从零开始手写 mybatis（四）- mybatis 事务管理机制详解

前景回顾

第一节 从零开始手写 mybatis（一）MVP 版本 中我们实现了一个最基本的可以运行的 mybatis。

第二节 从零开始手写 mybatis（二）mybatis interceptor 插件机制详解

第三节 从零开始手写 mybatis（三）jdbc pool 从零实现数据库连接池

本节我们一起来学习一下 mybatis 中的事务管理。

mybatis 中的事务管理

mybatis 事务有两种使用方式：

1. 使用JDBC的事务管理机制：即使用 java.Sql.Connection对象完成对事务的提交，回滚和关闭操作。

2. 使用MANAGED的事务管理机制：mybatis本身不会去实现事务管理的相关操作，而是交个外部容器来管理事务。当与spring整合使用后，一般使用spring来管理事务。

事务工厂 TransactionFactory

接口定义

这个是对事务的一个工厂，接口如下：

public interface TransactionFactory {

  /**
   * Sets transaction factory custom properties.
   * @param props
   */
  void setProperties(Properties props);

  /**
   * Creates a {@link Transaction} out of an existing connection.
   * @param conn Existing database connection
   * @return Transaction
   * @since 3.1.0
   */
  Transaction newTransaction(Connection conn);

  /**
   * Creates a {@link Transaction} out of a datasource.
   * @param dataSource DataSource to take the connection from
   * @param level Desired isolation level
   * @param autoCommit Desired autocommit
   * @return Transaction
   * @since 3.1.0
   */
  Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);

}

主要就是如何根据一个 DataSource 创建一个 Transaction。

实际上整体感觉意义不大。

最核心的还是要看一下 Transaction 的实现。

Transaction 接口

public interface Transaction {

  /**
   * Retrieve inner database connection
   * @return DataBase connection
   * @throws SQLException
   */
  Connection getConnection() throws SQLException;

  /**
   * Commit inner database connection.
   * @throws SQLException
   */
  void commit() throws SQLException;

  /**
   * Rollback inner database connection.
   * @throws SQLException
   */
  void rollback() throws SQLException;

  /**
   * Close inner database connection.
   * @throws SQLException
   */
  void close() throws SQLException;

  /**
   * Get transaction timeout if set
   * @throws SQLException
   */
  Integer getTimeout() throws SQLException;

}

这里最核心的实际上只有 commit() 和 rollback()，其他的都是可以忽略的。

针对 getTimeout() 我们就可以为 mybatis 提供一个操作的超时机制。

JdbcTransaction 实现

基于 jdbc 机制的一些处理。

public class JdbcTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(JdbcTransaction.class);

  protected Connection connection;
  protected DataSource dataSource;
  protected TransactionIsolationLevel level;
  protected boolean autoCommmit;

  public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
    dataSource = ds;
    level = desiredLevel;
    autoCommmit = desiredAutoCommit;
  }

  public JdbcTransaction(Connection connection) {
    this.connection = connection;
  }

  @Override
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

  @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();
    }
  }

  @Override
  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      connection.close();
    }
  }

  protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
    try {
      if (connection.getAutoCommit() != desiredAutoCommit) {
        if (log.isDebugEnabled()) {
          log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(desiredAutoCommit);
      }
    } catch (SQLException e) {
      // Only a very poorly implemented driver would fail here,
      // and there's not much we can do about that.
      throw new TransactionException("Error configuring AutoCommit.  "
          + "Your driver may not support getAutoCommit() or setAutoCommit(). "
          + "Requested setting: " + desiredAutoCommit + ".  Cause: " + e, e);
    }
  }

  protected void resetAutoCommit() {
    try {
      if (!connection.getAutoCommit()) {
        // MyBatis does not call commit/rollback on a connection if just selects were performed.
        // Some databases start transactions with select statements
        // and they mandate a commit/rollback before closing the connection.
        // A workaround is setting the autocommit to true before closing the connection.
        // Sybase throws an exception here.
        if (log.isDebugEnabled()) {
          log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");
        }
        connection.setAutoCommit(true);
      }
    } catch (SQLException e) {
      if (log.isDebugEnabled()) {
        log.debug("Error resetting autocommit to true "
          + "before closing the connection.  Cause: " + e);
      }
    }
  }

  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommmit);
  }

  @Override
  public Integer getTimeout() throws SQLException {
    return null;
  }

}

这里整体的实现实际上非常简单，就是主动设置了一下自动提交的属性。

ManagedDataSource

这个是另一个实现，实际上更加简单。

commit() 和 rollback() 实现都是空的。

public class ManagedTransaction implements Transaction {

  private static final Log log = LogFactory.getLog(ManagedTransaction.class);

  private DataSource dataSource;
  private TransactionIsolationLevel level;
  private Connection connection;
  private boolean closeConnection;

  public ManagedTransaction(Connection connection, boolean closeConnection) {
    this.connection = connection;
    this.closeConnection = closeConnection;
  }

  public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) {
    this.dataSource = ds;
    this.level = level;
    this.closeConnection = closeConnection;
  }

  @Override
  public Connection getConnection() throws SQLException {
    if (this.connection == null) {
      openConnection();
    }
    return this.connection;
  }

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

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

  @Override
  public void close() throws SQLException {
    if (this.closeConnection && this.connection != null) {
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + this.connection + "]");
      }
      this.connection.close();
    }
  }

  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }

  @Override
  public Integer getTimeout() throws SQLException {
    return null;
  }

}

作用

ManagedTransaction对事务的commit和rollback交给了容器去管理，自己本身并没有做任何处理。

mybatis 的使用方式

如果Mybatis是单独运行的，没有其他框架管理，此时mybatis内部会对下段代码实现。

con.setAutoCommit(false);
//此处命令通知数据库,从此刻开始从当前Connection通道推送而来的
//SQL语句属于同一个业务中这些SQL语句在数据库中应该保存到同一个
//Transaction中.这个Transaction的行为(commit,rollback)由当前Connection管理.
try{
    //推送sql语句命令……..;
    con.commit();//通知Transaction提交.
}catch(SQLException ex){
    con.rollback();//通知Transaction回滚.
}

整体来说这种写法比较原始，我们可以将本来交给 connection 处理的事务，统一调整为使用事务管理器处理。

spring 整合

当然针对 mybatis，大部分都是单个语句的执行。

用于使用 connection 时，实际上得到的是 mybatis 事务管理器封装之后的 connection。

实际上 spring 的整合，可能适用性更强一些。

个人实现

看完了 mybatis 的实现原理之后，我们的实现就变得非常简单。

我们可以简化上面的一些实现，保留核心的部分即可。

接口定义

我们只保留核心的 3 个接口。

/**
 * 事务管理
 */
public interface Transaction {

    /**
     * Retrieve inner database connection
     * @return DataBase connection
     */
    Connection getConnection();

    /**
     * Commit inner database connection.
     */
    void commit();

    /**
     * Rollback inner database connection.
     */
    void rollback();

}

ManageTransaction

这个实现，我们的 commit 和 rollback 什么都不做。

/**
 * 事务管理
 *
 * @since 0.0.18
 */
public class ManageTransaction implements Transaction {

    /**
     * 数据信息
     * @since 0.0.18
     */
    private final DataSource dataSource;

    /**
     * 隔离级别
     * @since 0.0.18
     */
    private final TransactionIsolationLevel isolationLevel;

    /**
     * 连接信息
     * @since 0.0.18
     */
    private Connection connection;

    public ManageTransaction(DataSource dataSource, TransactionIsolationLevel isolationLevel) {
        this.dataSource = dataSource;
        this.isolationLevel = isolationLevel;
    }

    public ManageTransaction(DataSource dataSource) {
        this(dataSource, TransactionIsolationLevel.READ_COMMITTED);
    }

    @Override
    public Connection getConnection() {
        try {
            if(this.connection == null) {
                Connection connection = dataSource.getConnection();
                connection.setTransactionIsolation(isolationLevel.getLevel());
                this.connection = connection;
            }

            return connection;
        } catch (SQLException throwables) {
            throw new MybatisException(throwables);
        }
    }

    @Override
    public void commit() {
        //nothing
    }

    @Override
    public void rollback() {
        //nothing
    }

}

JdbcTransaction.java

这里和上面的相比较，多出了 commit 和 rollback 的逻辑处理。

package com.github.houbb.mybatis.transaction.impl;

import com.github.houbb.mybatis.constant.enums.TransactionIsolationLevel;
import com.github.houbb.mybatis.exception.MybatisException;
import com.github.houbb.mybatis.transaction.Transaction;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;

/**
 * 事务管理
 *
 * @since 0.0.18
 */
public class JdbcTransaction implements Transaction {

    /**
     * 数据信息
     * @since 0.0.18
     */
    private final DataSource dataSource;

    /**
     * 隔离级别
     * @since 0.0.18
     */
    private final TransactionIsolationLevel isolationLevel;

    /**
     * 自动提交
     * @since 0.0.18
     */
    private final boolean autoCommit;

    /**
     * 连接信息
     * @since 0.0.18
     */
    private Connection connection;

    public JdbcTransaction(DataSource dataSource, TransactionIsolationLevel isolationLevel, boolean autoCommit) {
        this.dataSource = dataSource;
        this.isolationLevel = isolationLevel;
        this.autoCommit = autoCommit;
    }

    public JdbcTransaction(DataSource dataSource) {
        this(dataSource, TransactionIsolationLevel.READ_COMMITTED, true);
    }

    @Override
    public Connection getConnection(){
        try {
            if(this.connection == null) {
                Connection connection = dataSource.getConnection();
                connection.setTransactionIsolation(isolationLevel.getLevel());
                connection.setAutoCommit(autoCommit);
                this.connection = connection;
            }

            return connection;
        } catch (SQLException throwables) {
            throw new MybatisException(throwables);
        }
    }

    @Override
    public void commit() {
        try {
            //非自动提交，才执行 commit 操作
            if(connection != null && !this.autoCommit) {
                connection.commit();
            }
        } catch (SQLException throwables) {
            throw new MybatisException(throwables);
        }
    }

    @Override
    public void rollback() {
        try {
            //非自动提交，才执行 commit 操作
            if(connection != null && !this.autoCommit) {
                connection.rollback();
            }
        } catch (SQLException throwables) {
            throw new MybatisException(throwables);
        }
    }

}