JDBC Select * from Where x=(?)

Question

我正在尝试创建一种从数据库中检索客户并将其存储在对象中的方法。但由于某种原因，我的 sql 语句不起作用。

这是我的方法。

    public class CustomerDAO {

private String url = "jdbc:h2:tcp://localhost/project;IFEXISTS=TRUE";

Set<Customer> customerCollection = new TreeSet<>();


public Customer loginCustomer(String user,String pass) throws SQLException{

    String sql = "SELECT * FROM Customers WHERE USERNAME=(?) AND PASSWORD=(?) ;";

    try(
        //get connection form database
        Connection dbCon = JdbcConnection.getConnection(url);
        //create a statement
        PreparedStatement stmt = dbCon.prepareStatement(sql);                   
        ){

        stmt.setString(1, user);
        stmt.setString(2, pass);

        ResultSet rs = stmt.executeQuery();

        Customer c = new Customer(rs.getString("username"), rs.getString("name"), rs.getString("email"),
                                    rs.getString("address"), rs.getString("creditCardDetails"), rs.getString("password"));


        if(c.getUsername().contains(user))
            return c;
        else
            return null;


}catch (SQLException ex){
        throw new RuntimeException(ex);
    }

}

我用这个测试过

        public static void main(String[] args) throws SQLException {

    CustomerDAO dao = new CustomerDAO();

    Customer c = dao.loginCustomer("jesus", "1234");

    System.out.println(c.getName());
    System.out.println(c.getPassword());
    System.out.println(c.getAddress());



}

}

但我得到了这些错误。

        Exception in thread "main" java.lang.RuntimeException: org.h2.jdbc.JdbcSQLException: No data is available [2000-187]
    at DAO.CustomerDAO.loginCustomer(CustomerDAO.java:54)
    at web.NewMain.main(NewMain.java:25)
Caused by: org.h2.jdbc.JdbcSQLException: No data is available [2000-187]
    at org.h2.message.DbException.getJdbcSQLException(DbException.java:345)
    at org.h2.message.DbException.get(DbException.java:179)
    at org.h2.message.DbException.get(DbException.java:155)
    at org.h2.message.DbException.get(DbException.java:144)
    at org.h2.jdbc.JdbcResultSet.checkOnValidRow(JdbcResultSet.java:3186)
    at org.h2.jdbc.JdbcResultSet.get(JdbcResultSet.java:3192)
    at org.h2.jdbc.JdbcResultSet.get(JdbcResultSet.java:3209)
    at org.h2.jdbc.JdbcResultSet.getString(JdbcResultSet.java:302)
    at DAO.CustomerDAO.loginCustomer(CustomerDAO.java:43)
    ... 1 more
Java Result: 1

直接从数据库控制台运行语句可以正常工作。例如

SELECT * FROM Customers WHERE USERNAME='jesus' AND PASSWORD='1234' ;

Answer 1

您必须将光标移动到下一行： 添加

rs.next();

之后：

 ResultSet rs = stmt.executeQuery();

Answer 2

在调用结果集的 getter 方法之前，您需要将光标向前移动。看看http://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html#next%28%29

另外，为了避免空结果集，您可以将代码修改为

Customer c=null;    
if(rs.next()){
    c = new Customer(rs.getString("username"), rs.getString("name"), rs.getString("email"),
     rs.getString("address"), rs.getString("creditCardDetails"), rs.getString("password"));

}
if(null != c && c.getUsername().contains(user))
            return c;
        else
            return null;