JDBC 单例模式 封装 使用预编译（PreparedStatement接口）

使用预编译的PreparedStatement 类
使用上的区别？
private PreparedStatement preparedStatement;
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setObject(int parameterIndex, Object x);
preparedStatement.executeUpdate();
预编译PreparedStatement需要实现将SQL语句和语句中 的占位符相对应的value存入

If arbitrary parameter type conversions are required, the method setObject should be used with a target SQL type.

In the following example of setting a parameter, con represents an active connection:
使用范例：


PreparedStatement pstmt = con.prepareStatement("UPDATE EMPLOYEES
SET SALARY = ? WHERE ID = ?");
pstmt.setBigDecimal(1, 153833.00)
pstmt.setInt(2, 110592)


接口PreparedStatement接口中用到的方法
1. 方法 void setObject​(int parameterIndex, Object x)
API：
Parameters:
parameterIndex - the first parameter is 1, the second is 2, ...
x - the object containing the input parameter value

2.方法int executeUpdate()
API:
Returns:
either (1) the row count for SQL Data Manipulation Language (DML) statements or (2) 0 for SQL statements that return nothing

3.方法 ResultSet executeQuery()
Returns:
a ResultSet object that contains the data produced by the query; never null

ResultSetMetaData接口

ResultSetMetaData metaData =resultSet.getMetaData();
方法：
1. String getColumnName​(int column) —–>Get the designated column's name.
Parameters:
column - the first column is 1, the second is 2, ...
Returns: column name
2. int getColumnCount()—->
Returns the number of columns in this ResultSet object.

mport java.lang.reflect.Field;
import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class DBmanager {
   //数据库的用户名
    private final String USER="root";
    //密码
    private final String PASSWORD="19970909";
    //数据库的访问连接
    private final String URL="jdbc:mysql://localhost:3306/mydatabase?characterEncoding=utf8&useSSL=false";
    //数据库驱动
    private final String DRIVER="com.mysql.cj.jdbc.Driver";
    //获取单例
    private static DBmanager instance;
    //获取连接
    private Connection connection;


    //预编译的SQL执行空间
    private PreparedStatement preparedStatement;


    //单例模式，将构造方法设置为私有
    private DBmanager(){
        try {
            Class.forName(DRIVER);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    /**
     * 单例模式，获取实例的方法
     * @return
     */
    public static DBmanager getInstance(){
        if(instance==null){
            instance = new DBmanager();
        }
        return instance;
    }

    /**
     * 获取数据库连接的方法
     * @return
     * @throws SQLException
     */

    public Connection getConnection()throws SQLException{
        connection = DriverManager.getConnection(URL,USER,PASSWORD);
        return connection;
    }

    /**
     * 可执行 delete update insert 语句
     * @param sql SQL语句
     * @param params 装载value的list
     * @return
     * @throws SQLException
     */

    public int executeUpdateBySQL(String sql,List