目录

1. mybatis中mapper代理的生成过程
2. 与Spring集成时mapper代理的生成过程
3. 与SpringBoot集成时mapper代理的生成过程

mybatis中mapper代理的生成过程

构建代理类工厂

从入口点开始一步一步看，首先SqlSessionFactoryBuilder类中build()方法加载配置文件

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      // ...省略
    }
  }

将配置文件读取为XMLConfigBuilder对象，并调用parse()方法来解析文件，进到parse()中

  public Configuration parse() {
     // ...省略
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

可以看到具体的解析过程是在parseConfiguration方法中进行的。

  private void parseConfiguration(XNode root) {
    try {
       // ...省略
      //解析mapper
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

这里重点看一下最后解析mapper的方法mapperElement(root.evalNode("mappers"))，进到方法里，

  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          //   package 形式加载 ,加载package下的所有class文件
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            // 通过Mapper.xml 加载
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {
            // 通过Mapper.xml 加载
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            // 通过单个class文件加载
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

整个mapperElement()方法就是加载mapper的过程了，可以看到加载mapper
有两种形式：通过class文件和通过xml文件。
构建mapper代理的过程也就是从这开始的，那就一步一步分析。
看一下通过XML文件加载的过程，mybatis将mapper相关的配置读取为一个XMLMapperBuilder对象，并通过parse()方法进行解析，进到这个方法中

  public void parse() {
    if (!configuration.isResourceLoaded(resource)) {
      // 加载xml文件
      configurationElement(parser.evalNode("/mapper"));
      configuration.addLoadedResource(resource);
      // 加载mapper class文件
      bindMapperForNamespace();
    }
   // ...省略
  }

parse()方法做了主要做了两件事，加载xml文件和加载class文件。
看一下加载xml的过程

  private void configurationElement(XNode context) {
    try {
      // 获取xml文件的namespace
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      // 保存获取xml文件的namespace
      builderAssistant.setCurrentNamespace(namespace);
       // ...省略
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

本文是分析mapper代理的生成过程，所以加载xml的具体细节就不详细分析了，这里注意的是读取xml文件中namespace标签的值，并将值设置到builderAssistant对象中
现在回过头来看一下加载class文件的过程。进到bindMapperForNamespace()方法中去

  private void bindMapperForNamespace() {
    // 获取xml文件中设置的namespace值
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
      Class<?> boundType = null;
      try {
        // 加载类
        boundType = Resources.classForName(namespace);
      } catch (ClassNotFoundException e) {
        //ignore, bound type is not required
      }
      if (boundType != null) {
        if (!configuration.hasMapper(boundType)) {
          // Spring may not know the real resource name so we set a flag
          // to prevent loading again this resource from the mapper interface
          // look at MapperAnnotationBuilder#loadXmlResource
          configuration.addLoadedResource("namespace:" + namespace);
          // 添加到configuration中
          configuration.addMapper(boundType);
        }
      }
    }
  }

bindMapperForNamespace()通过xml文件中设置的namespace值加载对应的mapper接口，最后通过configuration.addMapper()添加到configuration中。

还记不记得刚才提到的加载mapper
有两种形式：通过class文件和通过xml文件。通过class文件的方式直接调用configuration.addMapper()将mapper接口加载到了configuration 中了。

Configuration是mybatis的全局配置类，所有的mybatis相关的信息都保存在Configuration中。
继续进到Configuration的addMapper方法中

  public <T> void addMapper(Class<T> type) {
    mapperRegistry.addMapper(type);
  }

Configuration把对应的mapper接口添加到mapperRegistry中，再进到mapperRegistry.addMapper()方法中

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
        // ...省略
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
         // ...省略
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

该方法首先判断是否是接口，如果是接口则将mapper接口添加到knownMappers中。
看一下knownMappers的定义

  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

knownMappers是一个HashMap，它保存的是所有的mapper接口和对应的mapper代理工厂。

到现在为止，mapper已经加载完了，但是并没有生成mapper的代理对象，只是生成了对应的代理工厂。

生成并使用代理对象

mybatis并没有在加载mapper接口的时候生成代理对象，而是在调用的时候生成的。
首先从入口开始

sqlSession.getMapper(XXX.class)

sqlSession默认是DefaultSqlSession。进到DefaultSqlSession的getMapper()方法中

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

继续到Configuration的getMapper中

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

继续到mapperRegistry.getMapper()中

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    // ...省略
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

从knownMappers中获取到对应mapper接口的代理工厂类MapperProxyFactory，然后通过MapperProxyFactory获取真正的代理对象。
进到MapperProxyFactory的newInstance()方法中

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
  
   protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

首先生成了MapperProxy类，再通过Proxy生成真正的代理类。
看一下MapperProxy类

public class MapperProxy<T> implements InvocationHandler, Serializable {
  //  ...省略
}

MapperProxy实现了InvocationHandler接口，mapper接口的具体处理逻辑也就是在这类中处理。

到此为止，代理对象才真正的生成。

与Spring集成时mapper代理的生成过程

mybatis与Spring集成时需要用到mybatis-spring的jar。

Spring注册mapper代理类

既然是与Spring集成，那么就要配置一下，将mybatis交给Spring管理。
spring的xml文件配置

    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
        <property name="driverClassName" value="driverClassName"/>
        <property name="url" value="url"/>
        <property name="username" value="username"/>
        <property name="password" value="password"/>
    </bean>

    <!--sqlSessionFactory-->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource"/>
        <!--绑定mybatis配置文件-->
        <property name="configLocation" value="classpath:mybatis-config.xml"/>
        <!--注册Mapper.xm映射器-->
        <property name="mapperLocations" value="classpath:cn/ycl/mapper/*.xml"/>
    </bean>

    <!--注册所有mapper-->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <!--basePackage 属性是映射器接口文件的包路径。-->
        <!--你可以使用分号或逗号 作为分隔符设置多于一个的包路径-->
        <property name="basePackage" value="cn/ycl/mapper"/>
        <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
    </bean>

将mybatis交给Spring只需要配置3个bean就可以了
1、 数据库相关的dataSource
2、 mybatis的sqlSessionFactory
3、 将mapper委托给Spring的工具类MapperScannerConfigurer
生成mapper代理的过程主要在MapperScannerConfigurer里，看一下MapperScannerConfigurer的定义

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

关键点在MapperScannerConfigurer 实现了BeanDefinitionRegistryPostProcessor，BeanDefinitionRegistryPostProcessor是Spring留的扩展点，可以往Spring中注册自定义的bean。

MapperScannerConfigurer中实现了BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry()方法，mapper的注册就是在该方法中注册的

  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    // ...省略
    
    // 实例化ClassPathMapperScanner，并对scanner相关属性进行配置
    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));
    }
    if (StringUtils.hasText(defaultScope)) {
      scanner.setDefaultScope(defaultScope);
    }
    // 注册扫描规则
    scanner.registerFilters();
    // 扫描并注册所有的mapper
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

postProcessBeanDefinitionRegistry()的主要逻辑是定义一个ClassPathMapperScanner对象，然后调用registerFilters()注册扫描规则,最后调用scan()方法。

在xml中定义MapperScannerConfigurerbean时可以设置一个annotationClass属性，值是一个注解类，调用registerFilters()时，registerFilters()会添加一个只扫描设置有annotationClass注解的类，这里没有设置，会扫描所有的接口。SpringBoot集成mybatis时会用到这个字段

看一下ClassPathMapperScanner类的定义

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

ClassPathMapperScanner继承了ClassPathBeanDefinitionScanner，ClassPathBeanDefinitionScanner是Spring中定义的，是一个从指定包内扫描所有bean定义的Spring工具。

看一下ClassPathMapperScanner的scan()方法

  public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);

    if (beanDefinitions.isEmpty()) {
        // ...省略
    } else {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

通过super.doScan(basePackages)已经扫描到了所有的mapper，继续processBeanDefinitions()方法

  private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    AbstractBeanDefinition definition;
    BeanDefinitionRegistry registry = getRegistry();
    // 遍历扫描到的所有bean
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (AbstractBeanDefinition) holder.getBeanDefinition();
      boolean scopedProxy = false;
      if (ScopedProxyFactoryBean.class.getName().equals(definition.getBeanClassName())) {
        definition = (AbstractBeanDefinition) Optional
            .ofNullable(((RootBeanDefinition) definition).getDecoratedDefinition())
            .map(BeanDefinitionHolder::getBeanDefinition).orElseThrow(() -> new IllegalStateException(
                "The target bean definition of scoped proxy bean not found. Root bean definition[" + holder + "]"));
        scopedProxy = true;
      }
      String beanClassName = definition.getBeanClassName();
      LOGGER.debug(() -> "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName
          + "' mapperInterface");

 
      // 增加一个构造方法，接口类型作为构造函数的入参
      definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); 

      // 将bean的类型转换成mapperFactoryBean
      definition.setBeanClass(this.mapperFactoryBeanClass);

      // 增加addToConfig属性
      definition.getPropertyValues().add("addToConfig", this.addToConfig);

  
      definition.setAttribute(FACTORY_BEAN_OBJECT_TYPE, beanClassName);

      boolean explicitFactoryUsed = false;
      // 增加sqlSessionFactory属性
      if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
        definition.getPropertyValues().add("sqlSessionFactory",
            new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
        explicitFactoryUsed = true;
      } else if (this.sqlSessionFactory != null) {
        definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
        explicitFactoryUsed = true;
      }

      // 增加sqlSessionTemplate属性
      if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
        if (explicitFactoryUsed) {
          LOGGER.warn(
              () -> "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
        }
        definition.getPropertyValues().add("sqlSessionTemplate",
            new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
        explicitFactoryUsed = true;
      } else if (this.sqlSessionTemplate != null) {
        if (explicitFactoryUsed) {
          LOGGER.warn(
              () -> "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
        }
        definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
        explicitFactoryUsed = true;
      }

      if (!explicitFactoryUsed) {
        LOGGER.debug(() -> "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
        definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }

      definition.setLazyInit(lazyInitialization);

      if (scopedProxy) {
        continue;
      }

      if (ConfigurableBeanFactory.SCOPE_SINGLETON.equals(definition.getScope()) && defaultScope != null) {
        definition.setScope(defaultScope);
      }

      if (!definition.isSingleton()) {
        BeanDefinitionHolder proxyHolder = ScopedProxyUtils.createScopedProxy(holder, registry, true);
        if (registry.containsBeanDefinition(proxyHolder.getBeanName())) {
          registry.removeBeanDefinition(proxyHolder.getBeanName());
        }
        registry.registerBeanDefinition(proxyHolder.getBeanName(), proxyHolder.getBeanDefinition());
      }

    }
  }

这个方法比较长，但是并不复杂，主要逻辑为将扫描的bean的类型修改成MapperFactoryBean类型，并增加一个将接口类型作为入参的构造函数，也就是说Spring获取mapper时都是通过FactoryBean生成的。最后通过调用egistry.registerBeanDefinition() 方法注册到Spring中。

看一下mybatis提供的MapperFactoryBean的定义

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

MapperFactoryBean实现了FactoryBean，FactoryBean是一个Spring提供的一个能生产对象的工厂Bean

MapperFactoryBean同时继承了SqlSessionDaoSupport，SqlSessionDaoSupport继承了DaoSupport，DaoSupport实现了InitializingBean。InitializingBean的作用是在Spring初始化bean对象时会首先调用InitializingBean的afterPropertiesSet()方法。

DaoSupport的afterPropertiesSet()中调用了checkDaoConfig()方法。

    public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {
        this.checkDaoConfig();

        try {
            this.initDao();
        } catch (Exception var2) {
            throw new BeanInitializationException("Initialization of DAO failed", var2);
        }
    }

具体checkDaoConfig()方法的实现逻辑在MapperFactoryBean 中

 protected void checkDaoConfig() {
    super.checkDaoConfig();

    notNull(this.mapperInterface, "Property 'mapperInterface' is required");

    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
        // ..省略
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

OK，到这又回到mybatis了。在前面中说了configuration.addMapper()方法只是生成了对应的代理工厂。

以上整个过程，即把mapper注册为Spring的bean，又将mapper设置到mybatis中的configuration中，所以，在使用时既可以使用Spring自动注入那一套，又可以使用mybatis中通过sqlSession来获取mapper的代理对象

Spring生成代理对象

Spring中所有的mapper对应的bean是mapper对应的MapperFactoryBean，那么在获取mapper bean时是通过MapperFactoryBean的getObject()方法生成的

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

MapperFactoryBean先获取到sqlsession，再通过getMapper()获取到的代理对象。到这里就回到了mybatis生成代理对象的过程了。

与SpringBoot集成时mapper代理的生成过程

mybatis与Spring集成时需要用到mybatis-spring-boot-starter的jar，mybatis-spring-boot-starter依赖mybatis-spring-boot-autoconfigure这个jar，而mybatis-spring-boot-autoconfigure这个jar又依赖mybatis-spring这个jar，所以最终其实还是mybatis集成Spring那一套

根据SpringBoot自动加载的原理直接看mybatis-spring-boot-autoconfigurejar下META-INF/spring.factories文件

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

SpringBoot会自动加载MybatisAutoConfiguration这个类，直接看这个类，MybatisAutoConfiguration定义了mybtis所需的各个bean。

    //生成SqlSessionFactory
    @Bean
    @ConditionalOnMissingBean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
        // ...省略
    }
    
    //生成SqlSessionTemplate
    @Bean
    @ConditionalOnMissingBean
    public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
     // ...省略
    }
    
    //扫描mapper
     @Configuration
    @Import({MybatisAutoConfiguration.AutoConfiguredMapperScannerRegistrar.class})
    @ConditionalOnMissingBean({MapperFactoryBean.class, MapperScannerConfigurer.class})
    public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {
        public MapperScannerRegistrarNotFoundConfiguration() {
        }

        public void afterPropertiesSet() {
            MybatisAutoConfiguration.logger.debug("Not found configuration for registering mapper bean using @MapperScan, MapperFactoryBean and MapperScannerConfigurer.");
        }
    }
     //扫描mapper
     public static class AutoConfiguredMapperScannerRegistrar implements BeanFactoryAware, ImportBeanDefinitionRegistrar {
        private BeanFactory beanFactory;

        public AutoConfiguredMapperScannerRegistrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
            if (!AutoConfigurationPackages.has(this.beanFactory)) {
                MybatisAutoConfiguration.logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.");
            } else {
                MybatisAutoConfiguration.logger.debug("Searching for mappers annotated with @Mapper");
                List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
                if (MybatisAutoConfiguration.logger.isDebugEnabled()) {
                    packages.forEach((pkg) -> {
                        MybatisAutoConfiguration.logger.debug("Using auto-configuration base package '{}'", pkg);
                    });
                }
                //生成MapperScannerConfigurer 
                BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
                builder.addPropertyValue("processPropertyPlaceHolders", true);
                // 注册扫描规则
                builder.addPropertyValue("annotationClass", Mapper.class);
                builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
                BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
                Stream.of(beanWrapper.getPropertyDescriptors()).filter((x) -> {
                    return x.getName().equals("lazyInitialization");
                }).findAny().ifPresent((x) -> {
                    builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}");
                });
                registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
            }
        }

        public void setBeanFactory(BeanFactory beanFactory) {
            this.beanFactory = beanFactory;
        }
    }