MyBatis基礎支援DataSource實現原始碼解析

package javax.sql;
public interface DataSource  extends CommonDataSource, Wrapper {
  // 最重要的方法
  Connection getConnection() throws SQLException;
  // 其他方法不再列出
}

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

public class PooledDataSource implements DataSource {
  // PooledDataSource真正管理連線狀態的是PoolState，後面會詳細說明
  private final PoolState state = new PoolState(this);
  // UnpooledDataSource上面說過和普通的Connection無異
  private final UnpooledDataSource dataSource;
  //正在使用連線的數量
  protected int poolMaximumActiveConnections = 10;
  //空閒連線數
  protected int poolMaximumIdleConnections = 5;
  //在被強制返回之前,池中連線被檢查的時間
  protected int poolMaximumCheckoutTime = 20000;
  //這是給連線池一個列印紀錄檔狀態機會的低層次設定,還有重新 嘗試獲得連線, 這些情況下往往需要很長時間 為了避免連線池沒有設定時靜默失 敗)。
  protected int poolTimeToWait = 20000;
  //傳送到資料的偵測查詢,用來驗證連線是否正常工作,並且準備 接受請求。預設是「NO PING QUERY SET」 ,這會引起許多資料庫驅動連線由一 個錯誤資訊而導致失敗
  protected String poolPingQuery = "NO PING QUERY SET";
  //開啟或禁用偵測查詢
  protected boolean poolPingEnabled = false;
  //用來設定 poolPingQuery 多次時間被用一次
  protected int poolPingConnectionsNotUsedFor = 0;
  private int expectedConnectionTypeCode;
}

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  String methodName = method.getName();
  //如果呼叫close的話，忽略它，反而將這個connection加入到池中
  if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) {
    dataSource.pushConnection(this);
    return null;
  } 
  return method.invoke(realConnection, args);
  // 其他邏輯省略....
}

public Connection getConnection() throws SQLException {
  return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}
private PooledConnection popConnection(String username, String password) throws SQLException {
  //最外面是while死迴圈，如果一直拿不到connection，則不斷嘗試
  while (conn == null) {
    synchronized (state) {
      if (!state.idleConnections.isEmpty()) {
        //如果有空閒的連線的話，返回第一個空閒連線
        conn = state.idleConnections.remove(0);
      } else {
        //如果沒有空閒的連線
        if (state.activeConnections.size() < poolMaximumActiveConnections) {
          //如果activeConnections太少,那就new一個PooledConnection
          conn = new PooledConnection(dataSource.getConnection(), this);
        } else {
          //如果activeConnections已經很多了，那不能再new了
          //取得activeConnections列表的第一個（最老的）
          PooledConnection oldestActiveConnection = state.activeConnections.get(0);
          long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
          if (longestCheckoutTime > poolMaximumCheckoutTime) {
            //如果checkout時間過長，則這個connection標記為overdue（過期）
            //刪掉最老的連線，然後再new一個新連線
            conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
            oldestActiveConnection.invalidate();
          } else {
            //如果checkout時間不夠長，沒辦法，只能等待，在此分支會記錄一些統計資訊
          }
        }
      }
      if (conn != null) {
        if (conn.isValid()) {
          //如果已經拿到connection，則記錄一些統計資訊
        } else {
          //如果沒拿到，統計資訊：壞連線+1
          state.badConnectionCount++;
          localBadConnectionCount++;
          conn = null;
          //如果好幾次都拿不到，就放棄了，丟擲異常
        }
      }
    }
  }
  return conn;
}

protected void pushConnection(PooledConnection conn) throws SQLException {
  synchronized (state) {
    //先從activeConnections中刪除此connection
    state.activeConnections.remove(conn);
    if (conn.isValid()) {
      if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) {
        //如果空閒的連線太少，
        state.accumulatedCheckoutTime += conn.getCheckoutTime();
        if (!conn.getRealConnection().getAutoCommit()) {
          conn.getRealConnection().rollback();
        }
        //new一個新的Connection，加入到idle列表
        PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this);
        state.idleConnections.add(newConn);
        //通知其他執行緒可以來搶connection了
        state.notifyAll();
      } else {
       //否則，即空閒的連線已經足夠了
        state.accumulatedCheckoutTime += conn.getCheckoutTime();
        //那就將connection關閉就可以了，獲取真正的connection物件並且關閉
        conn.getRealConnection().close();
        conn.invalidate();
      }
    } 
  }
}

public interface DataSourceFactory {
  //設定屬性,被XMLConfigBuilder所呼叫
  void setProperties(Properties props);
  //生產資料來源,直接得到javax.sql.DataSource
  DataSource getDataSource();
}

public class UnpooledDataSourceFactory implements DataSourceFactory {
  protected DataSource dataSource;
  public UnpooledDataSourceFactory() {
    this.dataSource = new UnpooledDataSource();
  }
  public DataSource getDataSource() {
    return dataSource;
  }
}

public class PooledDataSourceFactory extends UnpooledDataSourceFactory {
  //資料來源換成了PooledDataSource
  public PooledDataSourceFactory() {
    this.dataSource = new PooledDataSource();
  }
}