Spring源码解析之整合Mybatis

阅读须知

• 整合包版本：1.3.1
• 文章中使用/* */注释的方法会做深入分析做深入分析

正文

首先我们来看Spring整合Mybatis的配置：

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="configLocation" value="classpath:mybatis/mybatis-config.xml"/>
    <property name="dataSource" ref="dataSource"/>
    <property name="typeAliasesPackage" value="com.xxx.entity"/>
    <property name="mapperLocations" value="classpath:mapper/* .xml"/>
bean>

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="com.xxx.dao"/>
    <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
bean>

我们就以配置的解析入手，首先我们来看一下SqlSessionFactoryBean的层次结构：

我们发现了两个我们比较关心的接口，FactoryBean和InitializingBean，我们知道，实现了FactoryBean的bean会调用它的getObject方法创建bean，实现了InitializingBean的bean会在属性填充完成之后调用它的afterPropertiesSet方法，我们就来分析这个方法：
SqlSessionFactoryBean：

public void afterPropertiesSet() throws Exception {
    // 属性校验
    notNull(dataSource, "Property 'dataSource' is required");
    notNull(sqlSessionFactoryBuilder, "Property 'sqlSessionFactoryBuilder' is required");
    state((configuration == null && configLocation == null) || !(configuration != null && configLocation != null),
        "Property 'configuration' and 'configLocation' can not specified with together");
    /* 构建SqlSessionFactory */
    this.sqlSessionFactory = buildSqlSessionFactory();
}

SqlSessionFactoryBean：

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
    Configuration configuration;
    XMLConfigBuilder xmlConfigBuilder = null;
    // Configuration的几种配置
    if (this.configuration != null) {
        configuration = this.configuration;
        if (configuration.getVariables() == null) {
            configuration.setVariables(this.configurationProperties);
        } else if (this.configurationProperties != null) {
            configuration.getVariables().putAll(this.configurationProperties);
        }
    } else if (this.configLocation != null) {
        xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
        configuration = xmlConfigBuilder.getConfiguration();
    } else {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration");
        }
        configuration = new Configuration();
        if (this.configurationProperties != null) {
            configuration.setVariables(this.configurationProperties);
        }
    }
    // objectFactory配置
    if (this.objectFactory != null) {
        configuration.setObjectFactory(this.objectFactory);
    }
    // objectWrapperFactory配置
    if (this.objectWrapperFactory != null) {
        configuration.setObjectWrapperFactory(this.objectWrapperFactory);
    }
    // vfs配置
    if (this.vfs != null) {
        configuration.setVfsImpl(this.vfs);
    }
    // typeAliasesPackage配置
    if (hasLength(this.typeAliasesPackage)) {
        String[] typeAliasPackageArray = tokenizeToStringArray(this.typeAliasesPackage,
            ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
        for (String packageToScan : typeAliasPackageArray) {
            configuration.getTypeAliasRegistry().registerAliases(packageToScan,
                typeAliasesSuperType == null ? Object.class : typeAliasesSuperType);
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Scanned package: '" + packageToScan + "' for aliases");
            }
        }
    }
    // typeAliases配置
    if (!isEmpty(this.typeAliases)) {
        for (Class<?> typeAlias : this.typeAliases) {
            configuration.getTypeAliasRegistry().registerAlias(typeAlias);
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Registered type alias: '" + typeAlias + "'");
            }
        }
    }
    // plugins配置
    if (!isEmpty(this.plugins)) {
        for (Interceptor plugin : this.plugins) {
            configuration.addInterceptor(plugin);
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Registered plugin: '" + plugin + "'");
            }
        }
    }
    // typeHandlersPackage配置
    if (hasLength(this.typeHandlersPackage)) {
        String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
            ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
        for (String packageToScan : typeHandlersPackageArray) {
            configuration.getTypeHandlerRegistry().register(packageToScan);
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Scanned package: '" + packageToScan + "' for type handlers");
            }
        }
    }
    // typeHandlers配置
    if (!isEmpty(this.typeHandlers)) {
        for (TypeHandler<?> typeHandler : this.typeHandlers) {
            configuration.getTypeHandlerRegistry().register(typeHandler);
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Registered type handler: '" + typeHandler + "'");
            }
        }
    }
    // databaseIdProvider配置
    if (this.databaseIdProvider != null) {
        try {
            configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
        } catch (SQLException e) {
            throw new NestedIOException("Failed getting a databaseId", e);
        }
    }
    // cache配置
    if (this.cache != null) {
        configuration.addCache(this.cache);
    }
    if (xmlConfigBuilder != null) {
        try {
            // 如果配置了configLocation，则解析配置文件
            xmlConfigBuilder.parse();
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Parsed configuration file: '" + this.configLocation + "'");
            }
        } catch (Exception ex) {
            throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
        } finally {
            ErrorContext.instance().reset();
        }
    }
    if (this.transactionFactory == null) {
        // 如果没有transactionFactory配置，应用SpringManagedTransactionFactory
        this.transactionFactory = new SpringManagedTransactionFactory();
    }
    // 为Configuration设置环境
    configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));
    // mapperLocations配置
    if (!isEmpty(this.mapperLocations)) {
        for (Resource mapperLocation : this.mapperLocations) {
            if (mapperLocation == null) {
                continue;
            }

            try {
                // 解析mapper配置
                XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
                    configuration, mapperLocation.toString(), configuration.getSqlFragments());
                xmlMapperBuilder.parse();
            } catch (Exception e) {
                throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
            } finally {
                ErrorContext.instance().reset();
            }

            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Parsed mapper file: '" + mapperLocation + "'");
            }
        }
    } else {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Property 'mapperLocations' was not specified or no matching resources found");
        }
    }
    // 构建SqlSessionFactory
    return this.sqlSessionFactoryBuilder.build(configuration);
}

熟悉Mybatis的同学一定很容易看懂这个方法，就是对Mybatis的各种配置的设置和解析并构建SqlSessionFactory，不熟悉Mybatis的同学请看笔者关于Mybatis源码解析的文章。方法中有一个步骤是如果用户没有配置transactionFactory，默认将Mybatis的transactionFactory配置为SpringManagedTransactionFactory，我们在分析Mybatis源码的时候看到，在用户没有指定transactionFactory配置的时候，Mybatis使用ManagedTransactionFactory作为默认的TransactionFactory，Mybatis在创建SqlSession时，需要为其添加一个Executor执行器，构建Executor执行器时需要的Transaction对象就是通过TransactionFactory的newTransaction方法创建的，后续Executor执行sql命令时会通过Transaction的getConnection方法获取数据库连接，这里添加的SpringManagedTransactionFactory有什么作用呢？我们可以思考一下，Spring在开启事务的时候需要获取数据库连接，Mybatis执行的时候也要获取数据库连接，在一次调用过程中，两者配合使用时，如果想让Spring的事务作用于Mybatis的数据库操作，那么在这次调用的过程中两者肯定要共用同一个数据库连接，不然事务无法生效，SpringManagedTransactionFactory就可以解决共用数据库连接的问题，我们来分析这个过程：

public Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit) {
    return new SpringManagedTransaction(dataSource);
}

SpringManagedTransactionFactory的newTransaction方法会返回SpringManagedTransaction，所以Executor获取数据库连接时就会调用SpringManagedTransaction的getConnection方法：

public Connection getConnection() throws SQLException {
    if (this.connection == null) {
        openConnection(); /* 开启数据库连接 */
    }
    return this.connection;
}

SpringManagedTransaction：

private void openConnection() throws SQLException {
    /* 获取数据库连接 */
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    this.autoCommit = this.connection.getAutoCommit();
    this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);
    if (LOGGER.isDebugEnabled()) {
        LOGGER.debug(
            "JDBC Connection ["
            + this.connection
            + "] will"
            + (this.isConnectionTransactional ? " " : " not ")
            + "be managed by Spring");
    }
}

DataSourceUtils：

public static Connection getConnection(DataSource dataSource) throws CannotGetJdbcConnectionException {
    try {
        /* 获取数据库连接 */
        return doGetConnection(dataSource);
    }
    catch (SQLException ex) {
        throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);
    }
}

DataSourceUtils：

public static Connection doGetConnection(DataSource dataSource) throws SQLException {
    Assert.notNull(dataSource, "No DataSource specified");
    // 从当前线程中获取绑定的ConnectionHolder
    ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
    // 如果ConnectionHolder不为null并且持有数据库连接或者与事务同步
    if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
        // 将ConnectionHolder请求数+1
        conHolder.requested();
        // 如果ConnectionHolder没有持有数据库连接则获取数据库连接放入ConnectionHolder
        if (!conHolder.hasConnection()) {
            logger.debug("Fetching resumed JDBC Connection from DataSource");
            conHolder.setConnection(dataSource.getConnection());
        }
        // 返回数据库连接
        return conHolder.getConnection();
    }
    logger.debug("Fetching JDBC Connection from DataSource");
    // 当前线程没有ConnectionHolder，获取数据库连接
    Connection con = dataSource.getConnection();
    // 判断当前线程的事务同步是否处于活动状态
    if (TransactionSynchronizationManager.isSynchronizationActive()) {
        logger.debug("Registering transaction synchronization for JDBC Connection");
        // 在事务中使用相同的连接进一步JDBC操作，线程绑定对象将在事务完成时被同步删除
        ConnectionHolder holderToUse = conHolder;
        if (holderToUse == null) {
            // ConnectionHolder为null则创建新的ConnectionHolder
            holderToUse = new ConnectionHolder(con);
        }
        else {
            // 不为null则将数据库连接放入ConnectionHolder
            holderToUse.setConnection(con);
        }
        holderToUse.requested(); // 将ConnectionHolder请求数+1
        // 为当前线程注册一个新的事务同步
        TransactionSynchronizationManager.registerSynchronization(
            new ConnectionSynchronization(holderToUse, dataSource));
        // 将SqlSessionHolder标记为与事务同步
        holderToUse.setSynchronizedWithTransaction(true);
        if (holderToUse != conHolder) {
            // 如果是新的ConnectionHolder，则绑定到当前线程
            TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
        }
    }
    return con;
}

这里我们看到通过调用TransactionSynchronizationManager的getResource方法获取当前线程绑定的ConnectionHolder，TransactionSynchronizationManager这个类我们在分析Spring事务源码的时候看到过，用来管理每个线程的资源和事务同步，内部维护了很多ThreadLocal变量来保存一些线程相关的资源。方法中我们发现有一个注册事务同步的过程，这个事务同步是做什么用的呢？我们在分析Spring事务源码的时候提到过，在Spring事务回滚、提交、挂起等操作时会激活事务同步的相关方法，而这里添加的事务同步的作用主要是在Spring事务提交、回滚等操作后将ConnectionHolder的请求数量置为0、将一些属性置为初始状态、将数据库连接放回连接池、释放数据库连接等。

下面我们来分析另外一个bean MapperScannerConfigurer，它的作用是扫描用户配置的包自动注册MapperFactoryBean，这样就不用我们手动注册mapper，我们首先来看一下它的层次结构：

我们发现它同样实现了InitializingBean，但是它对afterPropertiesSet方法的实现仅仅是校验basePackage属性是否为null。我们来看它实现的另外一个接口BeanDefinitionRegistryPostProcessor，这个接口我们在分析Spring上下文初始化源码的时候介绍过，用于在创建bean之前增加或改变BeanDefinition，我们在介绍它的作用时列举的例子就是MapperScannerConfigurer，我们来看相关方法的实现：

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
        /* 处理properties配置 */
        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.registerFilters(); /* 注册过滤器 */
    /* 扫描 */
    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
}

这里为什么要给出处理properties配置的选项呢？properties配置不是由PropertyPlaceholderConfigurer这个bean来完成的么？（如果读者对PropertyPlaceholderConfigurer不熟悉，可以去任何一个使用Spring的工程中去看一下properties的相关配置，不论是直接配置PropertyPlaceholderConfigurer还是使用标签都与PropertyPlaceholderConfigurer有关）我们尝试翻译一下processPropertyPlaceHolders方法的注释来获得答案：“BeanDefinitionRegistries会在应用启动的时候调用，并且会早于BeanFactoryPostProcessors的调用，这就意味着PropertyResourceConfigurers还没有被加载所以任何对于属性文件的引用将会失效，为了避免这种情况的发生，此方法手动找出定义的PropertyResourceConfigurers并进行提前调用以保证对于属性的引用可以正常工作”。
MapperScannerConfigurer：

private void processPropertyPlaceHolders() {
    Map<String, PropertyResourceConfigurer> prcs = applicationContext.getBeansOfType(PropertyResourceConfigurer.class);
    if (!prcs.isEmpty() && applicationContext instanceof ConfigurableApplicationContext) {
        BeanDefinition mapperScannerBean = ((ConfigurableApplicationContext) applicationContext)
        .getBeanFactory().getBeanDefinition(beanName);
        // PropertyResourceConfigurer不公开任何显式执行属性占位符替换的方法，代替的是创建一个只包含当前mapper扫描器和后处理工厂的BeanFactory
        DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
        factory.registerBeanDefinition(beanName, mapperScannerBean);
        for (PropertyResourceConfigurer prc : prcs.values()) {
            // 提前执行PropertyResourceConfigurer的postProcessBeanFactory方法加载properties
            prc.postProcessBeanFactory(factory);
        }
        PropertyValues values = mapperScannerBean.getPropertyValues();
        // 更新需要替换的属性
        this.basePackage = updatePropertyValue("basePackage", values);
        this.sqlSessionFactoryBeanName = updatePropertyValue("sqlSessionFactoryBeanName", values);
        this.sqlSessionTemplateBeanName = updatePropertyValue("sqlSessionTemplateBeanName", values);
    }
}

ClassPathMapperScanner：

public void registerFilters() {
    boolean acceptAllInterfaces = true;
    // 如果指定了annotationClass配置，则添加注解类型过滤器，使用给定的注解和/或标记接口
    if (this.annotationClass != null) {
        addIncludeFilter(new AnnotationTypeFilter(this.annotationClass));
        acceptAllInterfaces = false;
    }
    // 如果指定了markerInterface配置，则添加可分配给定类型的过滤器，重写AssignableTypeFilter忽略实际标记接口上的匹配
    if (this.markerInterface != null) {
        addIncludeFilter(new AssignableTypeFilter(this.markerInterface) {
            @Override
            protected boolean matchClassName(String className) {
                return false;
            }
        });
        acceptAllInterfaces = false;
    }
    // 如果上面两个没有配置，则添加接受所有接口的过滤器
    if (acceptAllInterfaces) {
        addIncludeFilter(new TypeFilter() {
            @Override
            public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException {
                return true;
            }
        });
    }
    // 添加过滤掉package-info.java的过滤器
    addExcludeFilter(new TypeFilter() {
        @Override
        public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException {
            String className = metadataReader.getClassMetadata().getClassName();
            return className.endsWith("package-info");
        }
    });
}

ClassPathScanningCandidateComponentProvider：

public void addIncludeFilter(TypeFilter includeFilter) {
    this.includeFilters.add(includeFilter);
}

ClassPathScanningCandidateComponentProvider：

public void addExcludeFilter(TypeFilter excludeFilter) {
    this.excludeFilters.add(0, excludeFilter);
}

添加包括和排除过滤器就是将相关过滤器添加对应集合中，这些过滤器的使用会在稍后进行介绍。相关属性设置完成之后就可以开始进行文件的扫描了：
ClassPathBeanDefinitionScanner：

public int scan(String... basePackages) {
    int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
    doScan(basePackages); /* 包扫描 */
    if (this.includeAnnotationConfig) {
        // 注册注解处理器
        AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
    }
    return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}

ClassPathMapperScanner：

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    // 调用父类的doScan方法进行扫描
    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 {
        /* 处理扫描到的BeanDefinition */
        processBeanDefinitions(beanDefinitions);
    }
    return beanDefinitions;
}

扫描的过程其实我们在分析Spring注解扫描的源码的时候已经分析过了，这里的扫描流程同样是复用这个流程，流程中使用的过滤器就是上文中注册的过滤器，这里不再重复这个流程，这里给出文章链接，读者可以去回顾一下这个流程，传送门。下面我们来分析对扫描到的BeanDefinition的处理：
ClassPathMapperScanner：

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
        definition = (GenericBeanDefinition) holder.getBeanDefinition();
        if (logger.isDebugEnabled()) {
            logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() 
                + "' and '" + definiti..                                           on.getBeanClassName() + "' mapperInterface");
        }
        // mapper的接口是bean的原始类，但是，实际的bean类是MapperFactoryBean
          definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName());
          definition.setBeanClass(this.mapperFactoryBean.getClass());
          definition.getPropertyValues().add("addToConfig", this.addToConfig);
          boolean explicitFactoryUsed = false;
          // 下面是一些其他属性的添加
          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;
          }
          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) {
              if (logger.isDebugEnabled()) {
                  logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
              }
              definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
          }
      }
}

我们看到这里为扫描到的BeanDefinition设置了BeanClass为MapperFactoryBean，我们来看一下这个类的层次结构：

我们发现它同样实现了InitializingBean和FactoryBean，我们分析相关实现方法：
DaoSupport：

public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {
    checkDaoConfig(); /* 检查dao配置 */
    try {
        initDao(); // 子类扩展初始化dao，默认空实现
    }
    catch (Exception ex) {
        throw new BeanInitializationException("Initialization of DAO failed", ex);
    }
}

MapperFactoryBean：

protected void checkDaoConfig() {
    super.checkDaoConfig(); /* 调用父类方法进行dao配置检查 */
    // 检查mapper接口不能为null
    notNull(this.mapperInterface, "Property 'mapperInterface' is required");
    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
        try {
            // 如果Mybatis的Configuration配置中没有当前mapper，则添加
            configuration.addMapper(this.mapperInterface);
        } catch (Exception e) {
            logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
            throw new IllegalArgumentException(e);
        } finally {
            ErrorContext.instance().reset();
        }
    }
}

SqlSessionDaoSupport：

protected void checkDaoConfig() {
    notNull(this.sqlSession, "Property 'sqlSessionFactory' or 'sqlSessionTemplate' are required");
}

这里判断了sqlSession属性不能为null，我们在上文中分析处理扫描后的BeanDefinition的过程时，并没有看到为mapper的BeanDefinition增加sqlSession属性，那么MapperFactoryBean创建以后这个属性不就是应该为null么？这里的断言不就无法通过了么？我们知道，SqlSession是通过SqlSessionFactory创建的，处理扫描后的BeanDefinition时为其添加了sqlSessionFactory属性，所以我们尝试从sqlSessionFactory的setter方法中寻找答案：
SqlSessionDaoSupport：

public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (!this.externalSqlSession) {
        /* 创建SqlSessionTemplate */
        this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
    }
}

果然，sqlSession属性的赋值就在这。
SqlSessionTemplate：

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
    /* 调用重载构造方法 */
    this(sqlSessionFactory, sqlSessionFactory.getConfiguration().getDefaultExecutorType());
}

SqlSessionTemplate：

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType) {
    /* 调用重载构造方法 */
    this(sqlSessionFactory, executorType,
        new MyBatisExceptionTranslator(
            sqlSessionFactory.getConfiguration().getEnvironment().getDataSource(), true));
}

SqlSessionTemplate：

public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
    PersistenceExceptionTranslator exceptionTranslator) {
    notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
    notNull(executorType, "Property 'executorType' is required");
    this.sqlSessionFactory = sqlSessionFactory;
    this.executorType = executorType;
    this.exceptionTranslator = exceptionTranslator;
    // 创建sqlSession代理
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
}

我们发现这里创建了一个sqlSession的代理，它有什么用呢？我们知道，我们可以使用SqlSession执行mapper的方法，也可以用它来获取mapper，而SqlSessionTemplate实现了SqlSession并实现了相关方法，方法的功能则委托给了sqlSessionProxy来实现，eg：
SqlSessionTemplate：

public <T> T selectOne(String statement, Object parameter) {
    return this.sqlSessionProxy.<T> selectOne(statement, parameter);
}

动态代理的InvocationHandler角色是SqlSessionInterceptor，我们来看它的invoke方法：
SqlSessionInterceptor：

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    /* 获取sqlSession */
    SqlSession sqlSession = getSqlSession(
        SqlSessionTemplate.this.sqlSessionFactory,
        SqlSessionTemplate.this.executorType,
        SqlSessionTemplate.this.exceptionTranslator);
    try {
        // 执行sqlSession的对应方法
        Object result = method.invoke(sqlSession, args);
        // 如果sqlSession不是由Spring管理的，则提交sqlSession
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
            // 强制提交sqlSession，因为一些数据库在调用close方法之前需要提交/回滚。
            sqlSession.commit(true);
        }
        return result;
    } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
            closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);  // 关闭SqlSession
            sqlSession = null;
            Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
            if (translated != null) {
                unwrapped = translated;
            }
        }
        throw unwrapped;
    } finally {
        if (sqlSession != null) {
            closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory); // 关闭SqlSession
        }
    }
}

SqlSessionUtils：

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);
    notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);
    // 获取当前线程绑定的SqlSessionHolder
    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
    /* 从SqlSessionHolder中获取SqlSession */
    SqlSession session = sessionHolder(executorType, holder);
    if (session != null) {
        return session; // 当前线程存在则直接返回
    }
    if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("Creating a new SqlSession");
    }
    // 没有通过SqlSessionFactory获取新的SqlSession
    session = sessionFactory.openSession(executorType);
    /* 绑定SessionHolder到当前线程 */
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
    return session;
}

SqlSessionUtils：

private static SqlSession sessionHolder(ExecutorType executorType, SqlSessionHolder holder) {
    SqlSession session = null;
    // 如果SqlSessionHolder不为null并且与事务同步
    if (holder != null && holder.isSynchronizedWithTransaction()) {
        if (holder.getExecutorType() != executorType) {
            throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
        }
        // 将SqlSessionHolder请求数+1
        holder.requested();
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Fetched SqlSession [" + holder.getSqlSession() + "] from current transaction");
        }
        // 获取SqlSession返回
        session = holder.getSqlSession();
    }
    return session;
}

SqlSessionUtils：

private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
    PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
    SqlSessionHolder holder;
    // 判断当前线程的事务同步是否处于活动状态
    if (TransactionSynchronizationManager.isSynchronizationActive()) {
        Environment environment = sessionFactory.getConfiguration().getEnvironment();
        // 上文创建SqlSessionFactory时判断如果没有transactionFactory配置，应用SpringManagedTransactionFactory
        if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Registering transaction synchronization for SqlSession [" + session + "]");
            }
            // 创建SqlSessionHolder持有SqlSession
            holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
            // 绑定到当前线程
            TransactionSynchronizationManager.bindResource(sessionFactory, holder);
            // 为当前线程注册一个新的事务同步
            TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
            // 将SqlSessionHolder标记为与事务同步
            holder.setSynchronizedWithTransaction(true);
            // 将SqlSessionHolder请求数+1
            holder.requested();
        } else {
            if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
                if (LOGGER.isDebugEnabled()) {
                    LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because DataSource is not transactional");
                }
            } else {
                throw new TransientDataAccessResourceException(
                    "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
            }
        }
    } else {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("SqlSession [" + session + "] was not registered for synchronization because synchronization is not active");
        }
    }
}

这里添加的事务同步的作用主要是在Spring事务提交回滚等操作后调用SqlSession的commit、close等方法来做关闭和提交等操作。

我们继续来分析MapperFactoryBean对FactoryBean接口getObject方法也就是创建bean的方法的实现：

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

这里的getSqlSession方法返回的也就是我们上文分析的SqlSessionTemplate。
SqlSessionTemplate：

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

这个方法的内容我们应该很熟悉了，在分析Mybatis源码的时候已经看到过，调用Mybatis的Configuration对象的getMapper方法来获取mapper，不熟悉的同学可以阅读笔者关于Mybatis源码分析的文章，有很详细的说明，同样有mapper执行过程的源码分析。到这里，整个Spring整合Mybatis的源码分析就完成了。