springboot情操陶冶-@SpringBootApplication注解解析

Posted 2020-12-28 南柯问天

承接前文springboot情操陶冶-@Configuration注解解析，本文将在前文的基础上对@SpringBootApplication注解作下简单的分析

@SpringBootApplication

该注解是springboot最集中的一个注解，也是应用最广泛的注解。官方也多用此注解以启动spring服务，我们看下其中的源码

@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 {


	@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 {};

}

通过上述代码可知，其整合了@EnableAutoConfiguration和@ComponentScan两个主要的注解，并以此作了默认的指定。

1. 属性exclude和excludeName，其默认为空，指定的话最终由EnableAutoConfiguration.class来完成解析

2. 属性scanBasePackages和scanBasePackageClasses，其默认为空，指定的话最终由ComponentScan.class来完成解析

3. 默认情况下，以@SpringBootApplication注解的所在类的包名作为beanDefinition扫描路径。
   扫描过程中屏蔽META-INF\\spring.factories文件中org.springframework.boot.autoconfigure.EnableAutoConfiguration属性指定的class集合

4. 支持多个@ComponentScan注解同时与@SpringBootApplication注解搭配使用

@ComponentScan注解在前文中已了解的差不多了，笔者此处就针对@EnableAutoConfiguration注解作下详细的分析

@EnableAutoConfiguration

单纯从英文单词上看是开启自动注入的意思，具体的话笔者还是根据源码来解读。首先看下注解的源码

@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 {};

}

它用到了我们前文提及的@Import注解，所以我们关注所引入的AutoConfigurationImportSelector.class

AutoConfigurationImportSelector

其是DeferredImportSelector.class接口的实现类，此处我们直接查看复写的方法selectImports()

	@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		// spring.boot.enableautoconfiguration属性读取，默认为true
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		// read all META-INF\\spring-autoconfigure-metadata.properties files
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
				
		// 获取相应类上@EnableAutoConfiguration对应的属性
		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);
	}

对上述的步骤此处再作下总结

1. 优先判断环境变量spring.boot.enableautoconfiguration，如果指定为false，则不引入任何类。默认为true

2. 读取classpath环境下所有的META-INF\\spring-autoconfigure-metadata.properties文件，加载其中的所有属性保存至autoConfigurationMetadata

3. 获取相应类上@EnableAutoConfiguration对应的属性，其实也就是exclude属性和excludeName属性

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

5. 根据第三步读取的配置以及spring.autoconfigure.exclude环境变量指定的配置，得到exclusions集合

6. 确保exclusions集合是configurations集合的子集，以及exclusions集合内的class均是存在于classpath环境的。否则异常会抛出

7. 根据上述的exclusions集合筛选出未被过滤的configurations集合。

8. 根据第7点筛选出来的configurations集合，再在autoConfigurationMetadata的基础上针对ConditionalOnClass属性筛选一轮
   比如：org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration.ConditionalOnClass=org.springframework.cache.CacheManager
   表示如果要应用CacheAutoConfiguration，则得保证org.springframework.cache.CacheManager类存在

9. 触发AutoConfigurationImportEvent事件

10. 返回筛选过后的configurations集合

笔者此处罗列下spring-boot-autoconfigure-2.0.3.REALEASE包中的spring.factories中的EnableAutoConfiguration默认属性，属性太多，节选如下

# 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

当然用户也可以自定义去实现需要自动注入的配置类。

总结

@SpringBootApplication注解内含@EnableAutoConfiguration注解和@ComponentScan注解，所以这两个注解结合再搭配上@Configuration注解便可以实现springboot的相关功能了。
在这之中，@EnableAutoConfiguration注解最为重要，其扩展性很好，方便springboot无缝对接不同的插件(NoSql插件、Web插件、JMX协议插件等等)，读者需要好好分析。

下节预告

通过查阅上述的多个自动注解，发现用到了@Conditional和AutoConfigureAfter注解，都属于条件判断，在深入理解springboot整合其他插件前，必须对此两个注解有深刻的理解。