spring boot实战(第十三篇)自动配置原理分析

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：

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. }

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：

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. * Exclude specific auto-configuration classes such that they will never be applied.
10. * @return the classes to exclude
11. */
12. Class<?>[] exclude() default {};
13. }

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

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

EnableAutoConfigurationImportSelector

来看EnableAutoConfigurationImportSelector类

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

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

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

其结果为：

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：

　

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：

1. @ConfigurationProperties(prefix = "spring.rabbitmq")
2. public class RabbitProperties {
3. /**
4. * RabbitMQ host.
5. */
6. private String host = "localhost";
7. /**
8. * RabbitMQ port.
9. */
10. 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：

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

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，

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：

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

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)

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...