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：

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Configuration@EnableAutoConfiguration@ComponentScanpublic @interface SpringBootApplication {　　Class<?>[] exclude() default {};}

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：

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import({ EnableAutoConfigurationImportSelector.class,AutoConfigurationPackages.Registrar.class })public @interface EnableAutoConfiguration {/** * Exclude specific auto-configuration classes such that they will never be applied. * @return the classes to exclude */Class<?>[] exclude() default {};}

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

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

EnableAutoConfigurationImportSelector

来看EnableAutoConfigurationImportSelector类

public String[] selectImports(AnnotationMetadata metadata) {try {AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(EnableAutoConfiguration.class.getName(),true));Assert.notNull(attributes, "No auto-configuration attributes found. Is "+ metadata.getClassName()+ " annotated with @EnableAutoConfiguration?");// Find all possible auto configuration classes, filtering duplicatesList<String> factories = new ArrayList<String>(new LinkedHashSet<String>(SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,this.beanClassLoader)));// Remove those specifically disabledfactories.removeAll(Arrays.asList(attributes.getStringArray("exclude")));// Sortfactories = new AutoConfigurationSorter(this.resourceLoader).getInPriorityOrder(factories);return factories.toArray(new String[factories.size()]);}catch (IOException ex) {throw new IllegalStateException(ex);}}

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

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

其结果为：

# Auto Configureorg.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.cloud.CloudAutoConfiguration,\org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\org.springframework.boot.autoconfigure.jms.hornetq.HornetQAutoConfiguration,\org.springframework.boot.autoconfigure.jta.JtaAutoConfiguration,\org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchAutoConfiguration,\org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchDataAutoConfiguration,\org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\org.springframework.boot.autoconfigure.mongo.MongoDataAutoConfiguration,\org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\org.springframework.boot.autoconfigure.redis.RedisAutoConfiguration,\org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\org.springframework.boot.autoconfigure.velocity.VelocityAutoConfiguration,\org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\org.springframework.boot.autoconfigure.web.GzipFilterAutoConfiguration,\org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\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：

　

@Configuration@ConditionalOnClass({ RabbitTemplate.class, Channel.class })@EnableConfigurationProperties(RabbitProperties.class)@Import(RabbitAnnotationDrivenConfiguration.class)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：

@ConfigurationProperties(prefix = "spring.rabbitmq")public class RabbitProperties {/** * RabbitMQ host. */private String host = "localhost";/** * RabbitMQ port. */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：

@Configuration@ConditionalOnClass(EnableRabbit.class)class RabbitAnnotationDrivenConfiguration {@Autowired(required = false)private PlatformTransactionManager transactionManager;@Bean@ConditionalOnMissingBean(name = "rabbitListenerContainerFactory")public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory(ConnectionFactory connectionFactory) {SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();factory.setConnectionFactory(connectionFactory);if (this.transactionManager != null) {factory.setTransactionManager(this.transactionManager);}return factory;}@EnableRabbit@ConditionalOnMissingBean(name = RabbitListenerConfigUtils.RABBIT_LISTENER_ANNOTATION_PROCESSOR_BEAN_NAME)protected static class EnableRabbitConfiguration {}}

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，

@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：

@Configuration@ConditionalOnMissingBean(ConnectionFactory.class)protected static class RabbitConnectionFactoryCreator {@Beanpublic ConnectionFactory rabbitConnectionFactory(RabbitProperties config) {CachingConnectionFactory factory = new CachingConnectionFactory();String addresses = config.getAddresses();factory.setAddresses(addresses);if (config.getHost() != null) {factory.setHost(config.getHost());factory.setPort(config.getPort());}if (config.getUsername() != null) {factory.setUsername(config.getUsername());}if (config.getPassword() != null) {factory.setPassword(config.getPassword());}if (config.getVirtualHost() != null) {factory.setVirtualHost(config.getVirtualHost());}return factory;}}

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)

@Bean@ConditionalOnMissingBean(RabbitTemplate.class)public RabbitTemplate rabbitTemplate() {return new RabbitTemplate(this.connectionFactory);}

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...

转载请注明 
http://blog.csdn.net/liaokailin/article/details/49559951

欢迎关注，您的肯定是对我最大的支持