Java

Question

我有一个函数可以从 MySQL table

中获取数据

public ResultSet getAddressID(String city) throws SQLException{
        String q = "SELECT PK_ADDRESS_ID FROM tbl_addresses WHERE city =" + "\""+ city+ "\";";
        ResultSet rs = executeSearch(q);
        return rs;
    }

当我尝试 System.out.println(n.getAddressID("Sheffield")); 时 returns null。为什么即使我的 table 中有数据也会发生这种情况（见图）。

public ResultSet executeSearch(String q){
    openConnection();
    try{
        Statement statement = connection.createStatement();
        ResultSet resultSet = statement.executeQuery(q);
        closeConnection();
        return resultSet;
    }
    catch (Exception e){
        JOptionPane.showMessageDialog(null, e.getMessage());
    }
    finally {
        closeConnection();
        return null;
    }
}

Answer 1

问题似乎出在您的 executeSearch 方法中； finally 块将始终执行，因此通过在 finally 块中返回 null，您实际上 覆盖了 您在 try 块中返回的内容！

这可能是一个替代解决方案；请注意，我在方法的末尾返回，而不是在 try-catch-finally 块的任何部分内返回。

/**
 * Converts a provided ResultSet into a generic List so that the
 * ResultSet can be closed while the data persists.
 * Source: 
 */
public List convertResultSetToList(ResultSet rs) throws SQLException
{
    ResultSetMetaData md = rs.getMetaData();
    int columns = md.getColumnCount();
    List list = new ArrayList(50);

    while (rs.next())
    {
        HashMap row = new HashMap(columns);
        for(int i = 1; i <= columns; ++i)
        {
            row.put(md.getColumnName(i), rs.getObject(i));
        }
        list.add(row);
    }

    return list;
}

public List executeSearch(String q)
{
    List toReturn;
    openConnection();
    try {
        Statement statement = connection.createStatement();
        toReturn = this.convertResultSetToList(statement.executeQuery(q));
    }
    catch (Exception e) {
        JOptionPane.showMessageDialog(null, e.getMessage());
        toReturn = new ArrayList();
    }
    finally {
        closeConnection();
    }

    return toReturn;
}

Java - 空结果集

Java - NULL ResultSet

mysql

resultset