JDBC BaseDAO

import util.JDBCUtils;

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

/**
 * @program: Connection
 * @description:封装了针对数据表的通用的操作
 * @author: Mr.Fan
 * @create: 2021-05-30 13:17
 **/
public abstract class BaseDAO<T> {
    private Class<T> clazz = null;

    {//获取当前BaseDAO的子类继承的父类的泛型

        Type genericSuperclass = this.getClass().getGenericSuperclass();
        ParameterizedType paramType = (ParameterizedType) genericSuperclass;

        Type[] typeArguments = paramType.getActualTypeArguments();//获取了父类的泛型参数
        clazz = (Class<T>) typeArguments[0];//泛型的第一个参数
    }


    /**
     * 考虑数据库事务的增删改操作
     * @param sql
     * @param args
     */
    public void update(Connection conn, String sql, Object ...args) {

        PreparedStatement ps = null;
        try {

            //1.预编译sql语句
            ps = conn.prepareStatement(sql);
            //2.填充占位符
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1, args[i]);
            }
            //3.执行
            ps.execute();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(null, ps);
        }
    }


    /**
     * 返回数据表中的一条记录（考虑事务）
     * @param conn
     * @param sql
     * @param args
     * @return
     */
    public T getInstance(Connection conn, String sql, Object ...args){
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1, args[i]);
            }
            rs = ps.executeQuery();
            //获取结果集的元数据：ResultSetMetaData
            ResultSetMetaData metaData = rs.getMetaData();
            //获取结果集中的列数
            int columnCount = metaData.getColumnCount();

            if(rs.next()){
                T t = clazz.newInstance();

                for (int i = 0; i < columnCount; i++) {
                    Object columnValue = rs.getObject(i + 1);

                    //获取每个列的列名
                    String columnName = metaData.getColumnName(i+1);

                    //给t对象指定的columnName属性赋值为columnValue,通过反射
                    Field field = clazz.getDeclaredField(columnName);
                    field.setAccessible(true);
                    field.set(t, columnValue);

                    //给book对象的某个属性赋值为value
                }
                return t;

            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {

            JDBCUtils.closeResource(null, ps, rs);
        }
        return null;
    }

    /**
     * 获取多个对象的方法(考虑事务)
     * @param sql
     * @param args
     * @return
     */
    public List<T> getForList(Connection conn, String sql, Object...args){
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1, args[i]);
            }
            rs = ps.executeQuery();
            //获取结果集的元数据：ResultSetMetaData
            ResultSetMetaData metaData = rs.getMetaData();
            //获取结果集中的列数
            int columnCount = metaData.getColumnCount();
            //创建集合对象
            ArrayList<T> list = new ArrayList<>();
            while (rs.next()){
                T t = clazz.newInstance();

                for (int i = 0; i < columnCount; i++) {
                    Object columnValue = rs.getObject(i + 1);

                    //获取每个列的列名
                    String columnName = metaData.getColumnName(i+1);

                    //给t对象指定的columnName属性赋值为columnValue,通过反射
                    Field field = clazz.getDeclaredField(columnName);
                    field.setAccessible(true);
                    field.set(t, columnValue);

                    //给book对象的某个属性赋值为value
                }
                list.add(t);

            }
            return list;
        } catch (Exception e) {
            e.printStackTrace();
        }
        JDBCUtils.closeResource(null, ps, rs);
        return null;
    }

    /**
     * 用于查询特殊值的通用方法
     * @param conn
     * @param sql
     * @param args
     * @param <E>
     * @return
     */
    public <E> E getValue(Connection conn, String sql, Object...args) {
        PreparedStatement ps = null;
        ResultSet rs = null;


        try {
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++) {
                ps.setObject(i + 1, args[i]);

            }

            rs = ps.executeQuery();

            if(rs.next()){
                return  (E)rs.getObject(1);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        } finally {
            JDBCUtils.closeResource(null, ps, rs);
        }
        return null;

    }
}