Mybatis基于MapperScan注解的动态代理加载机制详解

package com.changshin.dao;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.changshin.entity.po.User;
/**
 * 后台管理用户表 dao   注意：只是一个接口没有对应的实现类
 *
 */
public interface UserDao extends BaseMapper<User> {
    public User selectUserNameById(Integer id);
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.changshin.dao.UserDao">
    <select id="selectUserNameById" parameterType="Integer" resultType="com.changshin.entity.po.User">
        select username from t_user where id = #{id}
    </select>
</mapper>

package com.changshin.config;
import com.baomidou.mybatisplus.extension.plugins.PaginationInterceptor;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.transaction.annotation.EnableTransactionManagement;
/**
 * MybatisPlus配置
 */
@Configuration
@EnableTransactionManagement(order = 2)
@MapperScan(value = "com.changshin.dao")
public class MybatisPlusConfig {
    /**
     * mybatis-plus分页插件
     */
    @Bean
    public PaginationInterceptor paginationInterceptor() {
        return new PaginationInterceptor();
    }
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)  //关注该行代码
@Repeatable(MapperScans.class)
public @interface MapperScan {

//实现了ImportBeanDefinitionRegistrar接口
public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {

  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
     //注意该方法
      registerBeanDefinitions(importingClassMetadata, mapperScanAttrs, registry,
          generateBaseBeanName(importingClassMetadata, 0));
    }
  }

void registerBeanDefinitions(AnnotationMetadata annoMeta, AnnotationAttributes annoAttrs,
      BeanDefinitionRegistry registry, String beanName) {
    //获取MapperScannerConfigurer的bean定义，注意该行方法
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    builder.addPropertyValue("processPropertyPlaceHolders", true);
 
    Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
    if (!Annotation.class.equals(annotationClass)) {
      builder.addPropertyValue("annotationClass", annotationClass);
    }
    //省略.....
}

public class MapperScannerConfigurer
    implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
    //省略....
}

@Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      //该方法可以加载包信息
      //PropertyValues values = mapperScannerBean.getPropertyValues();
      //this.basePackage = updatePropertyValue("basePackage", values);
      processPropertyPlaceHolders();
    }
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
    scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
    scanner.setResourceLoader(this.applicationContext);
    scanner.setBeanNameGenerator(this.nameGenerator);
    scanner.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
    if (StringUtils.hasText(lazyInitialization)) {
      scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization));
    }
    scanner.registerFilters();
    //扫描mapperscan里配置的com.changshin.dao包
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {
    //省略。。。。。
}

public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
        //注意该方法调用的并非是ClassPathBeanDefinitionScanner里的doScan方法，而是ClassPathMapperScanner重写的doScan方法
		doScan(basePackages);
		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}
		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

@Override
  public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    //调用父类ClassPathBeanDefinitionScanner的doScan加载bean定义
    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
    if (beanDefinitions.isEmpty()) {
      LOGGER.warn(() -> "No MyBatis mapper was found in '" + Arrays.toString(basePackages)
          + "' package. Please check your configuration.");
    } else {
      //注意该方法
      processBeanDefinitions(beanDefinitions);
    }
    return beanDefinitions;
  }

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
            //获取符合要求的bean定义
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
                    //将bean定义注册到registry中
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      String beanClassName = definition.getBeanClassName();
      //省略。。。。。
      //将bean对应的class替换成MapperFactoryBean
      //private Class<? extends MapperFactoryBean> mapperFactoryBeanClass = MapperFactoryBean.class;  
      definition.setBeanClass(this.mapperFactoryBeanClass);
      //省略。。。。。
      if (!explicitFactoryUsed) {
        LOGGER.debug(() -> "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
        //通过AUTOWIRE_BY_TYPE注入bean,注意此处在后面会用到,bean在初始化时会调用beanClass里的set方法
        definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }
      definition.setLazyInit(lazyInitialization);
    }
  }

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
    //省略。。。。。。。。。。。
}

public abstract class SqlSessionDaoSupport extends DaoSupport {
  private SqlSessionTemplate sqlSessionTemplate;
  /**
   * Set MyBatis SqlSessionFactory to be used by this DAO. Will automatically create SqlSessionTemplate for the given
   * SqlSessionFactory.
   *
   * @param sqlSessionFactory
   *          a factory of SqlSession
   */
  public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (this.sqlSessionTemplate == null || sqlSessionFactory != this.sqlSessionTemplate.getSqlSessionFactory()) {
      this.sqlSessionTemplate = createSqlSessionTemplate(sqlSessionFactory);
    }
  }
  /**
   * Create a SqlSessionTemplate for the given SqlSessionFactory. Only invoked if populating the DAO with a
   * SqlSessionFactory reference!
   * <p>
   * Can be overridden in subclasses to provide a SqlSessionTemplate instance with different configuration, or a custom
   * SqlSessionTemplate subclass.
   * 
   * @param sqlSessionFactory
   *          the MyBatis SqlSessionFactory to create a SqlSessionTemplate for
   * @return the new SqlSessionTemplate instance
   * @see #setSqlSessionFactory
   */
  @SuppressWarnings("WeakerAccess")
  protected SqlSessionTemplate createSqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    return new SqlSessionTemplate(sqlSessionFactory);
  }
  /**
   * Return the MyBatis SqlSessionFactory used by this DAO.
   *
   * @return a factory of SqlSession
   */
  public final SqlSessionFactory getSqlSessionFactory() {
    return (this.sqlSessionTemplate != null ? this.sqlSessionTemplate.getSqlSessionFactory() : null);
  }
  /**
   * Set the SqlSessionTemplate for this DAO explicitly, as an alternative to specifying a SqlSessionFactory.
   *
   * @param sqlSessionTemplate
   *          a template of SqlSession
   * @see #setSqlSessionFactory
   */
  public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
    this.sqlSessionTemplate = sqlSessionTemplate;
  }
  /**
   * Users should use this method to get a SqlSession to call its statement methods This is SqlSession is managed by
   * spring. Users should not commit/rollback/close it because it will be automatically done.
   *
   * @return Spring managed thread safe SqlSession
   */
  public SqlSession getSqlSession() {
    return this.sqlSessionTemplate;
  }
  /**
   * Return the SqlSessionTemplate for this DAO, pre-initialized with the SessionFactory or set explicitly.
   * <p>
   * <b>Note: The returned SqlSessionTemplate is a shared instance.</b> You may introspect its configuration, but not
   * modify the configuration (other than from within an {@link #initDao} implementation). Consider creating a custom
   * SqlSessionTemplate instance via {@code new SqlSessionTemplate(getSqlSessionFactory())}, in which case you're
   * allowed to customize the settings on the resulting instance.
   *
   * @return a template of SqlSession
   */
  public SqlSessionTemplate getSqlSessionTemplate() {
    return this.sqlSessionTemplate;
  }
  /**
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    notNull(this.sqlSessionTemplate, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
  }
}

    @Bean
    @ConditionalOnMissingBean
    public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        ExecutorType executorType = this.properties.getExecutorType();
        if (executorType != null) {
            return new SqlSessionTemplate(sqlSessionFactory, executorType);
        } else {
            return new SqlSessionTemplate(sqlSessionFactory);
        }
    }

    @Bean
    @ConditionalOnMissingBean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
        // TODO 使用 MybatisSqlSessionFactoryBean 而不是 SqlSessionFactoryBean
        MybatisSqlSessionFactoryBean factory = new MybatisSqlSessionFactoryBean();
        factory.setDataSource(dataSource);
        factory.setVfs(SpringBootVFS.class);
        if (StringUtils.hasText(this.properties.getConfigLocation())) {
            factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
        }
        applyConfiguration(factory);
        if (this.properties.getConfigurationProperties() != null) {
            factory.setConfigurationProperties(this.properties.getConfigurationProperties());
        }
        if (!ObjectUtils.isEmpty(this.interceptors)) {
            factory.setPlugins(this.interceptors);
        }
        if (this.databaseIdProvider != null) {
            factory.setDatabaseIdProvider(this.databaseIdProvider);
        }
        if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
            factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
        }
        if (this.properties.getTypeAliasesSuperType() != null) {
            factory.setTypeAliasesSuperType(this.properties.getTypeAliasesSuperType());
        }
        if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
            factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
        }
        if (!ObjectUtils.isEmpty(this.typeHandlers)) {
            factory.setTypeHandlers(this.typeHandlers);
        }
        // 注意该行代码获取xml配置路径
        Resource[] mapperLocations = this.properties.resolveMapperLocations();
        if (!ObjectUtils.isEmpty(mapperLocations)) {
            factory.setMapperLocations(mapperLocations);
        }
        // TODO 修改源码支持定义 TransactionFactory
        this.getBeanThen(TransactionFactory.class, factory::setTransactionFactory);
        // TODO 对源码做了一定的修改(因为源码适配了老旧的mybatis版本,但我们不需要适配)
        Class<? extends LanguageDriver> defaultLanguageDriver = this.properties.getDefaultScriptingLanguageDriver();
        if (!ObjectUtils.isEmpty(this.languageDrivers)) {
            factory.setScriptingLanguageDrivers(this.languageDrivers);
        }
        Optional.ofNullable(defaultLanguageDriver).ifPresent(factory::setDefaultScriptingLanguageDriver);
        // TODO 自定义枚举包
        if (StringUtils.hasLength(this.properties.getTypeEnumsPackage())) {
            factory.setTypeEnumsPackage(this.properties.getTypeEnumsPackage());
        }
        // TODO 此处必为非 NULL
        GlobalConfig globalConfig = this.properties.getGlobalConfig();
        // TODO 注入填充器
        this.getBeanThen(MetaObjectHandler.class, globalConfig::setMetaObjectHandler);
        // TODO 注入主键生成器
        this.getBeanThen(IKeyGenerator.class, i -> globalConfig.getDbConfig().setKeyGenerator(i));
        // TODO 注入sql注入器
        this.getBeanThen(ISqlInjector.class, globalConfig::setSqlInjector);
        // TODO 注入ID生成器
        this.getBeanThen(IdentifierGenerator.class, globalConfig::setIdentifierGenerator);
        // TODO 设置 GlobalConfig 到 MybatisSqlSessionFactoryBean
        factory.setGlobalConfig(globalConfig);
        //注意该行方法
        return factory.getObject();
    }

final SqlSessionFactory sqlSessionFactory = new MybatisSqlSessionFactoryBuilder().build(targetConfiguration);

SqlSessionFactory sqlSessionFactory = super.build(configuration);

 public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

  @Override
  public <T> T getMapper(Class<T> type) {
    return getConfiguration().getMapper(type, this);
  }

   public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        return mybatisMapperRegistry.getMapper(type, sqlSession);
    }

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        // TODO 这里换成 MybatisMapperProxyFactory 而不是 MapperProxyFactory
        final MybatisMapperProxyFactory<T> mapperProxyFactory = (MybatisMapperProxyFactory<T>) knownMappers.get(type);
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MybatisPlusMapperRegistry.");
        }
        try {
            //注意该行代码
            return mapperProxyFactory.newInstance(sqlSession);
        } catch (Exception e) {
            throw new BindingException("Error getting mapper instance. Cause: " + e, e);
        }
    }

 @SuppressWarnings("unchecked")
    protected T newInstance(MybatisMapperProxy<T> mapperProxy) {
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
    }
    public T newInstance(SqlSession sqlSession) {
        //该sqlSession是sqlSessionTemplate类型的对象
        final MybatisMapperProxy<T> mapperProxy = new MybatisMapperProxy<>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
    }