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
@ComponentScan
public @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 duplicates
List<String> factories = new ArrayList<String>(new LinkedHashSet<String>(
SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,
this.beanClassLoader)));

// Remove those specifically disabled
factories.removeAll(Arrays.asList(attributes.getStringArray("exclude")));

// Sort
factories = 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 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.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 {

@Bean
public 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...