我们回顾<MyBatis框架中Mapper映射配置的使用及原理解析(一) 配置与使用> 一文的示例
private static SqlSessionFactory getSessionFactory() {
SqlSessionFactory sessionFactory = null;
String resource = "mybatisConfig.xml";
try {
sessionFactory = new SqlSessionFactoryBuilder().build(Resources
.getResourceAsReader(resource));
} catch (IOException e) {
e.printStackTrace();
}
return sessionFactory;
}
@Test
public void findUserById() {
SqlSessionFactory sqlSessionFactory = getSessionFactory();
SqlSession sqlSession = sqlSessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
User user = userMapper.selectByPrimaryKey(1l);
System.out.println(user.getId() + " / " + user.getName());
}
SqlSessionFactoryBuilder 创建出SqlSessionFactory，然后从SqlSessionFactory中得到SqlSession，最后通过SqlSession得到Mapper接口对象进行数据库操作。
我们跟踪SqlSessionFactoryBuilder的源代码：
package org.apache.ibatis.session;
public class SqlSessionFactoryBuilder {
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
}
我们可以看到这个类用很多的构造方法，但主要分为三大类：
1、通过读取字符流（Reader）的方式构件SqlSessionFactory。
2、通过字节流（InputStream）的方式构件SqlSessionFacotry。
3、通过Configuration对象构建SqlSessionFactory。
第1、2种方式是通过配置文件方式，第3种是通过Java代码方式。
build方法返回SqlSessionFactory接口的实现对象DefaultSqlSessionFactory。
我们继续跟踪DefaultSqlSessionFactory的openSession()方法：
package org.apache.ibatis.session.defaults; /**
* @author Clinton Begin
*/
public class DefaultSqlSessionFactory implements SqlSessionFactory { private final Configuration configuration; public DefaultSqlSessionFactory(Configuration configuration) {
this.configuration = configuration;
} public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
} public SqlSession openSession(boolean autoCommit) {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit);
} public SqlSession openSession(ExecutorType execType) {
return openSessionFromDataSource(execType, null, false);
} public SqlSession openSession(TransactionIsolationLevel level) {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), level, false);
} public SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level) {
return openSessionFromDataSource(execType, level, false);
} public SqlSession openSession(ExecutorType execType, boolean autoCommit) {
return openSessionFromDataSource(execType, null, autoCommit);
} public SqlSession openSession(Connection connection) {
return openSessionFromConnection(configuration.getDefaultExecutorType(), connection);
} public SqlSession openSession(ExecutorType execType, Connection connection) {
return openSessionFromConnection(execType, connection);
} public Configuration getConfiguration() {
return configuration;
} private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
} private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
try {
boolean autoCommit;
try {
autoCommit = connection.getAutoCommit();
} catch (SQLException e) {
// Failover to true, as most poor drivers
// or databases won't support transactions
autoCommit = true;
}
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
final Transaction tx = transactionFactory.newTransaction(connection);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
} private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
if (environment == null || environment.getTransactionFactory() == null) {
return new ManagedTransactionFactory();
}
return environment.getTransactionFactory();
} private void closeTransaction(Transaction tx) {
if (tx != null) {
try {
tx.close();
} catch (SQLException ignore) {
// Intentionally ignore. Prefer previous error.
}
}
} }
这么多的openSession重载方法，都是通过传入不同的参数构造SqlSession实例，有通过设置事务是否自动提交"autoCommit"，有设置执行器类型"ExecutorType"来构造的，还有事务的隔离级别等等。
最后一个方法就告诉我们可以通过SqlSessionFactory来获取Configuration对象。
mybatis创建sqlsession经过了以下几个主要步骤：
1. 从核心配置文件mybatis-config.xml中获取Environment（这里面是数据源）；
2. 从Environment中取得DataSource；
3. 从Environment中取得TransactionFactory；
4. 从DataSource里获取数据库连接对象Connection；
5. 在取得的数据库连接上创建事务对象Transaction；
6. 创建Executor对象（该对象非常重要，事实上sqlsession的所有操作都是通过它完成的）；
7. 创建sqlsession对象。
从源码中可以知道DefaultSqlSession是SqlSession的实例。
new DefaultSqlSession(configuration, executor, autoCommit);
那么通过此文，我们就清楚的知道了SqlSessionFactory和SqlSession具体的创建过程，知道了他们的实现类是DefaultSqlSessionFactory和DefaultSqlSession。