介绍

JDBC2.0 提供了javax.sql.DataSource接口，它负责建立与数据库的连接，当在应用程序中访问数据库时
不必编写连接数据库的代码，直接引用DataSource获取数据库的连接对象即可。用于获取操作数据Connection对象。
在jdbc中我们获取数据库连接对象，需要先在DriverManager中注册，再通过DriverManager获取Connection对象。

        //1、注册mysql驱动
        Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://localhost:3306/zg_test?useUnicode=true&characterEncoding=UTF8&zeroDateTimeBehavior=convertToNull&serverTimezone=Asia/Shanghai";

        String username = "root";

        String password = "123456";

        //通过DriverManager获取Connection
        Connection connection = DriverManager.getConnection(url, username, password);

有了DataSource接口后，可以快速获得Connection对象，也方便实现数据库连接池。

public interface DataSource  extends CommonDataSource, Wrapper {

  /**
   * <p>Attempts to establish a connection with the data source that
   * this {@code DataSource} object represents.
   *
   * @return  a connection to the data source
   * @exception SQLException if a database access error occurs
   * @throws java.sql.SQLTimeoutException  when the driver has determined that the
   * timeout value specified by the {@code setLoginTimeout} method
   * has been exceeded and has at least tried to cancel the
   * current database connection attempt
   */
  Connection getConnection() throws SQLException;

  /**
   * <p>Attempts to establish a connection with the data source that
   * this {@code DataSource} object represents.
   *
   * @param username the database user on whose behalf the connection is
   *  being made
   * @param password the user's password
   * @return  a connection to the data source
   * @exception SQLException if a database access error occurs
   * @throws java.sql.SQLTimeoutException  when the driver has determined that the
   * timeout value specified by the {@code setLoginTimeout} method
   * has been exceeded and has at least tried to cancel the
   * current database connection attempt
   * @since 1.4
   */
  Connection getConnection(String username, String password)
    throws SQLException;
}

UnpooledDataSource

UnpooledDataSource是由mybatis提供的一种最简单的实现，没有使用到数据库连接池。
其实现和jdbc一摸一样。
实现了DataSource的getConnect方法，实际去执行doGetConnection方法。

  @Override
  public Connection getConnection() throws SQLException {
    return doGetConnection(username, password);
  }

doGetConnection

  private Connection doGetConnection(Properties properties) throws SQLException {
    initializeDriver();
    Connection connection = DriverManager.getConnection(url, properties);
    configureConnection(connection);
    return connection;
  }

doGetConnection也是注册驱动，然后通过DriverManager获取。

  private synchronized void initializeDriver() throws SQLException {
    if (!registeredDrivers.containsKey(driver)) {
      Class<?> driverType;
      try {
        if (driverClassLoader != null) {
           //和我们自己使用jdbc一样
          driverType = Class.forName(driver, true, driverClassLoader);
        } else {
          driverType = Resources.classForName(driver);
        }
        // DriverManager requires the driver to be loaded via the system ClassLoader.
        // http://www.kfu.com/~nsayer/Java/dyn-jdbc.html
        Driver driverInstance = (Driver)driverType.newInstance();
        DriverManager.registerDriver(new DriverProxy(driverInstance));
        registeredDrivers.put(driver, driverInstance);
      } catch (Exception e) {
        throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
      }
    }
  }