java - “结果集关闭后操作不可用”

Question

我在这里查看了发生这种情况的原因，我唯一能看到的原因是开放连接。在我的代码中，我关闭一个连接并注册一个新连接以重新打开。

try{

       //Registration of driver, connector
        String myDriver = "com.mysql.jdbc.Driver";
        String myURL = "jdbc:mysql://localhost:3306/maintenance";
        Class.forName(myDriver);
        Connection connection = DriverManager.getConnection(myURL, "admin", "");


        //Call MySQL for table names of users
        String query = "SELECT users FROM goods";

        Statement st = connection.createStatement();


        ResultSet res = st.executeQuery(query);

            while (res.next()) {

                String name = res.getString("users");


                if(name.equals(username)){
                    connection.close();
                    res.close();
                          try{
                         Connection connection2;
                         connection2 = DriverManager.getConnection("jdbc:mysql://localhost:3306/maintenance", "admin", "");
                         query = "SELECT idkeys FROM goods";
                         Statement st2;
                         st2 = connection2.createStatement();
                         ResultSet res2;
                         res2 = st2.executeQuery(query);
                         String myDriver2 = "com.mysql.jdbc.Driver";
                         Class.forName(myDriver2);
                         connection2 = DriverManager.getConnection("jdbc:mysql://localhost:3306/maintenance", "admin", "");
                         while (res2.next()) {

                             keyed = res2.getObject("idkeys");

                             if (keyed.equals(sanitizedPass)){
                                 checked = true;
                                 connection2.close();
                                 res2.close();

                             }
                         } 
                    } catch (Exception e2){
                        System.err.println(e2.getMessage());

                    }

                }

  }
  //Close connections  
  if (!checked){
  res.close();
  connection.close();
  }
    }
    catch(Exception e)
    {
        System.err.println(e.getMessage());  
   }

    return checked;

在我的代码中，至少据我所知，没有发生这样的事情。为什么还是会被抛出？

Answer 1

try {
    String myURL = "jdbc:mysql://localhost:3306/maintenance";
    Class.forName("com.mysql.jdbc.Driver");
    Connection connection = DriverManager.getConnection(myURL, "admin", "");

    String query = "SELECT users, idkeys FROM goods";
    Statement st = connection.createStatement();
    ResultSet res = st.executeQuery(query);

    while (res.next()) {
        String name = res.getString("users");
        if (name.equals(username)) {
            Object keyed = res.getObject("idkeys");
            if (keyed.equals(sanitizedPass)) {
                checked = true;
                break;
            }
        }
    }
     return checked;
} catch (Exception e) {
    e.printStackTrace(); // or do some reasonable error logging
} finally {
    // close the ResultSet, Statement and Connection here
}

1. 您不需要创建多个连接；一个就够了。
2. 即使您正在查询同一个表，您也会向数据库发送两个不同的查询；相反，在同一查询中返回两个字段(请参阅上面的查询)