springboot情操陶冶-@SpringBootApplication註解解析

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承接前文springboot情操陶冶[email protected]註解解析，本文將在前文的基礎上對@SpringBootApplication註解作下簡單的分析

@SpringBootApplication

該註解是springboot最集中的一個註解，也是應用最廣泛的註解。官方也多用此註解以啟動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兩個主要的註解，並以此作了預設的指定。

1. 屬性exclude和excludeName，其預設為空白，指定的話最終由EnableAutoConfiguration.class來完成解析

2. 屬性scanBasePackages和scanBasePackageClasses，其預設為空白，指定的話最終由ComponentScan.class來完成解析

3. 預設情況下，以@SpringBootApplication註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檔案中org.springframework.boot.autoconfigure.EnableAutoConfiguration屬性指定的class集合

4. 支援多個@ComponentScan註解同時與@SpringBootApplication註解搭配使用

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

@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。首先看下註解的源碼

@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註解，所以我們關注所引入的AutoConfigurationImportSelector.class

AutoConfigurationImportSelector

其是DeferredImportSelector.class介面的實作類別，此處我們直接查看複寫的方法selectImports()

    @Override    public String[] selectImports(AnnotationMetadata annotationMetadata) {        // spring.boot.enableautoconfiguration屬性讀取，預設為true        if (!isEnabled(annotationMetadata)) {            return NO_IMPORTS;        }        // read all META-INF\spring-autoconfigure-metadata.properties files        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader                .loadMetadata(this.beanClassLoader);                        // 擷取相應類上@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. 優先判斷環境變數spring.boot.enableautoconfiguration，如果指定為false，則不引入任何類。預設為true

2. 讀取classpath環境下所有的META-INF\spring-autoconfigure-metadata.properties檔案，載入其中的所有屬性儲存至autoConfigurationMetadata

3. 擷取相應類上@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檔案中的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環境變數指定的配置，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. 返回篩選過後的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預設屬性，屬性太多，節選如下

# Auto Configureorg.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

當然使用者也可以自訂去實現需要自動注入的配置類。

總結

@SpringBootApplication註解內含@EnableAutoConfiguration註解和@ComponentScan註解，所以這兩個註解結合再搭配上@Configuration註解便可以實現springboot的相關功能了。
在這之中，@EnableAutoConfiguration註解最為重要，其擴充性很好，方便springboot無縫對接不同的外掛程式(NoSql外掛程式、Web外掛程式、JMX協議外掛程式等等)，讀者需要好好分析。

下節預告

通過查閱上述的多個自動註解，發現用到了@Conditional和AutoConfigureAfter註解，都屬於條件判斷，在深入理解springboot整合其他外掛程式前，必須對此兩個註解有深刻的理解。

springboot情操陶冶[email protected]註解解析