四、BaseDao基础

重写代码遇到的bug：
示例代码如下：

package cn.kgc.Dao;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;


public class BaseDao {
    Connection connection = null;
    PreparedStatement pstmt = null;
    ResultSet rs = null;
    
    private boolean getConnection() {
        boolean flag = false;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/kgcnews?user=root&password=41312019";
            connection = DriverManager.getConnection(url);
            flag = true;
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return flag;
    }
    
    private void closeConnection() {
        try {
            if (rs != null) {
                rs.close();
            }
            if (pstmt != null) {
                pstmt.close();
            }
            if (connection != null) {
                connection.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        
    }
    
    public ResultSet executeQuery(String sql, Object[] params) {
        if (this.getConnection()) {
            try {
                pstmt = connection.prepareStatement(sql);
                for (int i = 0; i < params.length; i++) {
                    pstmt.setObject(i+1, params[i]);
                }
                rs = pstmt.executeQuery();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        this.closeConnection();
        return rs;
    }
    
    public int executeUpdate(String sql, Object[] params) {
        int lines = 0;
        if (this.getConnection()) {
            try {
                pstmt = connection.prepareStatement(sql);
                for (int i = 0; i < params.length; i++) {
                    pstmt.setObject(i+1, params[i]);
                }
                lines = pstmt.executeUpdate();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        this.closeConnection();
        return lines;
    }
    
    public static void main(String[] args) {
        BaseDao test = new BaseDao();
        String sql = "select id, title from news_detail where id=?";
        Object[] params = {2};
        ResultSet rs = test.executeQuery(sql, params);
        try {
            System.out.println(rs.next());
        } catch (SQLException e1) {
            e1.printStackTrace();
        }
        try {
            while (rs.next()) {
                int id = rs.getInt("id");
                System.out.println(id);
                String title = rs.getString("title");
                System.out.println(title);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

}

bug1：java.sql.SQLException: Operation not allowed after ResultSet closed，出现这个bug的原因是在executeQuery方法中我先关闭了资源在进行返回，但是一旦关闭了资源之后ResultSet里面的资源就丢失了，所以不应该关闭资源再返回。

bug2：结果界面只显示了个true，但是并不输出内容，这是因为在我的结果集rs里面只有一个元素，而在上面输入rs.next()去判断里面是否有值得时候使得指针移动了以为，所以下面的while就拿不出来值了。

bug3：将上述两bug解决完了之后，把main函数改为如下，但是还是报bug1，猜想原因可能是ResultSet返回值的地址一直没变，所以如果在拿值之前就关闭的话就会出错。只需把关闭资源的方法放到最后面即可。

public static void main(String[] args) {
        BaseDao test = new BaseDao();
        String sql = "select id, title from news_detail where id=?";
        Object[] params = {2};
        ResultSet rs = test.executeQuery(sql, params);
        test.closeConnection();
        try {
            while (rs.next()) {
                int id = rs.getInt("id");
                System.out.println(id);
                String title = rs.getString("title");
                System.out.println(title);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        
    }