MyBatis源码解析之Transaction事务模块

一、概述

MyBatis是一款优秀的持久层框架，它支持事务控制，能够帮助开发者方便地管理数据的事务。MyBatis的事务管理模块主要由Transaction接口、TransactionFactory接口、TransactionIsolationLevel枚举和JdbcTransaction、ManagedTransaction两个类实现。

二、Transaction接口

Transaction接口定义了事务的基本方法，包括提交事务、回滚事务和关闭事务。一个SqlSessionFactory实例可以创建一个Transaction实例。

public interface Transaction {
    void commit() throws SQLException;

    void rollback() throws SQLException;

    void close() throws SQLException;

    Integer getTimeout() throws SQLException;
}

三、TransactionFactory接口

TransactionFactory接口定义了创建Transaction实例的方法。

public interface TransactionFactory {
    Transaction newTransaction(Connection conn);
    /**
     * @since 3.1.0
     */
    Transaction newTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit);
    void setProperties(Properties props);
}

四、TransactionIsolationLevel枚举

TransactionIsolationLevel枚举定义了事务的隔离级别，包括NONE、READ_UNCOMMITTED、READ_COMMITTED、REPEATABLE_READ和SERIALIZABLE。

五、JdbcTransaction类

JdbcTransaction继承自BaseTransaction类，是基于JDBC实现的事务管理器。它的创建依赖于TransactionFactory实例。在JdbcTransaction对象创建的过程中，如果autoCommit为false，则将会开启事务。

示例代码：

public class JdbcTransaction extends BaseTransaction {

    public JdbcTransaction(Connection connection) {
        delegate = new JdbcTransactionImpl(connection);
    }

    public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
        Connection conn;
        try {
            conn = ds.getConnection();
            if (conn.getAutoCommit() != desiredAutoCommit) {
                conn.setAutoCommit(desiredAutoCommit);
            }
            delegate = new JdbcTransactionImpl(conn);
        } catch (SQLException e) {
            throw new TransactionException("Error initializing JDBC Connection.  Cause: " + e, e);
        }
    }

    private static class JdbcTransactionImpl extends BaseTransactionState implements TransactionState {
        private Connection connection;
        private TransactionIsolationLevel level;
        private boolean autoCommmit;

        public JdbcTransactionImpl(Connection connection) {
            this.connection = connection;
            try {
                this.level = TransactionIsolationLevel.fromValue(connection.getTransactionIsolation());
                this.autoCommmit = connection.getAutoCommit();
            } catch (SQLException e) {
                throw new TransactionException("Error configuring Transaction.  Cause: " + e, e);
            }
        }

        public void commit() throws SQLException {
            if (connection != null && !connection.isClosed()) {
                if (autoCommmit != connection.getAutoCommit()) {
                    connection.setAutoCommit(autoCommmit);
                }
                connection.commit();
            }
        }

        public void rollback() throws SQLException {
            if (connection != null && !connection.isClosed()) {
                if (autoCommmit != connection.getAutoCommit()) {
                    connection.setAutoCommit(autoCommmit);
                }
                connection.rollback();
            }
        }

        public void close() throws SQLException {
            connection.close();
        }

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

}

六、ManagedTransaction类

ManagedTransaction类也继承自BaseTransaction类，是基于容器环境下的事务管理器，它不会commit和rollback事务，而是依赖容器（如Spring）来完成这些操作。也就是说，它将交给上层容器来管理事务。

示例代码：

public class ManagedTransaction extends BaseTransaction {

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

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

    public ManagedTransaction(DataSource dataSource, TransactionIsolationLevel level, boolean autoCommit) {
        try {
            Connection conn = dataSource.getConnection();
            if (conn.getAutoCommit() != autoCommit) {
                conn.setAutoCommit(autoCommit);
            }
            this.connection = conn;
            this.level = level;
            this.closeConnection = true;
            this.delegate = new ManagedTransactionImpl(connection);
        } catch (SQLException e) {
            throw new TransactionException("Error getting a connection.  Cause: " + e, e);
        }
    }

    private class ManagedTransactionImpl extends BaseTransactionState implements TransactionState {
        public ManagedTransactionImpl(Connection connection) {
            // Do nothing
        }

        public void commit() throws SQLException {
            // Do nothing
        }

        public void rollback() throws SQLException {
            // Do nothing
        }

        public void close() throws SQLException {
            if (closeConnection && connection != null) {
                connection.close();
            }
        }

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

}

七、示例说明

示例一：

SqlSession sqlSession = sqlSessionFactory.openSession();
try {
    // 获取Transaction实例
    Transaction tx = sqlSession.getTransaction();
    // 关闭事务
    tx.close();
} catch (Exception e) {
    e.printStackTrace();
} finally {
    sqlSession.close();
}

示例二：

Transaction transaction = transactionFactory.newTransaction(dataSource, TransactionIsolationLevel.READ_COMMITTED, false);
try {
    // 执行一些数据库操作
    transaction.commit();
} catch (Exception e) {
    transaction.rollback();
} finally {
    transaction.close();
}

八、总结

MyBatis的Transaction事务模块是MyBatis框架的核心模块之一，它为MyBatis应用程序提供了事务管理功能。MyBatis的事务管理模块由Transaction接口、TransactionFactory接口、TransactionIsolationLevel枚举和JdbcTransaction、ManagedTransaction两个类实现，开发者可以根据自己的具体需求进行选择。在实际开发中，我们应该根据项目的实际情况选择合适的事务管理方式。