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：

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@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：

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@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

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@Import({ EnableAutoConfigurationImportSelector.class,

AutoConfigurationPackages.Registrar.class })

EnableAutoConfigurationImportSelector

来看EnableAutoConfigurationImportSelector类

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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

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SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,

this.beanClassLoader))

其结果为：

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# 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：

　

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@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：

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@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：

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@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，

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@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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@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)

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@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...