Mybatis事务（一）事务管理方式(JDBC和MANAGED)

Mybatis管理事务是分为两种方式:

(1)使用JDBC的事务管理机制,就是利用java.sql.Connection对象完成对事务的提交

(2)使用MANAGED的事务管理机制，这种机制mybatis自身不会去实现事务管理，而是让程序的容器（JBOSS,WebLogic）来实现对事务的管理

在Mybatis的配置文件中可以配置事务管理方式如下：

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<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>

<environments default="development">

<environment id="development">

<!--配置事务的管理方式-->

<transactionManager type="JDBC" />

<!-- 配置数据库连接信息 -->

<dataSource type="POOLED">

<property name="driver" value="com.mysql.jdbc.Driver" />

<property name="url" value="jdbc:mysql://localhost:3306/mybatis" />

<property name="username" value="root" />

<property name="password" value="XDP" />

</dataSource>

</environment>

</environments>

</configuration>

Mybatis提供了一个事务接口Transaction，以及两个实现类jdbcTransaction和ManagedTransaction，当spring与Mybatis一起使用时，spring提供了一个实现类SpringManagedTransaction

Transaction接口：提供的抽象方法有获取数据库连接getConnection，提交事务commit，回滚事务rollback和关闭连接close，源码如下：

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//事务接口

ublic interface Transaction {

/**

* Retrieve inner database connection

* @return DataBase connection

* @throws SQLException

*/

//获得数据库连接

Connection getConnection() throws SQLException;

/**

* 提交

* Commit inner database connection.

* @throws SQLException

*/

void commit() throws SQLException;

/**

* 回滚

* Rollback inner database connection.

* @throws SQLException

*/

void rollback() throws SQLException;

/**

* 关闭连接

* Close inner database connection.

* @throws SQLException

*/

void close() throws SQLException;

1)JdbcTransaction实现类：Transaction的实现类，通过使用jdbc提供的方式来管理事务，通过Connection提供的事务管理方法来进行事务管理，源码如下：

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public class JdbcTransaction implements Transaction {

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

/* 连接**/

protected Connection connection;

/* 数据源**/

protected DataSource dataSource;

/* 事务等级**/

protected TransactionIsolationLevel level;

/* 事务提交**/

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();

}

//返回连接

return connection;

}

@Override

public void commit() throws SQLException {

if (connection != null && !connection.getAutoCommit()) {

if (log.isDebugEnabled()) {

log.debug("Committing JDBC Connection [" + connection + "]");

}

//连接提交

connection.commit();

}

}

@Override

public void rollback() throws SQLException {

if (connection != null && !connection.getAutoCommit()) {

if (log.isDebugEnabled()) {

log.debug("Rolling back JDBC Connection [" + connection + "]");

}

//连接回滚

connection.rollback();

}

}

@Override

public void close() throws SQLException {

if (connection != null) {

resetAutoCommit();

if (log.isDebugEnabled()) {

log.debug("Closing JDBC Connection [" + connection + "]");

}

//关闭连接

connection.close();

}

}

protected void setDesiredAutoCommit(boolean desiredAutoCommit) {

try {

//事务提交状态不一致时修改

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. select操作没有commit和rollback事务

// Some databases start transactions with select statements 一些数据库在select操作是会开启事务

// 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);

}

}

}

//打开连接

protected void openConnection() throws SQLException {

if (log.isDebugEnabled()) {

log.debug("Opening JDBC Connection");

}

//从数据源中获得连接

connection = dataSource.getConnection();

if (level != null) {

connection.setTransactionIsolation(level.getLevel());

}

setDesiredAutoCommit(autoCommmit);

}

}

2)ManagedTransaction实现类：通过容器来进行事务管理，所有它对事务提交和回滚并不会做任何操作，源码如下：

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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;

}

//数据源，事务等级及是否关闭事务

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;

}

//提交操作无效

@Override

public void commit() throws SQLException {

// Does nothing

}

//回滚操作无效

@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 + "]");

}

//关闭连接

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());

}

}

}

注意：如果我们使用MyBatis构建本地程序，即不是WEB程序，若将type设置成"MANAGED"，那么，我们执行的任何update操作，即使我们最后执行了commit操作，数据也不会保留，不会对数据库造成任何影响。因为我们将MyBatis配置成了“MANAGED”，即MyBatis自己不管理事务，而我们又是运行的本地程序，没有事务管理功能，所以对数据库的update操作都是无效的。

3)SpringManagedTransaction实现类：它其实也是通过使用JDBC来进行事务管理的，当spring的事务管理有效时，不需要操作commit/rollback/close，spring事务管理会自动帮我们完成，源码如下：

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public class SpringManagedTransaction implements Transaction {

private static final Log LOGGER = LogFactory.getLog(SpringManagedTransaction.class);

private final DataSource dataSource;

private Connection connection;

private boolean isConnectionTransactional;

private boolean autoCommit;

//获得数据源

public SpringManagedTransaction(DataSource dataSource) {

notNull(dataSource, "No DataSource specified");

this.dataSource = dataSource;

}

/**

* {@inheritDoc}

* 返回数据库连接

*/

@Override

public Connection getConnection() throws SQLException {

if (this.connection == null) {

openConnection();

}

return this.connection;

}

/**

* Gets a connection from Spring transaction manager and discovers if this

* {@code Transaction} should manage connection or let it to Spring.

* <p>

* It also reads autocommit setting because when using Spring Transaction MyBatis

* thinks that autocommit is always false and will always call commit/rollback

* so we need to no-op that calls.

*从spring的事务管理中获得一个连接

*/

private void openConnection() throws SQLException {

this.connection = DataSourceUtils.getConnection(this.dataSource);

this.autoCommit = this.connection.getAutoCommit();

this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

if (LOGGER.isDebugEnabled()) {

LOGGER.debug(

"JDBC Connection ["

+ this.connection

+ "] will"

+ (this.isConnectionTransactional ? " " : " not ")

+ "be managed by Spring");

}

}

/**

* {@inheritDoc}

*/

@Override

public void commit() throws SQLException {

if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {

if (LOGGER.isDebugEnabled()) {

LOGGER.debug("Committing JDBC Connection [" + this.connection + "]");

}

this.connection.commit();

}

}

/**

* {@inheritDoc}

*/

@Override

public void rollback() throws SQLException {

if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {

if (LOGGER.isDebugEnabled()) {

LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]");

}

this.connection.rollback();

}

}

/**

* {@inheritDoc}

*/

@Override

public void close() throws SQLException {

DataSourceUtils.releaseConnection(this.connection, this.dataSource);

}

}

Mybatis的事务管理机制还是比较简单的，其并没有做过多的操作，只是封装一下方便别人调用而已。