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所以我有一个动态 Web 项目，它查询 MySQL 数据库并根据用户需求返回员工信息。

例如：htp://localhost:8080/Employees/123 //将返回员工 123 的信息 htp://localhost:8080/Employees //将返回所有员工的信息

问题是，如果数据库被更新并且有人要求新插入的员工，程序将抛出一个空指针。是否只有在数据库发生更改后才通知或检查更新？

这是有关 mySQL 的唯一相关代码，其余代码来自我正在使用的两个哈希映射。

package resources;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Connection;
import java.sql.ResultSetMetaData;
import java.sql.Statement;
import java.util.HashMap;

import pojo.Party;

public class DBConnection {

    private Connection con;
    private static Statement statement;
    private static ResultSet resultSet;
    public  static DBConnection connection;
    private static ResultSetMetaData meta;
    private static HashMap<String,Party> map;

    public Party party;

    private DBConnection()
    {
        try 
        {
            map = new HashMap<String,Party>();
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection(//not relevant, assume this works);
            statement = con.createStatement();
            readData();
        }
        catch (Exception e)
        {
            System.out.print("Error: "+e);
        }
    }
    public void readData()
    {
        try
        {
            String query = "(SELECT * FROM PureServlet)";
            resultSet = statement.executeQuery(query);
            meta = resultSet.getMetaData();
            String columnName, value, partyName;
            while(resultSet.next())
            {
                partyName = resultSet.getString("PARTY_NAME");
                map.put(partyName, new Party()); //this is the map that keeps track of all parties
                party = map.get(partyName);
              //getColumn...() irritatingly starts at 1 and not 0 thus j=1
                for(int j=1;j<=meta.getColumnCount();j++)
                {
                    columnName = meta.getColumnLabel(j);
                    value = resultSet.getString(columnName);
                    party.getPartyInfo().put(columnName, value); //this is the hashmap within the party that keeps 
                    //track of the individual values. The column Name = label, value is the value
                }
            }
        }
        catch (Exception e)
        {
            System.out.println(e);
        }
    }
    public static HashMap<String,Party> getPartyCollection()
    {
        if(connection == null)
        {
            connection = new DBConnection();
        }
        return map;
    }
}
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