SpringBoot源码学习之路（三、自动配置原理）

忘是亡心i 2022-05-19 04:51 238阅读 0赞

## 自动配置原理 ##

可以在application.properties/application.yml文件中指定各种属性，也可以在命令行开关中指定。来实现各式各样的功能配置，那究竟SpringBoot有多少自动配置功能呢？此处可以参考下[官方文档][Link 1]。  
![SpringBoot自动配置属性列表][SpringBoot]

### 一、自动配置原理： ###

--------------------

**1、SpringBoot启动的时候加载主配置类，开启了自动配置功能\[【 @EnableAutoConfiguration】**  
EnableAutoConfiguration.class:

@Target({ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import({AutoConfigurationImportSelector.class})
    public @interface EnableAutoConfiguration { 
        String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
        Class<?>[] exclude() default {};
    
        String[] excludeName() default {};
    }

--------------------

**2.EnableAutoConfiguration的作用：利用EnableAutoConfigurationImportSelector给容器中导入一些组件（实例）**  
可以查看EnableAutoConfigurationImportSelector . selectImports()方法的内容；

public String[] selectImports(AnnotationMetadata annotationMetadata) {
            if (!this.isEnabled(annotationMetadata)) {
                return NO_IMPORTS;
            } else {
                AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
                AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
                //分析getCandidateConfigurations（）方法如下
                List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
    
                configurations = this.removeDuplicates(configurations);
                Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
                this.checkExcludedClasses(configurations, exclusions);
                configurations.removeAll(exclusions);
                configurations = this.filter(configurations, autoConfigurationMetadata);
                this.fireAutoConfigurationImportEvents(configurations, exclusions);
                return StringUtils.toStringArray(configurations);
            }
        }

其中的this.getCandidateConfigurations(annotationMetadata, attributes);源码中：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
           /* *1.扫描所有jar包类路径下 META-INF/spring.factories *2.把扫描到的这些文件的内容包装成properties对象 *3.从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，然后把他们添加在容器中 */
            List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(),this.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;
        }

**其实其作用就是扫描所有jar包，将其类路径下 META-INF/spring.factories里面配置的所有EnableAutoConfiguration的值加入到了容器中；**  
查看spring-boot-autoconfigure-2.0.3.RELEASE.jar包的/spring.factories文件：

# 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.CassandraReactiveDataAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
    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.MongoReactiveDataAutoConfiguration,\
    org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
    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.RedisReactiveAutoConfiguration,\
    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.flyway.FlywayAutoConfiguration,\
    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.http.HttpMessageConvertersAutoConfiguration,\
    org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
    org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
    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.groovy.template.GroovyTemplateAutoConfiguration,\
    org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
    org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
    org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
    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.mongo.embedded.EmbeddedMongoAutoConfiguration,\
    org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
    org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
    org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
    org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
    org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
    org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
    org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
    org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
    org.springframework.boot.autoconfigure.security.oauth2.client.OAuth2ClientAutoConfiguration,\
    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.client.RestTemplateAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
    org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
    org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
    org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
    org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
    org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

其中类似 XxxxAutoConfiguration 类都是容器中的一个组件，都加入到容器中；用他们来做自动配置；

--------------------

**3、每一个自动配置类进行自动配置功能；**

--------------------

**4、以HttpEncodingAutoConfiguration（Http编码自动配置）为例解释自动配置原理；先来看看这个类的源码**  
HttpEncodingAutoConfiguration.class

@Configuration //表示这是一个配置类，也可以给容器中添加组件(配合@Bean使用)
    
    //启动指定类(HttpEncodingProperties.class)的ConfigurationProperties功能；
    //将配置文件中对应的值和HttpEncodingProperties绑定起来,并把HttpEncodingProperties加入到ioc容器中
    //HttpEncodingProperties.class类主要配置应用http请求和相应的编码格式
    @EnableConfigurationProperties({HttpEncodingProperties.class})
    
    //Spring底层@Conditional注解（Spring注解版），根据不同的条件，如果满足指定的条件，整个配置类里面的配置就会生效； 
    // 判断当前应用是否是web应用，如果是，当前配置类生效
    @ConditionalOnWebApplication(
        type = Type.SERVLET
    )
    
    //判断当前项目有没有这个类CharacterEncodingFilter；SpringMVC中进行乱码解决的过滤器；有当前配置生效
    @ConditionalOnClass({CharacterEncodingFilter.class})
    
    //判断配置文件中是否存在配置： spring.http.encoding.enabled；如果不存在，则使用默认值true
    @ConditionalOnProperty(
        prefix = "spring.http.encoding",
        value = {
       "enabled"},
        matchIfMissing = true
    )
    public class HttpEncodingAutoConfiguration { 
        private final HttpEncodingProperties properties;
    
        //只有一个有参构造器的情况下，参数的值就会从容器中拿
        public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
            this.properties = properties;
        }
    
        @Bean //给容器中添加一个组件，这个组件的某些值需要从properties中获取
        @ConditionalOnMissingBean
        public CharacterEncodingFilter characterEncodingFilter() {
            CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
            filter.setEncoding(this.properties.getCharset().name());
            filter.setForceRequestEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpEncodingProperties.Type.REQUEST));
            filter.setForceResponseEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpEncodingProperties.Type.RESPONSE));
            return filter;
        }
    
        @Bean
        public HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
            return new HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer(this.properties);
        }
    
        private static class LocaleCharsetMappingsCustomizer implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>, Ordered { 
            private final HttpEncodingProperties properties;
    
            LocaleCharsetMappingsCustomizer(HttpEncodingProperties properties) {
                this.properties = properties;
            }
    
            public void customize(ConfigurableServletWebServerFactory factory) {
                if (this.properties.getMapping() != null) {
                    factory.setLocaleCharsetMappings(this.properties.getMapping());
                }
    
            }
    
            public int getOrder() {
                return 0;
            }
        }
    }

**结论：**  
根据当前不同的条件判断，决定配置类HttpEncodingAutoConfiguration是否生效,一但这个配置类生效；这个配置类就会给容器中添加各种组件；这些组件的属性是从对应的properties类中获取的，这些类里面的每一个属性又是和配置文件绑定的；

--------------------

**5、所有在配置文件中能配置的属性都是在xxxxProperties类中封装着‘；配置文件能配置什么就可以参照某个功能对应的这个属性类xxxxProperties**

如：HttpEncodingAutoConfiguration.class上注解有引入配置类：

@EnableConfigurationProperties({
       HttpEncodingProperties.class})

查看配置类：HttpEncodingProperties.class源码：

@ConfigurationProperties(
        prefix = "spring.http.encoding"//从配置文件中获取指定的值和bean的属性进行绑定
    )
    public class HttpEncodingProperties { 
        public static final Charset DEFAULT_CHARSET;
        private Charset charset;
        private Boolean force;
        private Boolean forceRequest;
        private Boolean forceResponse;
        private Map<Locale, Charset> mapping;

所以当我们不使用SpringBoot的默认配置时，我们可以在配置文件中指定自己的配置值：  
如：

spring.http.encoding.charset=UTF-8

--------------------

### 二、自动配置精髓： ###

**1、SpringBoot启动会加载大量的自动配置类**

**2、我们看我们需要的功能有没有SpringBoot默认写好的自动配置类；**

**3、我们再来看这个自动配置类中到底配置了哪些组件；（只要我们要用的组件有，我们就不需要再来配置了）**

**4、给容器中自动配置类添加组件的时候，会从properties类中获取某些属性。我们就可以在配置文件中指定这些属性的值；**

**5、规律：**

<table> 
 <thead> 
  <tr> 
   <th>类名</th> 
   <th align="center">作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>xxxxAutoConfigurartion</td> 
   <td align="center">自动配置类,给容器中添加组件</td> 
  </tr> 
  <tr> 
   <td>xxxxProperties</td> 
   <td align="center">封装配置文件中相关属性；</td> 
  </tr> 
 </tbody> 
</table>

--------------------

### 三、自动配置中的其他细节 ###

#### 1、@Conditional派生注解（Spring注解版原生的@Conditional作用） ####

作用：必须是@Conditional指定的条件成立，才给容器中添加组件，配置配里面的所有内容才生效；

<table> 
 <thead> 
  <tr> 
   <th>@Conditional扩展注解</th> 
   <th>作用（判断是否满足当前指定条件）</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>@ConditionalOnJava</td> 
   <td>系统的java版本是否符合要求</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnBean</td> 
   <td>容器中存在指定Bean；</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnMissingBean</td> 
   <td>容器中不存在指定Bean；</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnExpression</td> 
   <td>满足SpEL表达式指定</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnClass</td> 
   <td>系统中有指定的类</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnMissingClass</td> 
   <td>系统中没有指定的类</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnSingleCandidate</td> 
   <td>容器中只有一个指定的Bean，或者这个Bean是首选Bean</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnProperty</td> 
   <td>系统中指定的属性是否有指定的值</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnResource</td> 
   <td>类路径下是否存在指定资源文件</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnWebApplication</td> 
   <td>当前是web环境</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnNotWebApplication</td> 
   <td>当前不是web环境</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnJndi</td> 
   <td>JNDI存在指定项</td> 
  </tr> 
 </tbody> 
</table>

**核心：自动配置类必须在一定的条件下才能生效；**  
所以我们启动时加载的大量配置类，但是并不是所有的生效的。

我们怎么知道哪些自动配置类生效？  
**我们可以通过启用 debug=true属性；来让控制台打印自动配置报告，这样我们就可以很方便的知道哪些自动配置类生效；**

debug=true

启动时就可以看到如下例子的报告信息（生效的配置类有哪些）：

============================
    CONDITIONS EVALUATION REPORT
    ============================
    
    
    Positive matches://生效的配置类
    -----------------
    
       CodecsAutoConfiguration matched:
          - @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
    
       CodecsAutoConfiguration.JacksonCodecConfiguration matched:
          - @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
    
       CodecsAutoConfiguration.JacksonCodecConfiguration#jacksonCodecCustomizer matched:
          - @ConditionalOnBean (types: com.fasterxml.jackson.databind.ObjectMapper; SearchStrategy: all) found bean 'jacksonObjectMapper' (OnBeanCondition)
    
    
    Negative matches://不生效的
    -----------------
    
       ActiveMQAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
    
       AopAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice', 'org.aspectj.weaver.AnnotatedElement' (OnClassCondition)
    
       ArtemisAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.artemis.jms.client.ActiveMQConnectionFactory' (OnClassCondition)

[Link 1]: https://docs.spring.io/spring-boot/docs/1.5.9.RELEASE/reference/htmlsingle/#common-application-properties
[SpringBoot]: /images/20220519/df137277d3ac4764b486682eceb979de.png