Java JDBC 效率:一个连接应该维持多久?

I'm still working on the same problem mention here. It seems to work fine especially after creating an AbstractModel class shown below:

public abstract class AbstractModel {

    protected static Connection myConnection = SingletonConnection.instance().establishConnection();
    protected static Statement stmt;
    protected static ResultSet rs;

    protected boolean loginCheck;                   // if userId and userLoginHistoryId are valid - true, else false
    protected boolean userLoggedIn;                 // if user is already logged in - true, else false

    public AbstractModel (int userId, Long userLoginHistoryId){
        createConnection();                                 // establish connection
            loginCheck = false;
        userLoggedIn = false;
        if (userId == 0 && userLoginHistoryId == 0){        // special case for login
            loginCheck = true;                              // 0, 0, false, false
            userLoggedIn = false;                           // set loginCheck to true, userLogged in to false
        } else {
            userLoggedIn = true;
            try{
                String query = "select "user_login_session_check"(" + userId + ", " + userLoginHistoryId + ");";
                System.out.println("query: " + query);
                stmt = myConnection.createStatement();
                rs = stmt.executeQuery(query);
                while (rs.next()){
                    loginCheck = rs.getBoolean(1);
                }
            } catch (SQLException e){
                System.out.println("SQL Exception: ");
                e.printStackTrace();
            }
        }

    }
    // close connection
    public void closeConnection(){
        try{
            myConnection.close();
        } catch (SQLException e){
            System.out.println("SQL Exception: ");
            e.printStackTrace();
        }

    }
    // establish connection
    public void createConnection(){
        myConnection = SingletonConnection.instance().establishConnection();
    }

    // login session check
    public boolean expiredLoginCheck (){
        if (loginCheck == false && userLoggedIn == true){
            closeConnection();
            return false;
        } else {
            return true;
        }
    }

}

我已经在上述问题的链接中发布了存储过程和单例模式实现.