spring Boot是一個偏執的開源框架，它可用於建立可執行的Spring應用程式，採用了習慣優於配置的方法。 
此框架的神奇之處在於@EnableAutoConfiguration註釋，此註釋自動載入應用程式所需的所有Bean——這依賴於Spring Boot在類路徑中的查詢。

一、@Enable*註釋

@Enable*註釋並不是新發明的註釋，早在Spring 3框架就引入了這些註釋，用這些註釋替代XML配置檔案。 
很多Spring開發者都知道@EnableTransactionManagement註釋，它能夠宣告事務管理；@EnableWebMvc註釋，它能啟用Spring MVC；以及@EnableScheduling註釋，它可以初始化一個排程器。 
這些註釋事實上都是簡單的配置，通過@Import註釋匯入。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({ EnableAutoConfigurationImportSelector.class,
        AutoConfigurationPackages.Registrar.class })
public @interface EnableAutoConfiguration {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     */
    Class<?>[] exclude() default {};

}
 

EnableAutoConfigurationImportSelector類使用了Spring Core包的SpringFactoriesLoader類的loadFactoryNamesof()方法。 
SpringFactoriesLoader會查詢META-INF/spring.factories檔案中包含的JAR檔案。 
當找到spring.factories檔案後，SpringFactoriesLoader將查詢配置檔案命名的屬性。在例子中，是org.springframework.boot.autoconfigure.EnableAutoConfiguration。 
讓我們來看看spring-boot-autoconfigure JAR檔案，它真的包含了一個spring.factories檔案，內容如下：

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration

在這個檔案中，可以看到一系列Spring Boot自動配置的列表。下面我們來看這些配置的細節，以MongoAutoConfiguration為例：

@Configuration
@ConditionalOnClass(Mongo.class)
@EnableConfigurationProperties(MongoProperties.class)
public class MongoAutoConfiguration {

    @Autowired
    private MongoProperties properties;

    private Mongo mongo;

    @PreDestroy
    public void close() throws UnknownHostException {
        if (this.mongo != null) {
            this.mongo.close();
        }
    }

    @Bean
    @ConditionalOnMissingBean
    public Mongo mongo() throws UnknownHostException {
        this.mongo = this.properties.createMongoClient();
        return this.mongo;
    }

}

這個類進行了簡單的Spring配置，聲明瞭MongoDB所需典型Bean。 
這個類跟其它很多類一樣，重度依賴於Spring Boot註釋： 
1）@ConditionOnClass啟用一個配置，在類路徑中只能存在一到幾個這樣的類。 
2）@EnableConfigurationProperties自動對映一個POJO到Spring Boot配置檔案（預設是application.properties檔案）的屬性集。 
3）@ConditionalOnMissingBean啟用一個Bean定義，但必須是這個Bean之前未定義過才有效。 
還可以使用@ AutoConfigureBefore註釋、@AutoConfigureAfter註釋來定義這些配置類的載入順序。

二、屬性對映

下面看MongoProperties類，它是一個Spring Boot屬性對映的例子：

@ConfigurationProperties(prefix = "spring.data.mongodb")
public class MongoProperties {

    private String host;
    private int port = DBPort.PORT;
    private String uri = "mongodb://localhost/test";
    private String database;

    // ... getters/ setters omitted
}

@ConfigurationProperties註釋將POJO關聯到指定字首的每一個屬性。例如，spring.data.mongodb.port屬性將對映到這個類的埠屬性。 
強烈建議Spring Boot開發者使用這種方式來刪除與配置屬性相關的瓶頸程式碼。

三、@Conditional註釋

Spring Boot的強大之處在於使用了Spring 4框架的新特性：@Conditional註釋，此註釋使得只有在特定條件滿足時才啟用一些配置。 
在Spring Boot的org.springframework.boot.autoconfigure.condition包中說明了使用@Conditional註釋能給我們帶來什麼，下面對這些註釋做一個概述：

• @ConditionalOnBean
• @ConditionalOnClass
• @ConditionalOnExpression
• @ConditionalOnMissingBean
• @ConditionalOnMissingClass
• @ConditionalOnNotWebApplication
• @ConditionalOnResource
• @ConditionalOnWebApplication

以@ConditionalOnExpression註釋為例，它允許在Spring的EL表示式中寫一個條件。

@Conditional(OnExpressionCondition.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
public @interface ConditionalOnExpression {

    /**
     * The SpEL expression to evaluate. Expression should return {@code true} if the
     * condition passes or {@code false} if it fails.
     */
    String value() default "true";

}

在這個類中，我們想利用@Conditional註釋，條件在OnExpressionCondition類中定義：

public class OnExpressionCondition extends SpringBootCondition {

    @Override
    public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        // ...
        // we first get a handle on the EL context via the ConditionContext

        boolean result = (Boolean) resolver.evaluate(expression, expressionContext);

        // ...
        // here we create a message the user will see when debugging

        return new ConditionOutcome(result, message.toString());
    }
}

在最後，@Conditional通過簡單的布林表示式（即ConditionOutcome方法）來決定。

四、應用程式上下文初始化器

spring.factories還提供了第二種可能性，即定義應用程式的初始化。這使得我們可以在應用程式載入前操縱Spring的應用程式上下文ApplicationContext。 
特別是，可以在上下文建立監聽器，使用ConfigurableApplicationContext類的addApplicationListener()方法。 
AutoConfigurationReportLoggingInitializer監聽到系統事件時，比如上下文重新整理或應用程式啟動故障之類的事件，Spring Boot可以執行一些工作。這有助於我們以除錯模式啟動應用程式時建立自動配置的報告。 
要以除錯模式啟動應用程式，可以使用-Ddebug標識，或者在application.properties檔案這新增屬性debug= true。

五、除錯Spring Boot自動配置

Positive matches:
-----------------

   MessageSourceAutoConfiguration
      - @ConditionalOnMissingBean (types: org.springframework.context.MessageSource; SearchStrategy: all) found no beans (OnBeanCondition)

   JmxAutoConfiguration
      - @ConditionalOnClass classes found: org.springframework.jmx.export.MBeanExporter (OnClassCondition)
      - SpEL expression on org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration: ${spring.jmx.enabled:true} (OnExpressionCondition)
      - @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: all) found no beans (OnBeanCondition)

   DispatcherServletAutoConfiguration
      - found web application StandardServletEnvironment (OnWebApplicationCondition)
      - @ConditionalOnClass classes found: org.springframework.web.servlet.DispatcherServlet (OnClassCondition)


Negative matches:
-----------------

   DataSourceAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType (OnClassCondition)

   DataSourceTransactionManagerAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.jdbc.core.JdbcTemplate,org.springframework.transaction.PlatformTransactionManager (OnClassCondition)

   MongoAutoConfiguration
      - required @ConditionalOnClass classes not found: com.mongodb.Mongo (OnClassCondition)

   FallbackWebSecurityAutoConfiguration
      - SpEL expression on org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration: !${security.basic.enabled:true} (OnExpressionCondition)

   SecurityAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.security.authentication.AuthenticationManager (OnClassCondition)

   EmbeddedServletContainerAutoConfiguration.EmbeddedJetty
      - required @ConditionalOnClass classes not found: org.eclipse.jetty.server.Server,org.eclipse.jetty.util.Loader (OnClassCondition)

   WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter#localeResolver
      - @ConditionalOnMissingBean (types: org.springframework.web.servlet.LocaleResolver; SearchStrategy: all) found no beans (OnBeanCondition)
      - SpEL expression: '${spring.mvc.locale:}' != '' (OnExpressionCondition)

   WebSocketAutoConfiguration
      - required @ConditionalOnClass classes not found: org.springframework.web.socket.WebSocketHandler,org.apache.tomcat.websocket.server.WsSci (OnClassCondition)

對於每個自動配置，可以看到它啟動或失敗的原因。

六、結論