github地址：https://github.com/dchack/Mybatis-source-code-learn （欢迎star）
TransactionFactory
官方文档：
在 MyBatis 中有两种类型的事务管理器（也就是 type=”[JDBC|MANAGED]”）：
JDBC – 这个配置就是直接使用了 JDBC 的提交和回滚设置，它依赖于从数据源得到的连接来管理事务作用域。
MANAGED – 这个配置几乎没做什么。它从来不提交或回滚一个连接，而是让容器来管理事务的整个生命周期（比如 JEE 应用服务器的上下文）。 默认情况下它会关闭连接，然而一些容器并不希望这样，因此需要将 closeConnection 属性设置为 false 来阻止它默认的关闭行为。例如:提示如果你正在使用 Spring + MyBatis，则没有必要配置事务管理器， 因为 Spring 模块会使用自带的管理器来覆盖前面的配置。
以上配置transactionManager属性来配置使用哪一种TransactionFactory的代码，肯定在MybatisXMLConfigBuilder中可以找到：
private TransactionFactory transactionManagerElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type");
Properties props = context.getChildrenAsProperties();
TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
factory.setProperties(props);
return factory;
}
throw new BuilderException("Environment declaration requires a TransactionFactory.");
}从TransactionFactory入手：
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);
}TransactionFactory接口描述实现者需要从Connection或DataSource生产org.apache.ibatis.transaction.Transaction出来。
接口实现类如下：
他们各自生产的Transaction分别是：
- JdbcTransaction
- ManagedTransaction
- SpringManagedTransaction
Transaction接口：
/**
* Wraps a database connection.
* Handles the connection lifecycle that comprises: its creation, preparation, commit/rollback and close.
*
* @author Clinton Begin
*/
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;
}抽象出了控制connection生命周期的核心接口：getConnection（create），commit，rollback，close。
JdbcTransaction的实现：
三个操作方法：commit，rollback，close，都是connection的封装而已，commit，rollback执行的条件需要已经生成好connection并且AutoCommit没有设置true，close方法会调用resetAutoCommit方法重置Connection的autoCommit属性为true：
@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();
}
}重置autoCommit属性方法:
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);
}
}
}在看下getConnection方法的实现：
@Override
public Connection getConnection() throws SQLException {
if (connection == null) {
openConnection();
}
return connection;
}
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);
}openConnection中设置了事务隔离级别（transaction isolation level）和autoCommmit。
事务隔离级别在TransactionIsolationLevel枚举中可以看到：
public enum TransactionIsolationLevel {
NONE(Connection.TRANSACTION_NONE),
READ_COMMITTED(Connection.TRANSACTION_READ_COMMITTED),
READ_UNCOMMITTED(Connection.TRANSACTION_READ_UNCOMMITTED),
REPEATABLE_READ(Connection.TRANSACTION_REPEATABLE_READ),
SERIALIZABLE(Connection.TRANSACTION_SERIALIZABLE);
private final int level;
private TransactionIsolationLevel(int level) {
this.level = level;
}
public int getLevel() {
return level;
}
}在java.sql.Connection中的定义和注释如下：
/**
* A constant indicating that transactions are not supported.
*/
int TRANSACTION_NONE = 0;
/**
* A constant indicating that
* dirty reads, non-repeatable reads and phantom reads can occur.
* This level allows a row changed by one transaction to be read
* by another transaction before any changes in that row have been
* committed (a "dirty read"). If any of the changes are rolled back,
* the second transaction will have retrieved an invalid row.
*/
int TRANSACTION_READ_UNCOMMITTED = 1;
/**
* A constant indicating that
* dirty reads are prevented; non-repeatable reads and phantom
* reads can occur. This level only prohibits a transaction
* from reading a row with uncommitted changes in it.
*/
int TRANSACTION_READ_COMMITTED = 2;
/**
* A constant indicating that
* dirty reads and non-repeatable reads are prevented; phantom
* reads can occur. This level prohibits a transaction from
* reading a row with uncommitted changes in it, and it also
* prohibits the situation where one transaction reads a row,
* a second transaction alters the row, and the first transaction
* rereads the row, getting different values the second time
* (a "non-repeatable read").
*/
int TRANSACTION_REPEATABLE_READ = 4;
/**
* A constant indicating that
* dirty reads, non-repeatable reads and phantom reads are prevented.
* This level includes the prohibitions in
* TRANSACTION_REPEATABLE_READ and further prohibits the
* situation where one transaction reads all rows that satisfy
* a WHERE condition, a second transaction inserts a row that
* satisfies that WHERE condition, and the first transaction
* rereads for the same condition, retrieving the additional
* "phantom" row in the second read.
*/
int TRANSACTION_SERIALIZABLE = 8;关于事务隔离级别
几个概念：
- 脏读：读取的数据可以取到其他未提交事务修改的数据
- 不可重复读：一个事务中多次读取相同的数据，因其他事务在中间修改了这个数据，导致第一个事务多次读的数据会不相同
- 幻读：就是在一个事务提交时发现之前查的条件发生了改变
隔离级别：
- 提交读（READ_COMMITTED）只能读取到已经提交的数据
- 未提交读（READ_UNCOMMITTED）允许脏读
- 可重复读（REPEATABLE_READ）在同一事务中保证多次读取的数据是一致的
- 串行读（SERIALIZABLE）每次读都需要获取表级锁，读写互相阻塞
mysql中查看隔离级别设置：
select @@global.tx_isolation;另外我们也看到JdbcTransaction中是需要autoCommmit设置true的，否则是不能完成事务功能的。
ManagedTransaction
从类注释上可以看到：ManagedTransaction是将事务的生命周期交给容器管理，可以理解它都是空实现，比如commit，rollback，close可以通过closeConnection字段来关闭。
SpringManagedTransaction
后续进入Mybatis扩展模块时展开。