mybatis分页插件pagehleper的基本使用

mybatis使用分页插件时，多数都是主要是使用pageInfo来作为分页的。

第一步：使用分页插件需要引入分页插件依赖，使用分页插件时需要注意版本，不同的版本有些区别

<dependency>
    <groupId>com.github.pagehelper</groupId>
    <artifactId>pagehelper</artifactId>
    <version>4.2.1</version>
</dependency>

第二步：在mybati-config.xml文件中配置插件的一些信息，如下

<plugins>
    <!-- com.github.pagehelper为PageHelper类所在包名 -->
    <plugin interceptor="com.github.pagehelper.PageHelper">
        <property name="dialect" value="mysql"/>
        <!-- 该参数默认为false -->
        <!-- 设置为true时，会将RowBounds第一个参数offset当成pageNum页码使用 -->
        <!-- 和startPage中的pageNum效果一样-->
        <property name="offsetAsPageNum" value="false"/>
        <!-- 该参数默认为false -->
        <!-- 设置为true时，使用RowBounds分页会进行count查询 -->
        <property name="rowBoundsWithCount" value="false"/>
        <!-- 设置为true时，如果pageSize=0或者RowBounds.limit = 0就会查询出全部的结果 -->
        <!-- （相当于没有执行分页查询，但是返回结果仍然是Page类型）-->
        <property name="pageSizeZero" value="true"/>
        <!-- 3.3.0版本可用 - 分页参数合理化，默认false禁用 -->
        <!-- 启用合理化时，如果pageNum<1会查询第一页，如果pageNum>pages会查询最后一页 -->
        <!-- 禁用合理化时，如果pageNum<1或pageNum>pages会返回空数据 -->
        <property name="reasonable" value="false"/>
        <!-- 3.5.0版本可用 - 为了支持startPage(Object params)方法 -->
        <!-- 增加了一个`params`参数来配置参数映射，用于从Map或ServletRequest中取值 -->
        <!-- 可以配置pageNum,pageSize,count,pageSizeZero,reasonable,不配置映射的用默认值 -->
        <!--<property name="params" value="pageNum=start;pageSize=limit;pageSizeZero=zero;reasonable=heli;count=contsql"/>-->
    </plugin>
</plugins>

第三步：实际使用如下

/**
 * <p>通过关键词查询商品接口</p>
 *
 * @author jason
 * @date 2020年8月15日16:07:59
 */
public interface QueryGoodsInfoWithKeyWords {

    /**
     * 通过关键词查询商品接口
     *
     * @param keyWordsCondition 用户输入的关键词,参数为关键词和分页参数以及二次查询条件
     * @return Results
     */
    PageInfo selectGoodsInfoByKeywords(KeyWordsCondition keyWordsCondition);

}

public class QueryGoodsInfoWithKeyWordsImpl implements QueryGoodsInfoWithKeyWords {
    @Override
    public PageInfo selectGoodsInfoByKeywords(KeyWordsCondition keyWordsCondition) {
        // md5加密密码 将入参加密成md5用作redis中的key，不管他的入参是多少个，只要输入了相同的条件，第二次直接从缓存里面取出就行
        String keyMD5 = DigestUtils.md5DigestAsHex(JSONObject.toJSONString(keyWordsCondition).getBytes());
        // 开启分页
        PageHelper.startPage(keyWordsCondition.getPage(), keyWordsCondition.getLimit());

        // 返回给前端的查询结果list
        List<GoodsSerachKeyWordsDTO> keyWordsDTOList = new ArrayList<>();

        // 查询缓存，如果缓存中有与之前相同的入参条件则直接返回
        if (redisUtil.existsKey(keyMD5)) {
            logger.info("是从缓存里面取出来的数据：" + JSONObject.toJSONString(redisUtil.getItemKey(keyMD5)));

            // 将json字符串列表list转换成keyWordsDTOList
            JSONArray jsonArray = JSONArray.fromObject(redisUtil.getItemKey(keyMD5));
            keyWordsDTOList = (List<GoodsSerachKeyWordsDTO>) JSONArray.toCollection(jsonArray, GoodsSerachKeyWordsDTO.class);

            PageInfo pageInfo = new PageInfo(keyWordsDTOList);

            return pageInfo;
        } 

    }

}

需要注意2点：

（1）在使用时需要使用 PageHelper.startPage()方法开启分页，只需要传入页码和每页条数即可，它会自动根据页码和每页条数自动计算偏移量。

（2）返回时把结果封装为PageInfo，如 PageInfo pageInfo = new PageInfo(你需要返回的数据list);

最后看一下PageInfo里面的源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.github.pagehelper;

import java.io.Serializable;
import java.util.Collection;
import java.util.List;

public class PageInfo<T> implements Serializable {
    private static final long serialVersionUID = 1L;
    private int pageNum; // 当前页
    private int pageSize;// 每页的条数
    private int size;// 当前页的数据条数
    private String orderBy; // 排序
    private int startRow;// 当前页第一个元素在数据库中的行号
    private int endRow;//  当前页最后一个元素在数据库中的行号
    private long total;// 总条数
    private int pages;// 总页数
    private List<T> list;// 结果集list
    private int prePage;// 前一页
    private int nextPage;// 下一页
    private boolean isFirstPage;// 是否是第一页
    private boolean isLastPage;// 是否是最后一页
    private boolean hasPreviousPage;// 是否有前一页
    private boolean hasNextPage;// 是否有下一页
    private int navigatePages;// 导航页码数
    private int[] navigatepageNums;// 所有导航页码
    private int navigateFirstPage;// 导航第一页
    private int navigateLastPage;// 导航最后一页

    public PageInfo() {
        this.isFirstPage = false;
        this.isLastPage = false;
        this.hasPreviousPage = false;
        this.hasNextPage = false;
    }

    public PageInfo(List<T> list) {
        this(list, 8);
    }

    public PageInfo(List<T> list, int navigatePages) {
        this.isFirstPage = false;
        this.isLastPage = false;
        this.hasPreviousPage = false;
        this.hasNextPage = false;
        if (list instanceof Page) {
            Page page = (Page)list;
            this.pageNum = page.getPageNum();
            this.pageSize = page.getPageSize();
            this.orderBy = page.getOrderBy();
            this.pages = page.getPages();
            this.list = page;
            this.size = page.size();
            this.total = page.getTotal();
            if (this.size == 0) {
                this.startRow = 0;
                this.endRow = 0;
            } else {
                this.startRow = page.getStartRow() + 1;
                this.endRow = this.startRow - 1 + this.size;
            }
        } else if (list instanceof Collection) {
            this.pageNum = 1;
            this.pageSize = list.size();
            this.pages = 1;
            this.list = list;
            this.size = list.size();
            this.total = (long)list.size();
            this.startRow = 0;
            this.endRow = list.size() > 0 ? list.size() - 1 : 0;
        }

        if (list instanceof Collection) {
            this.navigatePages = navigatePages;
            this.calcNavigatepageNums();
            this.calcPage();
            this.judgePageBoudary();
        }

    }

    private void calcNavigatepageNums() {
        int i;
        if (this.pages <= this.navigatePages) {
            this.navigatepageNums = new int[this.pages];

            for(i = 0; i < this.pages; ++i) {
                this.navigatepageNums[i] = i + 1;
            }
        } else {
            this.navigatepageNums = new int[this.navigatePages];
            i = this.pageNum - this.navigatePages / 2;
            int endNum = this.pageNum + this.navigatePages / 2;
            int i;
            if (i < 1) {
                i = 1;

                for(i = 0; i < this.navigatePages; ++i) {
                    this.navigatepageNums[i] = i++;
                }
            } else if (endNum > this.pages) {
                endNum = this.pages;

                for(i = this.navigatePages - 1; i >= 0; --i) {
                    this.navigatepageNums[i] = endNum--;
                }
            } else {
                for(i = 0; i < this.navigatePages; ++i) {
                    this.navigatepageNums[i] = i++;
                }
            }
        }

    }

    private void calcPage() {
        if (this.navigatepageNums != null && this.navigatepageNums.length > 0) {
            this.navigateFirstPage = this.navigatepageNums[0];
            this.navigateLastPage = this.navigatepageNums[this.navigatepageNums.length - 1];
            if (this.pageNum > 1) {
                this.prePage = this.pageNum - 1;
            }

            if (this.pageNum < this.pages) {
                this.nextPage = this.pageNum + 1;
            }
        }

    }

    private void judgePageBoudary() {
        this.isFirstPage = this.pageNum == 1;
        this.isLastPage = this.pageNum == this.pages;
        this.hasPreviousPage = this.pageNum > 1;
        this.hasNextPage = this.pageNum < this.pages;
    }

    public int getPageNum() {
        return this.pageNum;
    }

    public void setPageNum(int pageNum) {
        this.pageNum = pageNum;
    }

    public int getPageSize() {
        return this.pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    public int getSize() {
        return this.size;
    }

    public void setSize(int size) {
        this.size = size;
    }

    public String getOrderBy() {
        return this.orderBy;
    }

    public void setOrderBy(String orderBy) {
        this.orderBy = orderBy;
    }

    public int getStartRow() {
        return this.startRow;
    }

    public void setStartRow(int startRow) {
        this.startRow = startRow;
    }

    public int getEndRow() {
        return this.endRow;
    }

    public void setEndRow(int endRow) {
        this.endRow = endRow;
    }

    public long getTotal() {
        return this.total;
    }

    public void setTotal(long total) {
        this.total = total;
    }

    public int getPages() {
        return this.pages;
    }

    public void setPages(int pages) {
        this.pages = pages;
    }

    public List<T> getList() {
        return this.list;
    }

    public void setList(List<T> list) {
        this.list = list;
    }

    /** @deprecated */
    @Deprecated
    public int getFirstPage() {
        return this.navigateFirstPage;
    }

    /** @deprecated */
    @Deprecated
    public void setFirstPage(int firstPage) {
        this.navigateFirstPage = firstPage;
    }

    public int getPrePage() {
        return this.prePage;
    }

    public void setPrePage(int prePage) {
        this.prePage = prePage;
    }

    public int getNextPage() {
        return this.nextPage;
    }

    public void setNextPage(int nextPage) {
        this.nextPage = nextPage;
    }

    /** @deprecated */
    @Deprecated
    public int getLastPage() {
        return this.navigateLastPage;
    }

    /** @deprecated */
    @Deprecated
    public void setLastPage(int lastPage) {
        this.navigateLastPage = lastPage;
    }

    public boolean isIsFirstPage() {
        return this.isFirstPage;
    }

    public void setIsFirstPage(boolean isFirstPage) {
        this.isFirstPage = isFirstPage;
    }

    public boolean isIsLastPage() {
        return this.isLastPage;
    }

    public void setIsLastPage(boolean isLastPage) {
        this.isLastPage = isLastPage;
    }

    public boolean isHasPreviousPage() {
        return this.hasPreviousPage;
    }

    public void setHasPreviousPage(boolean hasPreviousPage) {
        this.hasPreviousPage = hasPreviousPage;
    }

    public boolean isHasNextPage() {
        return this.hasNextPage;
    }

    public void setHasNextPage(boolean hasNextPage) {
        this.hasNextPage = hasNextPage;
    }

    public int getNavigatePages() {
        return this.navigatePages;
    }

    public void setNavigatePages(int navigatePages) {
        this.navigatePages = navigatePages;
    }

    public int[] getNavigatepageNums() {
        return this.navigatepageNums;
    }

    public void setNavigatepageNums(int[] navigatepageNums) {
        this.navigatepageNums = navigatepageNums;
    }

    public int getNavigateFirstPage() {
        return this.navigateFirstPage;
    }

    public int getNavigateLastPage() {
        return this.navigateLastPage;
    }

    public void setNavigateFirstPage(int navigateFirstPage) {
        this.navigateFirstPage = navigateFirstPage;
    }

    public void setNavigateLastPage(int navigateLastPage) {
        this.navigateLastPage = navigateLastPage;
    }

    public String toString() {
        StringBuffer sb = new StringBuffer("PageInfo{");
        sb.append("pageNum=").append(this.pageNum);
        sb.append(", pageSize=").append(this.pageSize);
        sb.append(", size=").append(this.size);
        sb.append(", startRow=").append(this.startRow);
        sb.append(", endRow=").append(this.endRow);
        sb.append(", total=").append(this.total);
        sb.append(", pages=").append(this.pages);
        sb.append(", list=").append(this.list);
        sb.append(", prePage=").append(this.prePage);
        sb.append(", nextPage=").append(this.nextPage);
        sb.append(", isFirstPage=").append(this.isFirstPage);
        sb.append(", isLastPage=").append(this.isLastPage);
        sb.append(", hasPreviousPage=").append(this.hasPreviousPage);
        sb.append(", hasNextPage=").append(this.hasNextPage);
        sb.append(", navigatePages=").append(this.navigatePages);
        sb.append(", navigateFirstPage").append(this.navigateFirstPage);
        sb.append(", navigateLastPage").append(this.navigateLastPage);
        sb.append(", navigatepageNums=");
        if (this.navigatepageNums == null) {
            sb.append("null");
        } else {
            sb.append('[');

            for(int i = 0; i < this.navigatepageNums.length; ++i) {
                sb.append(i == 0 ? "" : ", ").append(this.navigatepageNums[i]);
            }

            sb.append(']');
        }

        sb.append('}');
        return sb.toString();
    }
}

参考博文：

（1） https://www.jianshu.com/p/50fcd7f127f0
（2）https://github.com/pagehelper/Mybatis-PageHelper/blob/master/wikis/en/HowToUse.md