MyBatis3源码解析(2)数据库连接

简介

基于上篇的示例感受，下面我们探索下MyBatis连接数据库的细节是如果实现的，在日常使用中是如何能和Druid数据库连接池等配合起来的

源码解析

基于上篇的示例代码：

public class MybatisTest {@Testpublic void test() {try(SqlSession session = buildSqlSessionFactory().openSession()) {PersonMapper personMapper = session.getMapper(PersonMapper.class);personMapper.createTable();personMapper.save(Person.builder().id(1L).name("1").build());Person person = personMapper.getPersonById(1);System.out.println(person);}}public static SqlSessionFactory buildSqlSessionFactory() {String JDBC_DRIVER = "org.h2.Driver";String DB_URL = "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1";String USER = "sa";String PASS = "";DataSource dataSource = new PooledDataSource(JDBC_DRIVER, DB_URL, USER, PASS);Environment environment = new Environment("Development", new JdbcTransactionFactory(), dataSource);Configuration configuration = new Configuration(environment);configuration.addMapper(PersonMapper.class);SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();return builder.build(configuration);}
}

当前我们想找的是与数据库建立连接的部分

通过阅读书籍：《MyBatis3源码深度解析》，我们大概知道执行是在Executor中，我们跟踪相关的代码

经过不懈的努力跟踪，得到下面的关键代码：

找到了在执行语句中，在Executor中获取连接的关键代码

public class SimpleExecutor extends BaseExecutor {private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {// 获取数据库连接Connection connection = this.getConnection(statementLog);Statement stmt = handler.prepare(connection, this.transaction.getTimeout());handler.parameterize(stmt);return stmt;}
}

继续跟踪，来到我们示例中定义的事务管理器：JdbcTransactionFactory

public class JdbcTransaction implements Transaction {public Connection getConnection() throws SQLException {if (this.connection == null) {this.openConnection();}return this.connection;}protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}// 从DataSource中获取this.connection = this.dataSource.getConnection();if (this.level != null) {this.connection.setTransactionIsolation(this.level.getLevel());}this.setDesiredAutoCommit(this.autoCommit);}
}

上面可以看到是从DataSource中获取的，来到我们定义的：PooledDataSource

public class PooledDataSource implements DataSource {public Connection getConnection() throws SQLException {return this.popConnection(this.dataSource.getUsername(), this.dataSource.getPassword()).getProxyConnection();}private PooledConnection popConnection(String username, String password) throws SQLException {while(conn == null) {synchronized(this.state) {PoolState var10000;if (!this.state.idleConnections.isEmpty()) {......} else if (this.state.activeConnections.size() < this.poolMaximumActiveConnections) {// 获取数据库连接池连接conn = new PooledConnection(this.dataSource.getConnection(), this);if (log.isDebugEnabled()) {log.debug("Created connection " + conn.getRealHashCode() + ".");}} else {......}if (conn != null) {......}}}......}
}

我们继续跟下去：

public class UnpooledDataSource implements DataSource {public Connection getConnection() throws SQLException {return this.doGetConnection(this.username, this.password);}private Connection doGetConnection(String username, String password) throws SQLException {Properties props = new Properties();if (this.driverProperties != null) {props.putAll(this.driverProperties);}if (username != null) {props.setProperty("user", username);}if (password != null) {props.setProperty("password", password);}return this.doGetConnection(props);}private Connection doGetConnection(Properties properties) throws SQLException {this.initializeDriver();// 看到了熟悉的原生的获取数据库连接的方式Connection connection = DriverManager.getConnection(this.url, properties);this.configureConnection(connection);return connection;}
}

到这里我们找到了源码中如何获取数据库连接的关键代码

其实就是对于原生数据库操作的封装

调用栈回顾

我们回过头来看看整个过程中的类调用栈：

MybatisTest：我们的测试代码
MapperProxy：MyBatis的Mapper的代理类
MapperMethod：SQL执行相关
DefaultSqlSession
CachingExecutor
BaseExecutor
SimpleExecutor
JdbcTransation
PooledDataSource
UnpooledDataSource

可以看到数据库连接是从DataSource中获取的，而PooledDataSource是MyBatis自己的一个数据库连接池，其中使用非池化的UnpooledDataSource

在我们的日常开发中，经常使用MyBatis+Druid等数据库连接池一起使用，从这里就可以看到，我们简单替换一下MyBatis初始化时候的DataSource即可

总结

本篇中，我们跟踪了数据库如何进行连接获取，看到日常开发中MyBatis+Druid等数据库连接的配置使用的背后原理