SpringBoot自动配置原理

SpringBoot作为一款全新的框架，有着诸多的优点，正受到越来越多的Java开发者的欢迎。

 

SpringBoot极大地简化了Spring应用的初始创建和开发过程。其优点如下：

1、内嵌Tomcat，无需部署WAR文件；

2、创建独立的Spring应用；

3、简化Maven配置；

4、自动配置Spring；

5、提供生产就绪型功能，如指标，健康检查和外部配置；

6、绝对没有代码生成并且对XML也没有配置要求。

下面就来深入源码分析下SpringBoot自动配置的原理（此文针对的SpringBoot版本为1.5.17.RELEASE）。

 首先从主类SpringbootApplication入手：

@SpringBootApplication
public class SpringbootApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringbootApplication.class, args);
    }

}

 类上的@SpringBootApplication注解定义如下：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {

其中的@EnableAutoConfiguration注解是关键所在，其定义如下：

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

@Import注解可以让普通的类导入到Spring的IOC容器中，由Spring进行管理。

EnableAutoConfigurationImportSelector类定义如下：

@Deprecated
public class EnableAutoConfigurationImportSelector
        extends AutoConfigurationImportSelector {

由于该类已被标记为过时，我们来看下它的父类AutoConfigurationImportSelector：

public class AutoConfigurationImportSelector
        implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
        BeanFactoryAware, EnvironmentAware, Ordered {

AutoConfigurationImportSelector类实现了多个接口，作为一个选择器实现了对自动配置类的选择导入功能，其类图关系如下：

AutoConfigurationImportSelector类中有一个重要的selectImports方法，其定义如下：

　　@Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);//1.1
            AnnotationAttributes attributes = getAttributes(annotationMetadata);//1.2
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                    attributes);//1.3
            configurations = removeDuplicates(configurations);//1.4
            configurations = sort(configurations, autoConfigurationMetadata);//1.5
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);//1.6
            checkExcludedClasses(configurations, exclusions);//1.7
            configurations.removeAll(exclusions);//1.8
            configurations = filter(configurations, autoConfigurationMetadata);//1.9
            fireAutoConfigurationImportEvents(configurations, exclusions);//2.0
            return configurations.toArray(new String[configurations.size()]);//2.1
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }

1.1 加载元数据并返回AutoConfigurationMetadata对象；

1.2 获取传入的元数据对应的属性值(AnnotationAttributes对象)；

1.3 返回候选自动配置类名的list集合；

1.4 移除list中的重复自动配置类名；

1.5 通过读取@AutoConfigureOrder、@link AutoConfigureBefore、@AutoConfigureAfter注解对自动配置类名进行优先级排序；

1.6 返回排除掉的自动配置类的set集合；

1.7 检查候选配置类中是否存在排除项，然后返回给SpringFactoriesLoader加载的候选自动配置类；

1.8 移除候选配置类当中的的所有排除项（不需要自动配置的类）；

1.9 利用在spring工厂注册过的AutoConfigurationImportFilter过滤器对候选自动配置类进行过滤；

2.0 通过AutoConfigurationImportListener监听器触发自动配置导入事件；

2.1 返回经过上述步骤得到的自动配置类的数组。

该方法最终返回了能进行自动配置的类的数组。

我们着重看下红色标注的getCandidateConfigurations方法：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

其中的getSpringFactoriesLoaderFactoryClass()方法正是返回EnableAutoConfiguration.class。

该方法里面调用了SpringFactoriesLoader.loadFactoryNames()方法扫描加载引入的jar包中包含META-INF/spring.factories文件中的自动配置类。

具体方法如下：

/**
 * The location to look for factories.
 * <p>Can be present in multiple JAR files.
 */
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

蓝色加粗的程序表示会将扫描到的文件内容包装成properties对象。

红色标注的文件spring.factories可以在spring-boot-autoconfigure等springboot相关的jar包中找到：

该文件里面内容都是键值对形式的，其中spring-boot-autoconfigure这个jar中的org.springframework.boot.autoconfigure.EnableAutoConfiguration正是我们要找的自动配置的类，包括Aop、WebMvc、Jpa、redis、jdbc等自动配置类。

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
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.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
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.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
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.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

当然这些自动配置类并不是都能自动配置进来的，里面会根据@Conditional等注解控制bean的创建行为。

只有满足了条件才能自动注入相关的bean。

以RabbitAutoConfiguration为例，其定义如下：

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

// Only available in rabbitmq-java-client 5.4.0 +
private static final boolean CAN_ENABLE_HOSTNAME_VERIFICATION = ReflectionUtils
      .findMethod(com.rabbitmq.client.ConnectionFactory.class,
            "enableHostnameVerification") != null;

　　@Bean
　　public CachingConnectionFactory rabbitConnectionFactory(RabbitProperties config)
   　　   throws Exception {
  　　 RabbitConnectionFactoryBean factory = new RabbitConnectionFactoryBean();
   　　if (config.determineHost() != null) {
      factory.setHost(config.determineHost());
　　　　.
　　　　.
　　　　.
   　　}

@ConditionalOnClass表示有某个类的情况下才会加载bean，这里需要同时有RabbitTemplate和Channel类，当所有条件满足的情况下，就能通过@Bean注解，让方法产生一个bean对象，交由Spring容器来进行管理，这样就能在SpringBoot项目启动的时候实现自动配置。

如果上述2个类提示找不到，可以在pom.xml引入以下依赖解决：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-amqp</artifactId>
</dependency>

@EnableConfigurationProperties(RabbitProperties.class)能实现属性类的自动配置，RabbitProperties类是配置Rabbitmq属性用的，其定义如下：

@ConfigurationProperties(prefix = "spring.rabbitmq")
public class RabbitProperties {

    /**
     * RabbitMQ host.
     */
    private String host = "localhost";

    /**
     * RabbitMQ port.
     */
    private int port = 5672;

    /**
     * Login user to authenticate to the broker.
     */
    private String username;

    /**
     * Login to authenticate against the broker.
     */
    private String password;

而这些属性值我们同样可以配置在resources下的application.properties文件里面，而配置的这些属性将会覆盖RabbitProperties类里面配置的默认值。

#自定义rabbitmq属性值
spring.rabbitmq.host=198.168.121.1
spring.rabbitmq.username=root
spring.rabbitmq.password=123456

至此，SpringBoot自动配置的源码分析告一段落，总结如下：

（1）SpringBoot在启动的时候会扫描所有jar包里面的META-INF/spring.properties，找出自动配置的类信息并将这些信息包装成properties对象。到时会从properties中获取EnableAutoConfiguration.class类名对应的值，然后使用@Bean注解将这些bean添加到Spring容器中。

（2）给Spring容器添加自动配置组件时，会从properties类中获取属性值，我们可以配置自定义的属性覆盖对应xxxProperties.class类里面的默认属性值，实现个性化配置。