深入理解 MyBatis - PooledDataSource

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我们在进行数据库链接操作时，会通过 JDBC 的 connection 进行数据库操作。但是频繁的创建和销毁 connection 会影响执行效率。因此 MyBatis 中存在连接池技术

• pooled
• unpooled

PooledDataSource 分析

数据库基础配置以及事务的隔离级别

 
  private String driver;
  private String url;
  private String username;
  private String password;

  private Boolean autoCommit;
  private Integer defaultTransactionIsolationLevel;

初始化时，会加载并注册驱动

  static {
    Enumeration<Driver> drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) &#123;
      Driver driver = drivers.nextElement();
      registeredDrivers.put(driver.getClass().getName(), driver);
    &#125;
  &#125;

获取数据库的连接时，会调用以下方法

1. 构造配置文件，用户名和密码，准备链接
2. 加载驱动，准备链接
3. 获取数据库连接
4. 对连接进行配置

public class UnpooledDataSource implements DataSource {

  private Connection doGetConnection(String username, String password) throws SQLException {
    Properties props = new Properties();
    if (driverProperties != null) {
      props.putAll(driverProperties);
    }
    if (username != null) {
      props.setProperty("user", username);
    }
    if (password != null) {
      props.setProperty("password", password);
    }
    return doGetConnection(props);
  }

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

初始化驱动

  private synchronized void initializeDriver() throws SQLException {
    if (!registeredDrivers.containsKey(driver)) {
      Class<?> driverType;
      try &#123;
        if (driverClassLoader != null) &#123;
          driverType = Class.forName(driver, true, driverClassLoader);
        &#125; else &#123;
          driverType = Resources.classForName(driver);
        &#125;
        // 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);
      &#125; catch (Exception e) &#123;
        throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
      &#125;
    &#125;
  &#125;

对链接进行配置，查看是否是自动提交，是否存在配置的事务隔离级别

  private void configureConnection(Connection conn) throws SQLException {
    if (autoCommit != null && autoCommit != conn.getAutoCommit()) {
      conn.setAutoCommit(autoCommit);
    }
    if (defaultTransactionIsolationLevel != null) {
      conn.setTransactionIsolation(defaultTransactionIsolationLevel);
    }
  }
}