Java SQLexception无效的游标状态 – 没有当前行
我一直在努力解决这个问题。 首先，我有一个insertProduct(Product p)方法，用于检查数据库中是否存在具有指定代码的产品。 如果是这样，此方法将显示错误消息。 否则，它应该将产品添加到数据库并将其打印到控制台。 我不确定我是否正确地做到了这一点。
其次， deleteProduct(Product p)方法应该删除insertProduct方法添加的产品。 现在的问题是，当我尝试添加产品时，我不断收到SQLexception，然后deleteProduct方法只是在每次运行程序时都会逐个删除数据库中的产品，直到不存在。 我不确定这两种方法都有什么问题。
控制台输出：
Derby has been started. Product list: bvbn Murach's Beginning Visual Basic .NET $49.50 cshp Murach's C# $49.50 java Murach's Beginning Java $49.50 jsps Murach's Java Servlets and JSP $49.50 mcb2 Murach's Mainframe COBOL $59.50 sqls Murach's SQL for SQL Server $49.50 zjcl Murach's OS/390 and z/OS JCL $62.50 First product: bvbn Murach's Beginning Visual Basic .NET $49.50 Last product: zjcl Murach's OS/390 and z/OS JCL $62.50 Product by code: cshp cshp Murach's C# $49.50 Insert test: java.sql.SQLException: Invalid cursor state - no current row. Product list: bvbn Murach's Beginning Visual Basic .NET $49.50 cshp Murach's C# $49.50 java Murach's Beginning Java $49.50 jsps Murach's Java Servlets and JSP $49.50 mcb2 Murach's Mainframe COBOL $59.50 sqls Murach's SQL for SQL Server $49.50 zjcl Murach's OS/390 and z/OS JCL $62.50 Delete test: zjcl Murach's OS/390 and z/OS JCL $62.50 Product list: bvbn Murach's Beginning Visual Basic .NET $49.50 cshp Murach's C# $49.50 java Murach's Beginning Java $49.50 jsps Murach's Java Servlets and JSP $49.50 mcb2 Murach's Mainframe COBOL $59.50 sqls Murach's SQL for SQL Server $49.50 Derby has been shut down.
码：
import java.sql.*; public class DBTesterApp { private static Connection connection = null; private static Product p = null ; public static void main(String args[]) { // get the connection and start the Derby engine connection = MurachDB.getConnection(); if (connection != null) System.out.println("Derby has been started.\n"); // select data from database printProducts(); printFirstProduct(); printLastProduct(); printProductByCode("cshp"); // modify data in the database p = new Product("test", "Test Product", 49.50); insertProduct(p); printProducts(); deleteProduct(p); printProducts(); // disconnect from the database if (MurachDB.disconnect()) System.out.println("Derby has been shut down.\n"); } public static void printProducts() { // Product p = null ; try (Statement statement = connection.createStatement(); ResultSet rs = statement.executeQuery("SELECT * FROM Products")) { System.out.println("Product list:"); while(rs.next()) { String code = rs.getString("ProductCode"); String description = rs.getString("Description"); double price = rs.getDouble("Price"); p = new Product(code, description, price); printProduct(p); } System.out.println(); } catch(SQLException e) { e.printStackTrace(); // for debugging } } public static void printFirstProduct() { try(Statement statement = connection.createStatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE); ResultSet rs = statement.executeQuery("SELECT * FROM Products")){ System.out.println("First product:"); rs.first(); rs.last(); if(rs.isFirst() == false){ rs.previous(); } if(rs.isLast() == false){ rs.next(); } rs.absolute(1); String code = rs.getString(1); String description = rs.getString(2); double price = rs.getDouble(3); p = new Product(code , description , price); printProduct(p); System.out.println(); } catch(SQLException e){ e.printStackTrace(); } } public static void printLastProduct() { try(Statement statement = connection.createStatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE); ResultSet rs = statement.executeQuery("SELECT * FROM Products")){ System.out.println("Last product:"); rs.first(); rs.last(); if(rs.isFirst() == false){ rs.previous(); } if(rs.isLast() == false){ rs.next(); } rs.absolute(7); String code = rs.getString(1); String description = rs.getString(2); double price = rs.getDouble(3); p = new Product(code, description, price); printProduct(p); System.out.println(); } catch(SQLException e){ e.printStackTrace(); } } public static void printProductByCode(String productCode) { String sql = "SELECT ProductCode, Description, Price " + "FROM Products " + "WHERE ProductCode = ?"; try(PreparedStatement ps = connection.prepareStatement(sql);){ ps.setString(1, productCode); ResultSet rs = ps.executeQuery(); if(rs.next()){ String description = rs.getString("Description"); double price = rs.getDouble("Price"); p = new Product(productCode, description, price); System.out.println("Product by code: " + productCode); printProduct(p); } else{ rs.close(); } } catch(SQLException e){ System.err.println(e); } System.out.println(); } public static void insertProduct(Product p) { System.out.println("Insert test: "); //check if product code exists in database try(Statement statement = connection.createStatement(); ResultSet rs = statement.executeQuery("SELECT * FROM Products")){ String code = rs.getString(1); if (p.getCode().equals(code) ){ System.out.println("Error: This product is already in the database!"); } else{ String sql = "INSERT INTO Products (productCode, Description, Price) " + "VALUES (?, ?, ?)"; try(PreparedStatement ps = connection.prepareStatement(sql)){ ps.setString(1, p.getCode()); ps.setString(2, p.getDescription()); ps.setDouble(3, p.getPrice()); ps.executeUpdate(); } catch(SQLException e){ System.err.println(e); } } //end else printProduct(p); System.out.println(); }//end try catch(SQLException e ){ System.out.println(e); } } private static void deleteProduct(Product p) { System.out.println("Delete test: "); String sql = "DELETE FROM Products " + "WHERE ProductCode = ?"; try(PreparedStatement ps = connection.prepareStatement(sql)){ ps.setString(1, p.getCode()); ps.executeUpdate(); } catch(SQLException e){ System.err.println(e); } // add code that deletes the specified product from the database // if a product with the specified code doesn't exist, display an error message printProduct(p); System.out.println(); } // use this method to print a Product object on a single line private static void printProduct(Product p) { String productString = StringUtils.padWithSpaces(p.getCode(), 8) + StringUtils.padWithSpaces(p.getDescription(), 44) + p.getFormattedPrice(); System.out.println(productString); } }
您需要先调用ResultSet.next()然后才能检索列值。
// check if product code exists in database try(Statement statement = connection.createStatement(); ResultSet rs = statement.executeQuery("SELECT * FROM Products")){ if (rs.next()) // THIS is MISSING! String code = rs.getString(1);
上面的代码将无exception地运行，但由于您选择了所有产品并且仅检查数据库返回的第一个代码，因此仍然会在逻辑上失败。 检查产品是否已存在的正确方法是
// check if product code exists in database try(Statement statement = connection.createStatement(); ResultSet rs = statement.executeQuery( "SELECT * FROM Products WHERE ProductCode = '" + p.getCode() + "'")){ if (rs.next()) { System.out.println("Error: This product is already in the database!"); return; }
在insertProduct方法中，需要在从ResultSet获取数据之前调用ResultSet上的next（）方法。