中屏蔽META-INF\spring.factories文件中org.springframework.boot.autoconfigure.EnableAutoConfiguration屬性指定的class集合

4. 支持多個@ComponentScan注解同時與@SpringBootApplication注解搭配使用

@ComponentScan注解在前文中已了解的差不多了，筆者此處就針對@EnableAutoConfiguration注解作下詳細的分析

@EnableAutoConfiguration

單純從英文單詞上看是開啟自動注入的意思，具體的話筆者還是根據源碼來解讀。首先看下注解的源碼

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

	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 * @return the classes to exclude
	 */
	Class<?>[] exclude() default {};

	/**
	 * Exclude specific auto-configuration class names such that they will never be
	 * applied.
	 * @return the class names to exclude
	 * @since 1.3.0
	 */
	String[] excludeName() default {};

}

它用到了我們前文提及的@Import注解，所以我們關注所引入的AutoConfigurationImportSelector.class

AutoConfigurationImportSelector

其是DeferredImportSelector.class接口的實現類，此處我們直接查看復寫的方法selectImports()

	@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		// spring.boot.enableautoconfiguration屬性讀取，默認為true
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		// read all META-INF\spring-autoconfigure-metadata.properties files
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
				
		// 獲取相應類上@EnableAutoConfiguration對應的屬性
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		
		// read all <org.springframework.boot.autoconfigure.EnableAutoConfiguration> properties from META-INF\spring.factories
		List<String> configurations = getCandidateConfigurations(annotationMetadata,
				attributes);
		configurations = removeDuplicates(configurations);
		
		// read exclude/excludeName property or spring.autoconfigure.exclude property
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		
		// make sure the assignable exclusions are present in classpath and in configurations collection, or will throw exception 
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		// filter the present configurations
		configurations = filter(configurations, autoConfigurationMetadata);
		// fire the AutoConfigurationImportEvent by AutoConfigurationImportListener.class
		fireAutoConfigurationImportEvents(configurations, exclusions);
		
		
		return StringUtils.toStringArray(configurations);
	}

對上述的步驟此處再作下總結

1. 優先判斷環境變量spring.boot.enableautoconfiguration，如果指定為false，則不引入任何類。默認為true

2. 讀取classpath環境下所有的META-INF\spring-autoconfigure-metadata.properties文件，加載其中的所有屬性保存至autoConfigurationMetadata

3. 獲取相應類上@EnableAutoConfiguration對應的屬性，其實也就是exclude屬性和excludeName屬性

4. 讀取classpath環境下所有的META-INF\spring.factories文件中的org.springframework.boot.autoconfigure.EnableAutoConfiguration屬性，得到configurations集合

5. 根據第三步讀取的配置以及spring.autoconfigure.exclude環境變量指定的配置，得到exclusions集合

6. 確保exclusions集合是configurations集合的子集，以及exclusions集合內的class均是存在於classpath環境的。否則異常會拋出

7. 根據上述的exclusions集合篩選出未被過濾的configurations集合。

8. 根據第7點篩選出來的configurations集合，再在autoConfigurationMetadata的基礎上針對ConditionalOnClass屬性篩選一輪
   比如：org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration.ConditionalOnClass=org.springframework.cache.CacheManager
   表示如果要應用CacheAutoConfiguration，則得保證org.springframework.cache.CacheManager類存在

9. 觸發AutoConfigurationImportEvent事件

10. 返回篩選過后的configurations集合

筆者此處羅列下spring-boot-autoconfigure-2.0.3.REALEASE包中的spring.factories中的EnableAutoConfiguration默認屬性，屬性太多，節選如下

# 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,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
...
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
...
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
...
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

當然用戶也可以自定義去實現需要自動注入的配置類。

總結

@SpringBootApplication注解內含@EnableAutoConfiguration注解和@ComponentScan注解，所以這兩個注解結合再搭配上@Configuration注解便可以實現springboot的相關功能了。
在這之中，@EnableAutoConfiguration注解最為重要，其擴展性很好，方便springboot無縫對接不同的插件(NoSql插件、Web插件、JMX協議插件等等)，讀者需要好好分析。

下節預告

通過查閱上述的多個自動注解，發現用到了@Conditional和AutoConfigureAfter注解，都屬於條件判斷，在深入理解springboot整合其他插件前，必須對此兩個注解有深刻的理解。