derby Java SQL异常无效游标状态无当前行
我一直很难弄明白这一点。首先，我有一个insertProduct(Product p)方法，用来检查数据库中是否存在具有指定代码的产品。如果是，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。我不确定我这样做是否正确
其次，deleteProduct(Product p)方法应该删除由insertProduct方法添加的产品。现在的问题是，当我尝试添加产品时，我一直得到一个SQL异常，然后deleteProduct方法每次运行程序时都会一个接一个地删除数据库中的产品，直到剩下非产品为止。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.
代码：
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);
}
}
# 1 楼答案
在检索列值之前，需要调用
ResultSet.next()上面的代码运行时不会出现异常，但在逻辑上仍然会失败，因为您正在选择所有产品，并且只检查数据库返回的第一个产品的代码。检查产品是否已存在的正确方法是
# 2 楼答案
在insertProduct方法中，在从ResultSet获取数据之前，需要调用ResultSet上的next（）方法