SpringBoot之@EnableAutoConfiguration原理及自定义扩展

╰+哭是因爲堅強的太久メ 2022-07-16 06:50 137阅读 0赞

[spring][] Boot是一个偏执的开源框架，它可用于创建可执行的Spring应用程序，采用了习惯优于配置的方法。   
此框架的神奇之处在于@EnableAutoConfiguration注释，此注释自动载入应用程序所需的所有Bean——这依赖于Spring Boot在类路径中的查找。

### 一、@Enable\*注释 ###

@Enable\*注释并不是新发明的注释，早在Spring 3框架就引入了这些注释，用这些注释替代XML配置文件。   
很多Spring开发者都知道@EnableTransactionManagement注释，它能够声明事务管理；@EnableWebMvc注释，它能启用Spring MVC；以及@EnableScheduling注释，它可以初始化一个调度器。   
这些注释事实上都是简单的配置，通过@Import注释导入。

@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类使用了Spring Core包的SpringFactoriesLoader类的loadFactoryNamesof()方法。   
SpringFactoriesLoader会查询META-INF/spring.factories文件中包含的JAR文件。   
当找到spring.factories文件后，SpringFactoriesLoader将查询配置文件命名的属性。在例子中，是org.springframework.boot.autoconfigure.EnableAutoConfiguration。   
让我们来看看spring-boot-autoconfigure JAR文件，它真的包含了一个spring.factories文件，内容如下：

# 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

在这个文件中，可以看到一系列Spring Boot自动配置的列表。下面我们来看这些配置的细节，以MongoAutoConfiguration为例：

@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;
        }
    
    }

这个类进行了简单的Spring配置，声明了MongoDB所需典型Bean。   
这个类跟其它很多类一样，重度依赖于Spring Boot注释：   
1）@ConditionOnClass激活一个配置，在类路径中只能存在一到几个这样的类。   
2）@EnableConfigurationProperties自动映射一个POJO到Spring Boot配置文件（默认是application.properties文件）的属性集。   
3）@ConditionalOnMissingBean启用一个Bean定义，但必须是这个Bean之前未定义过才有效。   
还可以使用@ AutoConfigureBefore注释、@AutoConfigureAfter注释来定义这些配置类的载入顺序。

### 二、属性映射 ###

下面看MongoProperties类，它是一个Spring Boot属性映射的例子：

@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注释将POJO关联到指定前缀的每一个属性。例如，spring.data.mongodb.port属性将映射到这个类的端口属性。   
强烈建议Spring Boot开发者使用这种方式来删除与配置属性相关的瓶颈代码。

### 三、@Conditional注释 ###

Spring Boot的强大之处在于使用了Spring 4框架的新特性：@Conditional注释，此注释使得只有在特定条件满足时才启用一些配置。   
在Spring Boot的org.springframework.boot.autoconfigure.condition包中说明了使用@Conditional注释能给我们带来什么，下面对这些注释做一个概述：

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

以@ConditionalOnExpression注释为例，它允许在Spring的EL表达式中写一个条件。

@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";
    
    }

在这个类中，我们想利用@Conditional注释，条件在OnExpressionCondition类中定义：

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());
        }
    }

在最后，@Conditional通过简单的布尔表达式（即ConditionOutcome方法）来决定。

### 四、应用程序上下文初始化器 ###

spring.factories还提供了第二种可能性，即定义应用程序的初始化。这使得我们可以在应用程序载入前操纵Spring的应用程序上下文ApplicationContext。   
特别是，可以在上下文创建监听器，使用ConfigurableApplicationContext类的addApplicationListener()方法。   
AutoConfigurationReportLoggingInitializer监听到系统事件时，比如上下文刷新或应用程序启动故障之类的事件，Spring Boot可以执行一些工作。这有助于我们以调试模式启动应用程序时创建自动配置的报告。   
要以调试模式启动应用程序，可以使用-Ddebug标识，或者在application.properties文件这添加属性debug= true。

### 五、调试Spring Boot自动配置 ###

Spring Boot的官方文档（[http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/\#howto-troubleshoot-auto-configuration][http_docs.spring.io_spring-boot_docs_current_reference_htmlsingle_howto-troubleshoot-auto-configuration]）有助于理解自动配置期间发生了什么。当以调试模式运行时，Spring Boot会产生一个报告，如下：

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)

对于每个自动配置，可以看到它启动或失败的原因。

### 六、结论 ###

Spring Boot很好地利用了Spring 4框架的功能，可以创建一个自动配置的可执行JAR。   
不要忘记，可以使用自己的配置来替代自动配置，见： [http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/\#using-boot-replacing-auto-configuration][http_docs.spring.io_spring-boot_docs_current_reference_htmlsingle_using-boot-replacing-auto-configuration]   
over!

[spring]: http://lib.csdn.net/base/javaee
[http_docs.spring.io_spring-boot_docs_current_reference_htmlsingle_howto-troubleshoot-auto-configuration]: http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-troubleshoot-auto-configuration
[http_docs.spring.io_spring-boot_docs_current_reference_htmlsingle_using-boot-replacing-auto-configuration]: http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#using-boot-replacing-auto-configuration