前景回顾
第一节 从零开始手写 mybatis（一）MVP 版本 中咱们实现了一个最根本的能够运行的 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
本节咱们一起来学习一下 mybatis 中的事务管理。
mybatis 中的事务管理
mybatis 事务有两种应用形式：
- 应用JDBC的事务管理机制：即应用 java.Sql.Connection对象实现对事务的提交，回滚和敞开操作。
- 应用MANAGED的事务管理机制：mybatis自身不会去实现事务管理的相干操作，而是交个内部容器来治理事务。当与spring整合应用后，个别应用spring来治理事务。
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);}次要就是如何依据一个 DataSource 创立一个 Transaction。
实际上整体感觉意义不大。
最外围的还是要看一下 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; }这里最外围的实际上只有 commit() 和 rollback()，其余的都是能够疏忽的。
针对 getTimeout() 咱们就能够为 mybatis 提供一个操作的超时机制。
JdbcTransaction 实现
基于 jdbc 机制的一些解决。
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，就是被动设置了一下主动提交的属性。
ManagedDataSource
这个是另一个实现，实际上更加简略。
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交给了容器去治理，本人自身并没有做任何解决。
mybatis 的应用形式
如果Mybatis是独自运行的，没有其余框架治理，此时mybatis外部会对下段代码实现。
con.setAutoCommit(false);//此处命令告诉数据库,从此刻开始从以后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留到同一个//Transaction中.这个Transaction的行为(commit,rollback)由以后Connection治理.try{ //推送sql语句命令……..; con.commit();//告诉Transaction提交.}catch(SQLException ex){ con.rollback();//告诉Transaction回滚.}整体来说这种写法比拟原始，咱们能够将原本交给 connection 解决的事务，对立调整为应用事务管理器解决。
spring 整合
当然针对 mybatis，大部分都是单个语句的执行。
用于应用 connection 时，实际上失去的是 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。
集体实现
看完了 mybatis 的实现原理之后，咱们的实现就变得非常简单。
咱们能够简化下面的一些实现，保留外围的局部即可。
接口定义
咱们只保留外围的 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，咱们的 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; /** * 连贯信息 * @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
这里和下面的相比拟，多出了 commit 和 rollback 的逻辑解决。
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; /** * 主动提交 * @since 0.0.18 */ private final boolean autoCommit; /** * 连贯信息 * @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 { //非主动提交，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 { //非主动提交，才执行 commit 操作 if(connection != null && !this.autoCommit) { connection.rollback(); } } catch (SQLException throwables) { throw new MybatisException(throwables); } }}