java 웹 페이지 원리 상세 설명

public class Page {
 private int currentPage;
 private int totalPage;
 private int count;
 private int PageSize;
 private List<Product> list;
 private String category;
 
 } 

package com.learning.web.servlet;

import java.io.IOException;
import java.util.List;

import javax.enterprise.inject.New;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.learning.domain.Page;
import com.learning.domain.Product;
import com.learning.service.ProductService;


@WebServlet("/showProductByPage")
public class ShowProductByPage extends HttpServlet {
 private static final long serialVersionUID = 1L;
 
 
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  int currentPage=1;
  int pageSize=4;
  // 
  String currentPageString=request.getParameter("currentPage");
  if (currentPageString!=null) {
  currentPage=Integer.parseInt(currentPageString);
  }
  String category = request.getParameter("category");
  if ("".equals(category)) {
  category=null;
  } 
  ProductService productService=new ProductService();
  Page page=productService.showProductByPage(currentPage,pageSize,category);
  request.setAttribute("page", page);
  request.getRequestDispatcher("/product_list.jsp").forward(request, response);
 }
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 doGet(request, response);
 }

}

public Page showProductByPage(int currentPage, int pageSize, String category) {
 try {
  Page page=new Page();
  int count=productDao.count(category);
  page.setCount(count);
  page.setList(productDao.findProductsByPage(currentPage,pageSize,category));
  int totalPage=(int) Math.ceil(1.0*count/pageSize);
  page.setPageSize(pageSize);
  page.setCurrentPage(currentPage);
  page.setTotalPage(totalPage);
  page.setCategory(category);
  return page;
 } catch (SQLException e) {
  e.printStackTrace();
 }
 return null;
 }

 public int count(String category) throws SQLException {
 QueryRunner queryRunner=new QueryRunner(C3P0Util.getDataSource());
 String sql=" select count(*) from products ";
 if (category!=null) {
  sql+=" where category='"+category+"'";
 }
 long l= (Long)queryRunner.query(sql, new ScalarHandler(1));
 return (int) l;
 }



 public List<Product> findProductsByPage(int currentPage, int pageSize, String category) throws SQLException {
 QueryRunner queryRunner=new QueryRunner(C3P0Util.getDataSource());
 String sql=" select * from products ";
 if (category!=null) {
  sql+=" where category='"+category+"'";
 }
 sql+=" limit ?,?";
 return queryRunner.query(sql, new BeanListHandler<Product>(Product.class),(currentPage-1)*pageSize,pageSize);
 }