簡單介紹

之前使用者使用的是3個註解註解他們的main類。分別是@Configuration,@EnableAutoConfiguration,@ComponentScan。由於這些註解一般都是一起使用，處於簡介的考慮spring boot提供了一個統一的註解@SpringBootApplication。

• @ComponentScan會自動掃描指定包下的全部標有@Component的類，並註冊成bean，當然包括@Component下的子註解@Service,@Repository,@Controller。
• @Configuration後來被換成了其子註解@SpringBootConfiguration。但是二者沒有差別，都是標註當前類是配置類。會將當前當前類內宣告的一個或多個以@Bean註解標記的方法的例項納入到spring容器中，並且例項名就是方法名。
• @EnableAutoConfiguration。被稱為SpringBoot的精髓。這個註釋告訴SpringBoot“猜”你將如何想配置Spring。SpringBoot分析的依據你已經新增jar依賴項。如果spring-boot-starter-web已經新增Tomcat和Spring MVC,這個註釋自動將假設您正在開發一個web應用程式並新增相應的spring設定。@EnableAutoConfiguration可以從逐層的往下搜尋各個加註解的類，這也是為什麼Spring建議將main類放在root包下的原因。

程式碼分析

@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 {

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 * @return the classes to exclude
	 */
	@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "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
	 */
	@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")
	String[] excludeName() default {};

	/**
	 * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
	 * for a type-safe alternative to String-based package names.
	 * @return base packages to scan
	 * @since 1.3.0
	 */
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};

	/**
	 * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
	 * scan for annotated components. The package of each class specified will be scanned.
	 * <p>
	 * Consider creating a special no-op marker class or interface in each package that
	 * serves no purpose other than being referenced by this attribute.
	 * @return base packages to scan
	 * @since 1.3.0
	 */
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};

}

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.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 {};

}