前言

Spring Boot中引入了自动配置，让开发者利用起来更加的简便、快捷，本篇讲利用RabbitMQ的自动配置为例讲分析下Spring Boot中的自动配置原理。

在上一篇末尾讲述了Spring Boot 默认情况下会为ConnectionFactory、RabbitTemplate等bean，在前面的文章中也讲到嵌入的Tomcat默认配置为8080端口

这些都属于Spring Boot自动配置的范畴，当然其自动配置相当多。

EnableAutoConfiguration注解

在创建Application时我们使用了SpringBootApplication注解，在
spring boot实战(第九篇)Application创建源码分析
中曾有所分析，再来看下其定义：

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1. @Target(ElementType.TYPE)  
2. @Retention(RetentionPolicy.RUNTIME)  
3. @Documented  
4. @Inherited  
5. @Configuration  
6. @EnableAutoConfiguration  
7. @ComponentScan  
8. public @interface SpringBootApplication {　　  
9.     Class<?>[] exclude() default {};  
10.   
11. }  

该注解上存在元注解@EnableAutoConfiguration，这就是Spring Boot自动配置实现的核心入口；其定义为：

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1. @Target(ElementType.TYPE)  
2. @Retention(RetentionPolicy.RUNTIME)  
3. @Documented  
4. @Inherited  
5. @Import({ EnableAutoConfigurationImportSelector.class,  
6.         AutoConfigurationPackages.Registrar.class })  
7. public @interface EnableAutoConfiguration {  
8.   
9.     /**  
10.      * Exclude specific auto-configuration classes such that they will never be applied.  
11.      * @return the classes to exclude  
12.      */  
13.     Class<?>[] exclude() default {};  
14.   
15. }  

很显然能看出有一特殊的注解
@Import，该注解在spring boot实战(第十篇)Spring boot Bean加载源码分析中有讲解到，加载bean时会解析Import注解，因此需要讲目光聚集在这段代码

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1. @Import({ EnableAutoConfigurationImportSelector.class,  
2.         AutoConfigurationPackages.Registrar.class })  

EnableAutoConfigurationImportSelector

来看EnableAutoConfigurationImportSelector类

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1. public String[] selectImports(AnnotationMetadata metadata) {  
2.         try {  
3.             AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata  
4.                     .getAnnotationAttributes(EnableAutoConfiguration.class.getName(),  
5.                             true));  
6.   
7.             Assert.notNull(attributes, “No auto-configuration attributes found. Is ”  
8.                     + metadata.getClassName()  
9.                     + ” annotated with @EnableAutoConfiguration?”);  
10.   
11.             // Find all possible auto configuration classes, filtering duplicates  
12.             List<String> factories = new ArrayList<String>(new LinkedHashSet<String>(  
13.                     SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,  
14.                             this.beanClassLoader)));  
15.   
16.             // Remove those specifically disabled  
17.             factories.removeAll(Arrays.asList(attributes.getStringArray(“exclude”)));  
18.   
19.             // Sort  
20.             factories = new AutoConfigurationSorter(this.resourceLoader)  
21.                     .getInPriorityOrder(factories);  
22.   
23.             return factories.toArray(new String[factories.size()]);  
24.         }  
25.         catch (IOException ex) {  
26.             throw new IllegalStateException(ex);  
27.         }  
28.     }  

看如下代码，获取类路径下spring.factories下key为EnableAutoConfiguration全限定名对应值

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1. SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,  
2.                             this.beanClassLoader))  

其结果为：

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1. # Auto Configure  
2. org.springframework.boot.autoconfigure.EnableAutoConfiguration=\  
3. org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\  
4. org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\  
5. org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration,\  
6. org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\  
7. org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\  
8. org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\  
9. org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\  
10. org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\  
11. org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\  
12. org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\  
13. org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\  
14. org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\  
15. org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\  
16. org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\  
17. org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\  
18. org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\  
19. org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\  
20. org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\  
21. org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\  
22. org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\  
23. org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\  
24. org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\  
25. org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\  
26. org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\  
27. org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\  
28. org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\  
29. org.springframework.boot.autoconfigure.jms.hornetq.HornetQAutoConfiguration,\  
30. org.springframework.boot.autoconfigure.jta.JtaAutoConfiguration,\  
31. org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchAutoConfiguration,\  
32. org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchDataAutoConfiguration,\  
33. org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\  
34. org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\  
35. org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\  
36. org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\  
37. org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\  
38. org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\  
39. org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\  
40. org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\  
41. org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\  
42. org.springframework.boot.autoconfigure.mongo.MongoDataAutoConfiguration,\  
43. org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\  
44. org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\  
45. org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\  
46. org.springframework.boot.autoconfigure.redis.RedisAutoConfiguration,\  
47. org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\  
48. org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\  
49. org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\  
50. org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\  
51. org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\  
52. org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\  
53. org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\  
54. org.springframework.boot.autoconfigure.velocity.VelocityAutoConfiguration,\  
55. org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\  
56. org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\  
57. org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\  
58. org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\  
59. org.springframework.boot.autoconfigure.web.GzipFilterAutoConfiguration,\  
60. org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\  
61. org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\  
62. org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\  
63. org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\  
64. org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\  
65. org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration  

以上为Spring Boot中所有的自动配置相关类；在启动过程中会解析对应类配置信息，以RabbitMQ为例，则会去解析RabbitAutoConfiguration

RabbitAutoConfiguration

首先来看RabbitAutoConfiguration类上的注解：

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1. @Configuration  
2. @ConditionalOnClass({ RabbitTemplate.class, Channel.class })  
3. @EnableConfigurationProperties(RabbitProperties.class)  
4. @Import(RabbitAnnotationDrivenConfiguration.class)  
5. public class RabbitAutoConfiguration {  

• @Configuration: 应该不需要解释

• @ConditionalOnClass：表示存在对应的Class文件时才会去解析RabbitAutoConfiguration，否则直接跳过不解析，这也是为什么在不导入RabbitMQ依赖Jar时工程能正常启动的原因

• @EnableConfigurationProperties：表示对@ConfigurationProperties的内嵌支持，默认会将对应Class这是为bean，例如这里值为RabbitProperties.class，其定义为：

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1. @ConfigurationProperties(prefix = “spring.rabbitmq”)  
2. public class RabbitProperties {  
3.   
4.     /**  
5.      * RabbitMQ host.  
6.      */  
7.     private String host = “localhost”;  
8.   
9.     /**  
10.      * RabbitMQ port.  
11.      */  
12.     private int port = 5672;   …. //省略部分代码}  

RabbitProperties提供对RabbitMQ的配置信息，其前缀为spring.rabbitmq，因此在上篇中配置的host、port等信息会配置到该类上，随后@
EnableConfigurationProperties会将RabbitProperties注册为一个bean。

• @Import为导入配置，RabbitAnnotationDrivenConfiguration具体实现如下：

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1. @Configuration  
2. @ConditionalOnClass(EnableRabbit.class)  
3. class RabbitAnnotationDrivenConfiguration {  
4.   
5.     @Autowired(required = false)  
6.     private PlatformTransactionManager transactionManager;  
7.   
8.     @Bean  
9.     @ConditionalOnMissingBean(name = “rabbitListenerContainerFactory”)  
10.     public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory(  
11.             ConnectionFactory connectionFactory) {  
12.         SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();  
13.         factory.setConnectionFactory(connectionFactory);  
14.         if (this.transactionManager != null) {  
15.             factory.setTransactionManager(this.transactionManager);  
16.         }  
17.         return factory;  
18.     }  
19.   
20.     @EnableRabbit  
21.     @ConditionalOnMissingBean(name = RabbitListenerConfigUtils.RABBIT_LISTENER_ANNOTATION_PROCESSOR_BEAN_NAME)  
22.     protected static class EnableRabbitConfiguration {  
23.   
24.     }  
25.   
26. }  

这里又涉及到一个重要的注解：@ConditionalOnMissingBean，其功能为如果存在指定name的bean，则该注解标注的bean不创建，
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1. @ConditionalOnMissingBean(name = “rabbitListenerContainerFactory”)  

表示的意思为：如果存在名称为rabbitListenerContainerFactory的bean，则该部分代码直接忽略，这是Spring Boot人性化体现之一，开发者申明的bean会放在第一位，实在是太懂礼貌了~~

本篇中涉及到的注解比较多，其具体实现原理在以后有时间再具体分析。

再回到RabbitAutoConfiguration类的具体实现

首先来看：
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1. @Configuration  
2.     @ConditionalOnMissingBean(ConnectionFactory.class)  
3.     protected static class RabbitConnectionFactoryCreator {  
4.   
5.         @Bean  
6.         public ConnectionFactory rabbitConnectionFactory(RabbitProperties config) {  
7.             CachingConnectionFactory factory = new CachingConnectionFactory();  
8.             String addresses = config.getAddresses();  
9.             factory.setAddresses(addresses);  
10.             if (config.getHost() != null) {  
11.                 factory.setHost(config.getHost());  
12.                 factory.setPort(config.getPort());  
13.             }  
14.             if (config.getUsername() != null) {  
15.                 factory.setUsername(config.getUsername());  
16.             }  
17.             if (config.getPassword() != null) {  
18.                 factory.setPassword(config.getPassword());  
19.             }  
20.             if (config.getVirtualHost() != null) {  
21.                 factory.setVirtualHost(config.getVirtualHost());  
22.             }  
23.             return factory;  
24.         }  
25.   
26.     }  

创建了默认的ConnectionFactory，需要注意的时，这里的ConnectionFactory无回调的设置(解答了上篇中的疑问)

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1. @Bean  
2.     @ConditionalOnMissingBean(RabbitTemplate.class)  
3.     public RabbitTemplate rabbitTemplate() {  
4.         return new RabbitTemplate(this.connectionFactory);  
5.     }  

创建了默认的RabbitTemplate；下面创建的RabbitMessagingTemplate实现对
RabbitTemplate的包装。

在RabbitAutoConfiguration类中还剩AmqpAdmin的创建没有讲解，这部分就留到后面的文章再来讲述…

本文转自 http://blog.csdn.net/liaokailin/article/details/49559951