摘要：用于主類(lèi)上最最最核心的注解，表示這是一個(gè)項(xiàng)目，用于開(kāi)啟的各項(xiàng)能力。下面我們來(lái)分析一下這個(gè)注解的組成以及作用通過(guò)上面的代碼我們可以看出來(lái)是一個(gè)組合注解，主要由和這三個(gè)注解組成的。通過(guò)源碼可以看出也是一個(gè)組合注解。

??SpringBoot項(xiàng)目一般都會(huì)有Application的入口類(lèi)，入口類(lèi)中會(huì)有main方法，這是一個(gè)標(biāo)準(zhǔn)的java應(yīng)用程序的入口方法。@SpringBootApplication用于Spring主類(lèi)上最最最核心的注解，表示這是一個(gè)SpringBoot項(xiàng)目，用于開(kāi)啟SpringBoot的各項(xiàng)能力。

??下面我們來(lái)分析一下@SpringBootApplication這個(gè)注解的組成以及作用

/**
 * Indicates a {@link Configuration configuration} class that declares one or more
 * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration
 * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience
 * annotation that is equivalent to declaring {@code @Configuration},
 * {@code @EnableAutoConfiguration} and {@code @ComponentScan}.
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @since 1.2.0
 */
@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

??通過(guò)上面的代碼我們可以看出來(lái)@SpringBootApplication是一個(gè)組合注解，主要由@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan這三個(gè)注解組成的。

/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring"s
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 * 

 * Application should only ever include one {@code @SpringBootConfiguration} and
 * most idiomatic Spring Boot applications will inherit it from
 * {@code @SpringBootApplication}.
 *
 * @author Phillip Webb
 * @since 1.4.0
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

??通過(guò)源碼可以看出@SpringBootConfiguration也是一個(gè)組合注解。@SpringBootConfiguration是SpringBoot項(xiàng)目的配置注解，標(biāo)識(shí)這是一個(gè)被裝載的Bean，在SpringBoot項(xiàng)目中推薦使用@SpringBootConfiguration替代@Configuration

??@EnableAutoConfiguration的作用是開(kāi)啟自動(dòng)配置功能。以前我們需要配置的東西，SpringBoot幫我們自動(dòng)配置；@EnableAutoConfiguration告訴SpringBoot開(kāi)啟自動(dòng)配置功能；這樣自動(dòng)配置才能生效

/**
 * Enable auto-configuration of the Spring Application Context, attempting to guess and
 * configure beans that you are likely to need. Auto-configuration classes are usually
 * applied based on your classpath and what beans you have defined. For example, if you
 * have {@code tomcat-embedded.jar} on your classpath you are likely to want a
 * {@link TomcatServletWebServerFactory} (unless you have defined your own
 * {@link ServletWebServerFactory} bean).
 * 

 * When using {@link SpringBootApplication}, the auto-configuration of the context is
 * automatically enabled and adding this annotation has therefore no additional effect.
 * 

 * Auto-configuration tries to be as intelligent as possible and will back-away as you
 * define more of your own configuration. You can always manually {@link #exclude()} any
 * configuration that you never want to apply (use {@link #excludeName()} if you don"t
 * have access to them). You can also exclude them via the
 * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied
 * after user-defined beans have been registered.
 * 

 * The package of the class that is annotated with {@code @EnableAutoConfiguration},
 * usually via {@code @SpringBootApplication}, has specific significance and is often used
 * as a "default". For example, it will be used when scanning for {@code @Entity} classes.
 * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you"re
 * not using {@code @SpringBootApplication}) in a root package so that all sub-packages
 * and classes can be searched.
 * 

 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are
 * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class).
 * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most
 * often using {@link ConditionalOnClass @ConditionalOnClass} and
 * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @see ConditionalOnBean
 * @see ConditionalOnMissingBean
 * @see ConditionalOnClass
 * @see AutoConfigureAfter
 * @see SpringBootApplication
 */
@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 {};

}

??當(dāng)我們啟用了@EnableAutoConfiguration注解，啟動(dòng)自動(dòng)配置后，該注解會(huì)使SpringBoot根據(jù)項(xiàng)目中依賴(lài)的jar包自動(dòng)配置項(xiàng)目的配置項(xiàng)。例如：引入了Spring-boot-starter-web的依賴(lài)，項(xiàng)目中也就會(huì)引入SpringMVC的依賴(lài)，SpringBoot就會(huì)自動(dòng)配置tomcat和SpringMVC

??@ComponentScan注解會(huì)自動(dòng)掃描包路徑下的所有@Controller、@Service、@Repository、@Component的類(lèi)，不配置包路徑的話(huà)，SpringBoot中默認(rèn)掃描@SpringBootApplication注解所在類(lèi)的同級(jí)目錄以及子目錄下的相關(guān)注解。