MyBatis事务管理

MyBatis的事务管理是由TransactionFactory和Transaction两个接口定义的，TransactionFactory负责生成Transaction，这是一个典型的工厂模式。

官方提供了事务管理的两种模式：

Managed：对应ManagedTransactionFactory和ManagedTransaction
JDBC：对应JdbcTransactionFactory和JdbcTransaction

重点看一下Transaction中提交和回滚的实现：

java 复制代码

public class ManagedTransaction implements Transaction {
  @Override
  public void commit() throws SQLException {
    // Does nothing
  }

  @Override
  public void rollback() throws SQLException {
    // Does nothing
  }
 }

java 复制代码

public class JdbcTransaction implements Transaction {
  @Override
  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      connection.commit();
    }
  }

  @Override
  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      connection.rollback();
    }
  }
}

可以看出，这两者的主要区别在于ManagedTransaction不会进行实际的事务提交和回滚，而是交由外部进行控制，而JdbcTransaction是我们能进行实际事务提交和回滚的，所以如果我们要手动控制事务，应该指定事务管理器为JdbcTransactionFactory，例如在mybatis的配置文件中：

xml 复制代码

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED"></dataSource>
    </environment>
  </environments>
</configuration>

那Transaction和实际执行查询的SqlSession又是什么关系呢？原来是创建SqlSession的时候会交由Executor管理，一起传给SqlSession：

java 复制代码

public class DefaultSqlSessionFactory implements SqlSessionFactory {
  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);
      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();
    }
  }
}

当我们执行SqlSession的commit()时，实际上是调用Executor的commit（），进而调用Transaction的commit()：

java 复制代码

public class DefaultSqlSession implements SqlSession {
  @Override
  public void commit(boolean force) {
    try {
      executor.commit(isCommitOrRollbackRequired(force));
      dirty = false;
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
}

java 复制代码

public abstract class BaseExecutor implements Executor {
  @Override
  public void commit(boolean required) throws SQLException {
    if (closed) {
      throw new ExecutorException("Cannot commit, transaction is already closed");
    }
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
  }
}