分页查询

参考资料

<黑马苍穹外卖，菜品分页查询业务>

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个人粗略理解，可能存在错误！！！

Controller接收前端传来的Page 以及 PageSize

ServiceImpl中通过PageHelper类将Page和PageSize参数传递给Page

ServiceImpl代码

public PageResult pageQuery(DishPageQueryDTO dishPageQueryDTO) {
    PageHelper.startPage(dishPageQueryDTO.getPage(), dishPageQueryDTO.getPageSize());
    Page<DishVO> page = dishMapper.pageQuery(dishPageQueryDTO);
    return new PageResult(page.getTotal(), page.getResult());
}

PageHelper

public class PageHelper extends PageMethod

PageMethod，startPage（）

public abstract class PageMethod {
    protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal();
    
    protected static void setLocalPage(Page page) {
        LOCAL_PAGE.set(page);
    }
    
    public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
        Page<E> page = new Page(pageNum, pageSize, count);
        page.setReasonable(reasonable);
        page.setPageSizeZero(pageSizeZero);
        Page<E> oldPage = getLocalPage();
        if (oldPage != null && oldPage.isOrderByOnly()) {
            page.setOrderBy(oldPage.getOrderBy());
        }

        setLocalPage(page);
        return page;
    }

Page继承了ArrayList

public class Page<E> extends ArrayList<E> implements Closeable {
    private static final long serialVersionUID = 1L;
    private int pageNum;
    private int pageSize;
    private long startRow;
    private long endRow;
    private long total;
    private int pages;

MySQL分页查询

SELECT * FROM table_name
ORDER BY some_column
-- pageSize 是每页显示的记录数，currentPage 是当前页码
LIMIT pageSize OFFSET (currentPage - 1) * pageSize;