1. 什么是事务
事务是指的是一个业务上的最小不可再分单元，通常一个事务对应了一个完整的业务，而一个完整的业务需要批量的DML语句共同联合完成。一般，同一个事务中的SQL语句是保存到数据库中的同一个Transaction对象中,原因是Transaction具有一致性的特征,也就是说事务中如果有任何一条sql语句运行失败,那么这个事务中所有的SQL语句都会被判定为无效SQL。
2. MyBatis事务管理策略
2.1 亲自实现JDBC管理方式
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.
}
2.2 委托实现JDBC管理方式
如果实现Spring+MyBatis，可以将对Transasaction 管理交给Spring框架来管理实现。
开发人员可以在mybatis-configuration.xml 文件中配置，采用指定的管理方式。
当<transactionManager type="JDBC"/>时，就会调用JdbcTransaction。
当<transactionManager type="MANAGED"/>时，调用ManagedTransaction。
3. MyBatis中Transaction接口
mybatis 支持的两种事务类型管理器，Transactions接口中对两种事务管理方式,进行行为约束。有四种方法对事物管理，具体介绍看下面代码。
public interface Transaction {
//JDBC中事务手动管理，需要依靠connection对象，getConnection方法约束取得Connection对象方式。
//一般来说,要么是手动new一个Connection，要么就是从数据库连池获得。
Connection getConnection() throws SQLException;
//设置在什么情况下执行commit()命令
void commit() throws SQLException;
//设置在什么情况下执行rollback()命令
void rollback() throws SQLException;
//业务完毕后，处理Connection对象。
//一般有两种形式，将这个Connection对象销毁，将Connection返回数据库连接池中。
void close() throws SQLException;
//Connection向数据库索要一个Transaction对象时的最大等待时间。
Integer getTimeout() throws SQLException;
}
4. Transaction的接口实现类
Transaction接口中有两个实现类。
4.1 JdbcTransaction
JdbcTransaction直接使用JDBC的提交和回滚事务管理机制 。它依赖与从dataSource中取得的连接connection 来管理transaction 的作用域，connection对象的获取被延迟到调用getConnection()方法。如果autocommit设置为on，开启状态的话，它会忽略commit和rollback。
public class JdbcTransaction implements Transaction {
private static final Log log = LogFactory.getLog(JdbcTransaction.class);
protected Connection connection;
protected DataSource dataSource;
protected TransactionIsolationLevel level;
// MEMO: We are aware of the typo. See #941
protected boolean autoCommmit;
//数据源、隔离级别、是否自动提交
public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
dataSource = ds;
level = desiredLevel;
autoCommmit = desiredAutoCommit;
}
//根据当前的数据库连接connection创建transaction
public JdbcTransaction(Connection connection) {
this.connection = connection;
}
//返回一个Connection对象，其中openConnection方法是得到Connection 具体实现。
@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();
}
}
//close方法中我们可以看到connection被回收到数据库连接池前，执行了一个resetAutoCommit()。
@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);
}
}
}
/**
* openConnection方法中做了三件事情：
* 1. 数据库连接池得到了一个Connection
* 2. 设置数据库的事务隔离级别
* 3. 调用了con.setAutoCommit(false),再setDesireAutoCommit(autoCommit)。
**/
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;
}
}
4.2 ManagedTransaction
我们可以在这个类中看到它的commit方法和rollback方法没有具体实现。
ManagedTransaction是让容器来管理事务Transaction的整个生命周期，使用ManagedTransaction的commit和rowblkrollback功能不会对事务有任何影响，它没有具体实现，它将事务管理权交给容器来实现。
public class ManagedTransaction implements Transaction {
private static final Log log = LogFactory.getLog(ManagedTransaction.class);
private DataSource dataSource;
private TransactionIsolationLevel level;
private Connection connection;
private final 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;
}
}
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