JDBC 6—— 针对不同表的通用查询操作

package com.JDBCStudy3.PreparedStatement.crud;

import java.awt.List;
import java.lang.reflect.Field;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.util.ArrayList;

import org.junit.Test;

import JDBC_util.JDBCutils;

/*
 * 使用PrepareStatement实现针对于不同表的通用的查询操作
 * */
public class PrepaerdStatementQueryTest<T> {

    @Test
    public void testGetInstance() {
        String sql = "select id,name,email from customers where id = ?";
        Customer customer = getInstance(Customer.class, sql, 12);
        System.out.println(customer);

        sql = "select order_id orderId,order_name orderName from `order` where order_id = ?";
        Order order = getInstance(Order.class, sql, 1);
        System.out.println(order);
    }

    /*
     * 针对不同的表的通用查询操作，返回表中的一条记录
     */
    @SuppressWarnings("finally")
    public <T> T getInstance(Class<T> clazz, String sql, Object... args) {
        Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            conn = JDBCutils.getConnection();

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

            // 执行，获取结果集
            rs = ps.executeQuery();
            // 获取结果集的元数据
            ResultSetMetaData rsmd = rs.getMetaData();
            // 获取列数
            int columnCount = rsmd.getColumnCount();

            if (rs.next()) {
                @SuppressWarnings("deprecation")
                T t = clazz.newInstance();
                for (int i = 0; i < columnCount; i++) {
                    // 获取每个列的列值：通过ResultSet
                    Object columnValue = rs.getObject(i + 1);
                    /*
                     * 通过ResultSetMetaData 获取列的列名：getColumnName() -- 不推荐使用 获取列的别名：getColumnLabel()
                     */
                    // String columnName = rsmd.getColumnName(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i);
                    // 通过反射，将对象指定名columnName的属性赋值为指定的值columnValue
                    Field field = Order.class.getDeclaredField((String) columnLabel);
                    field.setAccessible(true);
                    field.set(t, columnValue);
                }
                return t;
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCutils.closeResource(conn, ps, rs);
            return null;
        }
    }

    public void testGetForList() {
        String sql = "select id,name,email from customers where id < ?";
        ArrayList<Customer> list = getForList(Customer.class, sql, 12);
        list.forEach(System.out::println);
        
        sql = "select order_id orderId,order_name orderName from `order` where order_id < ?";
        ArrayList<Order> list1 = getForList(Order.class, sql, 5);
        list1.forEach(System.out::println);
    }

    @SuppressWarnings("finally")
    public <T> ArrayList<T> getForList(Class<T> clazz, String sql, Object... args) {
        Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            conn = JDBCutils.getConnection();

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

            // 执行，获取结果集
            rs = ps.executeQuery();
            // 获取结果集的元数据
            ResultSetMetaData rsmd = rs.getMetaData();
            // 获取列数
            int columnCount = rsmd.getColumnCount();
            // 创建集合对象
            ArrayList<T> list = new ArrayList<T>();
            while (rs.next()) {
                @SuppressWarnings("deprecation")
                T t = clazz.newInstance();
                for (int i = 0; i < columnCount; i++) {
                    // 获取每个列的列值：给t对象指定的属性赋值
                    Object columnValue = rs.getObject(i + 1);
                    /*
                     * 通过ResultSetMetaData 获取列的列名：getColumnName() -- 不推荐使用 获取列的别名：getColumnLabel()
                     */
                    // String columnName = rsmd.getColumnName(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i);
                    // 通过反射，将对象指定名columnName的属性赋值为指定的值columnValue
                    Field field = Order.class.getDeclaredField((String) columnLabel);
                    field.setAccessible(true);
                    field.set(t, columnValue);
                }
                list.add(t);
            }
            return list;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCutils.closeResource(conn, ps, rs);
            return null;
        }

    }

}