浅谈MyBatis 事务管理

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;
}

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);
}

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
}

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);
}

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

public class SqlSessionTemplate implements SqlSession, DisposableBean {
  private final SqlSessionFactory sqlSessionFactory;
  private final ExecutorType executorType;
  private final SqlSession sqlSessionProxy;
  private final PersistenceExceptionTranslator exceptionTranslator;

  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;

    // 因为 SqlSession 接口声明的方法也不少，
    // 在每个方法里添加事务相关的拦截比较麻烦，
    // 不如创建一个内部的代理对象进行统一处理。
    this.sqlSessionProxy = (SqlSession) newProxyInstance(
      SqlSessionFactory.class.getClassLoader(),
      new Class[] { SqlSession.class },
      new SqlSessionInterceptor());
  }

  public int update(String statement) {
    // 在代理对象上执行方法调用
    return this.sqlSessionProxy.update(statement);
  }

  // 对方法调用进行拦截，加入事务控制逻辑
  private class SqlSessionInterceptor implements InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      // 获取与数据库关联的会话
      SqlSession sqlSession = SqlSessionUtils.getSqlSession(
        SqlSessionTemplate.this.sqlSessionFactory,
        SqlSessionTemplate.this.executorType,
        SqlSessionTemplate.this.exceptionTranslator);
      try {
        // 执行 SQL 操作
        Object result = method.invoke(sqlSession, args);
        if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          // 如果 sqlSession 不是 Spring 管理的，则要自行提交事务
          sqlSession.commit(true);
        }
        return result;
      } catch (Throwable t) {
        Throwable unwrapped = unwrapThrowable(t);
        if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {

          SqlSessionUtils.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) {
          SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }
}

// SqlSessionUtils
public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {
  // 从 Spring 的事务管理机制那里获取当前事务关联的会话
  SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

  SqlSession session = sessionHolder(executorType, holder);
  if (session != null) {
    // 已经有一个会话则复用
    return session;
  }

  // 创建新的 会话
  session = sessionFactory.openSession(executorType);

  // 注册到 Spring 的事务管理机制里
  registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

  return session;
}

private static void registerSessionHolder(SqlSessionFactory sessionFactory, ExecutorType executorType,
    PersistenceExceptionTranslator exceptionTranslator, SqlSession session) {
  SqlSessionHolder holder;
  if (TransactionSynchronizationManager.isSynchronizationActive()) {
    Environment environment = sessionFactory.getConfiguration().getEnvironment();

    if (environment.getTransactionFactory() instanceof SpringManagedTransactionFactory) {
      holder = new SqlSessionHolder(session, executorType, exceptionTranslator);
      TransactionSynchronizationManager.bindResource(sessionFactory, holder);

      // 重点：注册会话管理的回调钩子，真正的关闭动作是在回调里完成的。
      TransactionSynchronizationManager.registerSynchronization(new SqlSessionSynchronization(holder, sessionFactory));
      holder.setSynchronizedWithTransaction(true);

      // 维护会话的引用计数
      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 {
  }
}

public static void closeSqlSession(SqlSession session, SqlSessionFactory sessionFactory) {
  // 从线程本地变量里获取 Spring 管理的会话
  SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);
  if ((holder != null) && (holder.getSqlSession() == session)) {
    // Spring 管理的不直接关闭，由回调钩子来关闭
    holder.released();
  } else {
    // 非 Spring 管理的直接关闭
    session.close();
  }
}

private static final class SqlSessionSynchronization extends TransactionSynchronizationAdapter {
  private final SqlSessionHolder holder;
  private final SqlSessionFactory sessionFactory;
  private boolean holderActive = true;

  public SqlSessionSynchronization(SqlSessionHolder holder, SqlSessionFactory sessionFactory) {
    this.holder = holder;
    this.sessionFactory = sessionFactory;
  }

  public int getOrder() {
    return DataSourceUtils.CONNECTION_SYNCHRONIZATION_ORDER - 1;
  }

  public void suspend() {
    if (this.holderActive) {
      TransactionSynchronizationManager.unbindResource(this.sessionFactory);
    }
  }

  public void resume() {
    if (this.holderActive) {
      TransactionSynchronizationManager.bindResource(this.sessionFactory, this.holder);
    }
  }

  public void beforeCommit(boolean readOnly) {
    if (TransactionSynchronizationManager.isActualTransactionActive()) {
      try {
        this.holder.getSqlSession().commit();
      } catch (PersistenceException p) {
        if (this.holder.getPersistenceExceptionTranslator() != null) {
          DataAccessException translated = this.holder
            .getPersistenceExceptionTranslator()
            .translateExceptionIfPossible(p);
          if (translated != null) {
            throw translated;
          }
        }
        throw p;
      }
    }
  }

  public void beforeCompletion() {
    if (!this.holder.isOpen()) {
      TransactionSynchronizationManager.unbindResource(sessionFactory);
      this.holderActive = false;

      // 真正关闭数据库会话
      this.holder.getSqlSession().close();
    }
  }

  public void afterCompletion(int status) {
    if (this.holderActive) {
      TransactionSynchronizationManager.unbindResourceIfPossible(sessionFactory);
      this.holderActive = false;

      // 真正关闭数据库会话
      this.holder.getSqlSession().close();
    }
    this.holder.reset();
  }
}

// DefaultSqlSessionFactory
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();

    // 获取事务工厂实现
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

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

    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } 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();
  }
}

private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
  if (environment == null || environment.getTransactionFactory() == null) {
    return new ManagedTransactionFactory();
  }
  return environment.getTransactionFactory();
}