Spring Boot自动装配

分手后的思念是犯贱 2022-03-22 14:28 292阅读 0赞

# Spring Boot自动装配 #

## 题记 ##

在日常的开发过程中`Spring Boot`自动装配的特性给我们开发减少了很多重复性的工作，具体时如果实现自动装配的呢，我们会在下一篇文章做详细的分析，以及用源码解析跟实战方式带大家了解`Spring Boot` “自动化”的原因，我们将透过现象去发现本质。

Spring Boot介绍：
    Create stand-alone Spring applications 
    -- 创建一个独立的Spring应用
    Embed Tomcat, Jetty or Undertow directly (no need to deploy WAR files) 
    -- 嵌入式的web服务器Tomcat、Jetty、Undertow(无需部署 war 文件)
    Provide opinionated 'starter' dependencies to simplify your build configuration
    -- 提供建议的 "启动" 依赖关系, 以简化生成配置
    Automatically configure Spring and 3rd party libraries whenever possible
    -- 尽可能自动配置 spring 和第三方库
    Provide production-ready features such as metrics, health checks and externalized configuration
    -- 具备为生产准备的特性，如指标、运行状况检查和外部化配置
    Absolutely no code generation and no requirement for XML configuration
    -- 尽可能的无需代码生成并且无XML配置

话不多说，开始进入本次分享的主题，在`Spring Boot`中`@EnableAutoConfiguration`的使用显得十分普遍，使用越多遇到的问题可能也就越多。为什么笔者会这样说呢，因为在程序开发的过程中，引入新技术一般会有两个原因：

第一：更便捷，能明显提高开发效率，否则一般的团队很少会主动尝试新事物。毕竟新的就意味着有风险
    第二：对程序有显著地提升（此处是指可以是性能，也可以是健壮性，可用性，并发处理能力等等）

### 注解分析 ###

<table> 
 <thead> 
  <tr> 
   <th>注解名称</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>@Component</td> 
   <td>基础注解（元注解），可被嵌套</td> 
  </tr> 
  <tr> 
   <td>@Configuration</td> 
   <td>声明定义Bean，嵌套了@Component</td> 
  </tr> 
  <tr> 
   <td>@Import</td> 
   <td>导入类（如AutoConfigurationImportSelector.class）</td> 
  </tr> 
  <tr> 
   <td>@ImportResource</td> 
   <td>导入文件("classpath:/com/acme/database-config.xml")</td> 
  </tr> 
  <tr> 
   <td>@AutoConfigureAfter</td> 
   <td>条件注解控制相对顺序（在条件注入之后注入），Order实现绝对顺序控制</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnClass</td> 
   <td>当类存在时装配</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnMissingClass</td> 
   <td>当类不存在时装配</td> 
  </tr> 
  <tr> 
   <td>@EnableAutoConfiguration</td> 
   <td>使用该注解实现自动装配</td> 
  </tr> 
  <tr> 
   <td>@SpringBootConfiguration</td> 
   <td>包含@Configuration，启动类的标注</td> 
  </tr> 
  <tr> 
   <td>@PropertySource</td> 
   <td>properties文件加载</td> 
  </tr> 
  <tr> 
   <td>@ConditionalOnSystemProperty</td> 
   <td>properties文件中存在满足条件的属性时装配</td> 
  </tr> 
  <tr> 
   <td>@EnableConfigurationProperties</td> 
   <td>是带有@ConfigurationProperties注解的类生效</td> 
  </tr> 
 </tbody> 
</table>

在实际的使用中h还会有很多注解的定义与使用，在我们的日常开发中需要根据实际情况去选择使用。

### 自动装配 ###

由于`Spring Boot`自动装配的特性，所以在`jar`引入后，我们会得到`Spring`容器为我们量身定做的实例，即我们所谓的`Bean`，这个过程中当然不可或缺的有`RestTemplate`，使用`JDBC`或者`MyBatis`等时会有`JdbcTemplate`生成。

//在Spring Boot中通过@EnableAutoConfiguration实现自动装配对象的选择
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    //具体选择器实现
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
        
        ...
    }

该示例即为`RestTemplate`自动注入，包含条件注解：在`HttpMessageConvertersAutoConfiguration`自动注入之后；在`RestTemplate.class`存在时注入

@Configuration
    @AutoConfigureAfter(HttpMessageConvertersAutoConfiguration.class)
    @ConditionalOnClass(RestTemplate.class)
    public class RestTemplateAutoConfiguration {
        
        ...
            @Bean
    }

该示例即为`JdbcTemplate`自动注入，包含条件注解：在`DataSourceAutoConfiguration`自动注入之后；在`JdbcTemplate.class`和`DataSource.class`并且`DataSource`为单例时，存在时注入

@Configuration
    @ConditionalOnClass({ DataSource.class, JdbcTemplate.class })
    @ConditionalOnSingleCandidate(DataSource.class)
    @AutoConfigureAfter(DataSourceAutoConfiguration.class)
    @EnableConfigurationProperties(JdbcProperties.class)
    public class JdbcTemplateAutoConfiguration {
        
        @Bean
        @Bean
            @Primary
            @ConditionalOnMissingBean(JdbcOperations.class)
            public JdbcTemplate jdbcTemplate() {
        
                ...
            }
    }

`RestTemplateAutoConfiguration`和`JdbcTemplateAutoConfiguration`在spring-boot-autoconfiguration包下的`META-INF/spring.factories`中声明

# Auto Configure
    org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
    org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
    org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration

而后被`AutoConfigurationImportSelector`选择加载，该类实现了`ImportSelector`接口，具体加载的位置在`invokeBeanFactoryPostProcessors(beanFactory);`注册的时候，实现`Bean`的注册；而在`Bean`注册之前会有`BeanClass`，`Resource`，`Environment`等的属性注入；`Ordered`接口主要是确定加载的顺序。

public class AutoConfigurationImportSelector     implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
        
        ...
            @Override
            public String[] selectImports(AnnotationMetadata annotationMetadata) {
        
            if (!isEnabled(annotationMetadata)) {
        
                return NO_IMPORTS;
            }
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                                                                     attributes);
            configurations = removeDuplicates(configurations);
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = filter(configurations, autoConfigurationMetadata);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return StringUtils.toStringArray(configurations);
        }
    }

如上所述可以实现，`RestTemplateAutoConfiguration`和`JdbcTemplateAutoConfiguration`的自动注入得以实现。

### 可自动装配列表 ###

# 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

### 实现自动装配 ###

首先梳理自动装配的流程

定义`HelloWorldAutoConfiguration`

/**  * @ClassName: HelloWorldAutoConfiguration  * @Description: HelloWorld 自动装配  * @Author: 尚先生  * @CreateDate: 2019/1/28  * @Version: 1.0  */
    @Configuration // Spring 模式注解装配
    @EnableHelloWorld // Spring @Enable 模块装配
    public class HelloWorldAutoConfiguration {
        
        @Bean
        public String helloWorld() { // 方法名即 Bean 名称
            return "Hello,World sxs";
        }
    }

配置注解标记类

/**  * @ClassName: EnableHelloWorld  * @Description: HelloWorld 自动装配注解  * @Author: 尚先生  * @CreateDate: 2019/1/28  * @Version: 1.0  */
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE)
    @Documented
    @Import(HelloWorldConfiguration.class)
    public @interface EnableHelloWorld {
        
    }

配置META-INF/spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
    com.sxs.springboot.configuration.HelloWorldAutoConfiguration

### 测试实现结果 ###

/**  * @ClassName: EnableHelloWorldBootstrap  * @Description: HelloWorld 自动装配引导类（测试类）  * @Author: 尚先生  * @CreateDate: 2019/1/28  * @Version: 1.0  */
    @EnableHelloWorld
    public class EnableHelloWorldBootstrap {
        
    
        public static void main(String[] args) {
        
            ConfigurableApplicationContext context = new SpringApplicationBuilder(EnableHelloWorldBootstrap.class)
                    .web(WebApplicationType.NONE)
                    .run(args);
            // helloWorld Bean 是否存在
            String helloWorld = context.getBean("helloWorld", String.class);
            System.out.println("helloWorld Bean : " + helloWorld);
            // 关闭上下文
            context.close();
        }
    }

### 测试结果 ###

helloWorld Bean : Hello,World sxs

### 自动装配的过程 ###

1. 通过各种注解实现了类与类之间的依赖关系，容器在启动的时候Application.run，会调用EnableAutoConfigurationImportSelector.class的selectImports方法（其实是其父类的方法）
    
    2. selectImports方法最终会调用SpringFactoriesLoader.loadFactoryNames方法来获取一个全面的常用BeanConfiguration列表
    
    3. loadFactoryNames方法会读取FACTORIES_RESOURCE_LOCATION（也就是spring-boot-autoconfigure.jar 下面的spring.factories），获取到所有的Spring相关的Bean的全限定名ClassName，大概120多个
    
    4. selectImports方法继续调用filter(configurations, autoConfigurationMetadata);这个时候会根据这些BeanConfiguration里面的条件，来一一筛选，最关键的是
    @ConditionalOnClass，这个条件注解会去classpath下查找，jar包里面是否有这个条件依赖类，所以必须有了相应的jar包，才有这些依赖类，才会生成IOC环境需要的一些默认配置Bean
    
    5. 最后把符合条件的BeanConfiguration注入默认的EnableConfigurationPropertie类里面的属性值，并且注入到IOC环境当中

### 往期文章 ###

博客地址：https://blog.csdn.net/shang_xs
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