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。
属性exclude和excludeName，其默认为空，指定的话最终由EnableAutoConfiguration.class来完成解析
属性scanBasePackages和scanBasePackageClasses，其默认为空，指定的话最终由ComponentScan.class来完成解析
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
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
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
获取相应类上@EnableAutoConfiguration对应的属性，其实也就是exclude属性和excludeName属性
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
根据上述的exclusions集合筛选出未被过滤的configurations集合。
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类存在触发AutoConfigurationImportEvent事件
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。
