1. 运行环境 Enviroment

当 MyBatis 与不同的应用结合时，需要不同的事务管理机制。与 Spring 结合时，由 Spring 来管理事务；单独使用时需要自行管理事务，在容器里运行时可能由容器进行管理。

MyBatis 用 Enviroment 来表示运行环境，其封装了三个属性：

public class Configuration {
  // 一个 MyBatis 的配置只对应一个环境
  protected Environment environment;
  // 其他属性 .....
}

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;
}

2. 事务抽象

MyBatis 把事务管理抽象出 Transaction 接口，由 TransactionFactory 接口的实现类负责创建。

public interface Transaction {
  Connection getConnection() throws SQLException;
  void commit() throws SQLException;
  void rollback() throws SQLException;
  void close() throws SQLException;
  Integer getTimeout() throws SQLException;
}

public interface TransactionFactory {
  void setProperties(Properties props);
  Transaction newTransaction(Connection conn);
  Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);
}

Executor 的实现持有一个 SqlSession 实现，事务控制是委托给 SqlSession 的方法来实现的。

public abstract class BaseExecutor implements Executor {
  protected Transaction transaction;

  public void commit(boolean required) throws SQLException {
    if (closed) {
      throw new ExecutorException("Cannot commit, transaction is already closed");
    }
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
  }

  public void rollback(boolean required) throws SQLException {
    if (!closed) {
      try {
        clearLocalCache();
        flushStatements(true);
      } finally {
        if (required) {
          transaction.rollback();
        }
      }
    }
  }

  // 省略其他方法、属性
}

3. 与 Spring 集成的事务管理

3.1 配置 TransactionFactory

与 Spring 集成时，通过 SqlSessionFactoryBean 来初始化 MyBatis 。

protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
  Configuration configuration;

  XMLConfigBuilder xmlConfigBuilder = null;
  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 {
    configuration = new Configuration();
    configuration.setVariables(this.configurationProperties);
  }

  if (this.objectFactory != null) {
    configuration.setObjectFactory(this.objectFactory);
  }

  if (this.objectWrapperFactory != null) {
    configuration.setObjectWrapperFactory(this.objectWrapperFactory);
  }

  if (this.vfs != null) {
    configuration.setVfsImpl(this.vfs);
  }

  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 (!isEmpty(this.typeAliases)) {
    for (Class<?> typeAlias : this.typeAliases) {
      configuration.getTypeAliasRegistry().registerAlias(typeAlias);
    }
  }

  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);
    }
  }

  if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
    try {
      configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
    } catch (SQLException e) {
      throw new NestedIOException("Failed getting a databaseId", e);
    }
  }

  if (this.cache != null) {
    configuration.addCache(this.cache);
  }

  if (xmlConfigBuilder != null) {
    try {
      xmlConfigBuilder.parse();
    } catch (Exception ex) {
      throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  // 创建 SpringManagedTransactionFactory
  if (this.transactionFactory == null) {
    this.transactionFactory = new SpringManagedTransactionFactory();
  }

  // 封装成 Environment
  configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));

  if (!isEmpty(this.mapperLocations)) {
    for (Resource mapperLocation : this.mapperLocations) {
      if (mapperLocation == null) {
        continue;
      }

      try {
        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);
}