java - JDBC 预处理语句 select from table where

Question

我正在尝试编写一个 JDBC SQLite 准备语句，它将返回 customerID 与传入的行匹配的行。我的方法返回空指针异常。

我不太了解 JDBC、SQLite 或准备好的语句，但据我所知，我拥有我需要的一切，但无法弄清楚为什么它不起作用。我的代码如下:

public static void CustomersSelectWhere(JPanel customers, int CustID)
      {              
         CustomersTable();
          String [] entries = new String[7]; 
        Connection c = null;
        PreparedStatement pstmt = null;
        try {
          Class.forName("org.sqlite.JDBC");
          c = DriverManager.getConnection("jdbc:sqlite:WalkerTechCars.db");
          c.setAutoCommit(false);

          String query = "SELECT * FROM CUSTOMERS WHERE CUSTID=? " ;

          ResultSet rs = pstmt.executeQuery( query );

          pstmt = c.prepareStatement(query);
          pstmt.setInt(1, CustID);
          pstmt.executeUpdate();
          c.commit();



          while ( rs.next() ) {
             int custId = rs.getInt("custID");
             String phone = rs.getString("phone");
             String  surname = rs.getString("surname");
             String  firstname = rs.getString("firstname");
             String home  = rs.getString("home");
             String  address = rs.getString("address");
             String  postcode = rs.getString("postcode");

             customers.add(customersTableSingle(Integer.toString(custId), firstname, surname, phone, home, address, postcode, false, customers ));


          }
          rs.close();
          pstmt.close();
          c.close();
        } catch ( Exception e ) {
          System.err.println( e.getClass().getName() + ": " + e.getMessage() );
          System.exit(0);
        }

      }

}

Answer 1

你已经定义了

PreparedStatement pstmt = null;

准备好的语句应该是从连接创建的。因为你的准备语句是空的，你正试图调用 ResultSet rs = pstmt.executeQuery( query );这就是NPE的原因。