spring事务源码分析结合mybatis源码(三)

下面将结合mybatis源码来分析下，这种持久化框架是如何对connection使用，来达到spring事务的控制。

想要在把mybatis跟spring整合都需要这样一个jar包：mybatis-spring-x.x.x.jar，这里面定义了一些主要的整合信息。

在spring配置文件中需要配置如下两个bean：

    <!-- mybatis配置 -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dynamicDataSource" />
        <property name="configLocation" value="classpath:mybatis.xml"></property>
        <!-- mybatis配置文件 -->
        <property name="mapperLocations" value="classpath:com/blackbread/dao/mapper/*.xml" />
    </bean>
    <!--mapper scanning -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.blackbread.dao" />
        <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
    </bean>

首先让我们来看org.mybatis.spring.SqlSessionFactoryBean，在这个类需要注入跟之前tx配置中一样的dataSource。

SqlSessionFactoryBean类实现了InitializingBean接口，所以会执行afterPropertiesSet方法，在afterPropertiesSet方法中会执行buildSqlSessionFactory方法生成一个sqlSessionFactory对象，让我们看下buildSqlSessionFactory方法：由于主要看的是跟spring tx结合的方式，所以代码看不上很细，如有疏漏，望不吝赐教。

      protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
            Configuration configuration;
            XMLConfigBuilder xmlConfigBuilder = null;
            //初始化一个configuration
            if (this.configLocation != null) {
              xmlConfigBuilder = new XMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties);
              configuration = xmlConfigBuilder.getConfiguration();
            } else {
              configuration = new Configuration();
              configuration.setVariables(this.configurationProperties);
            }
            if (this.objectFactory != null) {
              configuration.setObjectFactory(this.objectFactory);
            }
            if (this.objectWrapperFactory != null) {
              configuration.setObjectWrapperFactory(this.objectWrapperFactory);
            }
            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);
              }
            }
            //设置别名
            if (!isEmpty(this.typeAliases)) {
              for (Class<?> typeAlias : this.typeAliases) {
                configuration.getTypeAliasRegistry().registerAlias(typeAlias);
              }
            }
            //装入插件，mybatis的插件都是以拦截器的形式进行的好像，比如分页插件，这里是载入spring中注入的
            if (!isEmpty(this.plugins)) {
              for (Interceptor plugin : this.plugins) {
                configuration.addInterceptor(plugin);
              }
            }
            if (hasLength(this.typeHandlersPackage)) {
              String[] typeHandlersPackageArray = tokenizeToStringArray(this.typeHandlersPackage,
                  ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
              for (String packageToScan : typeHandlersPackageArray) {
                configuration.getTypeHandlerRegistry().register(packageToScan);
              }
            }

            if (!isEmpty(this.typeHandlers)) {
              for (TypeHandler<?> typeHandler : this.typeHandlers) {
                configuration.getTypeHandlerRegistry().register(typeHandler);
              }
            }
            //这里将解析mybatis.xml文件，载入所有配置，插件、setting等
            if (xmlConfigBuilder != null) {
              try {
                xmlConfigBuilder.parse();
              } catch (Exception ex) {
                throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
              } finally {
                ErrorContext.instance().reset();
              }
            }
            //这个很重要，这里定义了用的transactionFactory为SpringManagedTransactionFactory，这个在获取
            //connection等地方都有用到，是mybatis跟spring的主要链接
            if (this.transactionFactory == null) {
              this.transactionFactory = new SpringManagedTransactionFactory();
            }
            //新建一个Environment对象，并将新建的transactionFactory放入其中
            Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);
            configuration.setEnvironment(environment);
            if (this.databaseIdProvider != null) {
              try {
                configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
              } catch (SQLException e) {
                throw new NestedIOException("Failed getting a databaseId", e);
              }
            }
            if (!isEmpty(this.mapperLocations)) {
              for (Resource mapperLocation : this.mapperLocations) {
                if (mapperLocation == null) {
                  continue;
                }
                try {
                //这里主要是解析配置的sql mapper配置文件
                  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();
                }
              }
            } else {
              }
            }
            return this.sqlSessionFactoryBuilder.build(configuration);
          }

看着好长的一段啊，其实这里做的工作就是解析配置文件生成configuration对象而已。在

 xmlMapperBuilder.parse();

这里将解析sql mapper文件中的映射关系生成MappedStatement对象，并执行 configuration.addMappedStatement(statement);放入到configuration对象中，有兴趣的同学可以仔细看下。

这里主要需要注意的一块就是this.transactionFactory = new SpringManagedTransactionFactory();这里就是mybatis跟spring连接到一起的地方。

接着我们看一下org.mybatis.spring.mapper.MapperScannerConfigurer对象的初始化过程，这个对象实现了BeanDefinitionRegistryPostProcessor接口，在postProcessBeanDefinitionRegistry方法中初始化一个对象ClassPathMapperScanner，并讲执行scan--->doScan方法，

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
          for (BeanDefinitionHolder holder : beanDefinitions) {
            GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
            definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
            //实际就是将扫描到的接口包装成MapperFactoryBean的实现类
            definition.setBeanClass(MapperFactoryBean.class);
            definition.getPropertyValues().add("addToConfig", this.addToConfig);
            boolean explicitFactoryUsed = false;
            //注入sqlSessionFactory对象，这个也很重要
            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) {
              definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
            }
          }
        return beanDefinitions;
      }

这段代码其实主要就是从basePackage中扫描到相应的接口类，并且注册到spring中，并且定义此对象的FactoryBean为：MapperFactoryBean，将返回如下对象

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

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

最终其实就是生成Handler为MapperProxy的基于mapperInterface的代理类。

同时添加属性：sqlSessionFactory。

这个操作很重要，在后面有关联操作。

这里让我们看下MapperFactoryBean类，这个类继承自SqlSessionDaoSupport而在SqlSessionDaoSupport中有如下方法：

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

也就是上面调用的添加sqlSessionFactory属性的set操作，在这个方法中初始话sqlSession，利用的是SqlSessionTemplate对象。

接下来让我们看下SqlSessionTemplate的初始化过程：

 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;
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
        SqlSessionFactory.class.getClassLoader(),
        new Class[] { SqlSession.class },
        new SqlSessionInterceptor());
  }

SqlSessionTemplate其实是实现了SqlSession接口的，在初始化的时候将生成一个sqlSessionProxy 代理类，可以查看下SqlSessionTemplate里面的所有与数据库相关的操作都是通过sqlSessionProxy 这个代理类实现的。

接着看下sqlSessionProxy 的实际handler：

 private class SqlSessionInterceptor implements InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          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);
          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);
        }
      }
    }
  }

这里首先需要获取一个SqlSession对象：

public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
            SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
            if (holder != null && holder.isSynchronizedWithTransaction()) {
              if (holder.getExecutorType() != executorType) {
                throw new TransientDataAccessResourceException("Cannot change the ExecutorType when there is an existing transaction");
              }
              holder.requested();
              return holder.getSqlSession();
            }
            SqlSession session = sessionFactory.openSession(executorType);
            if (TransactionSynchronizationManager.isSynchronizationActive()) {
              Environment environment = sessionFactory.getConfiguration().getEnvironment();
              if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
                holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
                TransactionSynchronizationManager.bindResource(sessionFactory, holder);
                TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
                holder.setSynchronizedWithTransaction(true);
                holder.requested();
              } else {
                if (TransactionSynchronizationManager.getResource(environment.getDataSource()) == null) {
                } else {
                  throw new TransientDataAccessResourceException(
                      "SqlSessionFactory must be using a SpringManagedTransactionFactory in order to use Spring transaction synchronization");
                }
              }
            } else {
            }
            return session;
          }

这里将会获取一个SqlSessionHolder并判断是否已经存在，如果不存在将会初始化一个新的，我们这里只分析第一次调用过程，也就是将会执行

 SqlSession session = sessionFactory.openSession(executorType);
--->openSessionFromDataSource

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          final Environment environment = configuration.getEnvironment();
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
          return new DefaultSqlSession(configuration, executor);
        } catch (Exception e) {
          closeTransaction(tx); // may have fetched a connection so lets call close()
          throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
        } finally {
          ErrorContext.instance().reset();
        }
      }

这里是主要的跟spring结合部分，让我们仔细分析下，首先这里将获取TransactionFactory： final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);这里将得到我们之前初始化时候加入的SpringManagedTransactionFactory。然后将初始化当前的tx：

tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);

然后将生成一个Executor ,将final Executor executor = configuration.newExecutor(tx, execType, autoCommit);这里在之前指定了execType为Simple，所以在这里将生成一个SimpleExecutor： executor = new SimpleExecutor(this, transaction);并将transaction加入属性。

到这里SqlSession的初始化也就完成了，接下来就是通过反射进行实际方法的执行了：

  Object result = method.invoke(sqlSession, args);

以一个update操作来说明：

public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

这里首先将从configuration中根据操作的statement获取映射内容MappedStatement ，

getMappedStatement(String id)---->getMappedStatement(String id, boolean validateIncompleteStatements)
接着将执行executor.update(ms, wrapCollection(parameter))，也就是实际的数据库操作了，记得之前初始化的executor么，这里就是对应的SimpleExecutor

 public int update(MappedStatement ms, Object parameter) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
    if (closed) throw new ExecutorException("Executor was closed.");
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

这里主要是看prepareStatement方法：

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection);
    handler.parameterize(stmt);
    return stmt;
  }

然后看Connection 方法：

protected Connection getConnection(Log statementLog) throws SQLException {
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog);
    } else {
      return connection;
    }
  }

到这里看到了真正的Connection获取方法：  transaction.getConnection();也就是通过之前注入的transaction中获取connection，而这个transaction也就是对应的SpringManagedTransaction，他的调用过程getConnection()---->openConnection()

 private void openConnection() throws SQLException {
    this.connection = DataSourceUtils.getConnection(this.dataSource);
    this.autoCommit = this.connection.getAutoCommit();
    this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);
  }

这里其实就是调用了DataSourceUtils.getConnection(this.dataSource);来获取一个Connection。

看看DataSourceUtils的getConnection(DataSource dataSource)--->doGetConnection(DataSource dataSource)

    public static Connection doGetConnection(DataSource dataSource) throws SQLException {
        //从TransactionSynchronizationManager中获取ConnectionHolder，这个对象也就是之前我们第一次分析spring tx的时候
        //持有ConnectionHolder的对象了
        ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
        //由于在前面的切面中已经开启事务，并且初始化了ConnectionHolder所以这里将直接返回ConnectionHolder中的connection
        if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {
            conHolder.requested();
            if (!conHolder.hasConnection()) {
                conHolder.setConnection(dataSource.getConnection());
            }
            return conHolder.getConnection();
        }
        Connection con = dataSource.getConnection();
        if (TransactionSynchronizationManager.isSynchronizationActive()) {
            ConnectionHolder holderToUse = conHolder;
            if (holderToUse == null) {
                holderToUse = new ConnectionHolder(con);
            }
            else {
                holderToUse.setConnection(con);
            }
            holderToUse.requested();
            TransactionSynchronizationManager.registerSynchronization(
                    new ConnectionSynchronization(holderToUse, dataSource));
            holderToUse.setSynchronizedWithTransaction(true);
            if (holderToUse != conHolder) {
                TransactionSynchronizationManager.bindResource(dataSource, holderToUse);
            }
        }
        return con;
    }

是不是感觉这段代码很眼熟，对了，因为这里有我们第一篇里面非常熟悉的TransactionSynchronizationManager，在spring tx中也是通过这个类中的resources （ThreadLocal对象）对ConnectionHolder进行持有的。

在这里将获取到之前持有的ConnectionHolder对象，并从中获取到Connection 对象然后返回，这样就保证了spring tx中控制的Connection 跟实际调用的Connection为同一个Connection，也就可以通过spring tx对事务进行管理了。

后续的对数据的操作有兴趣的可以自己读一下，感觉mybatis的源码没有spring的那么清晰，还是需要仔细分析下才能整合到一起。

看的比较粗略，难免有疏漏地方，望不吝赐教。