基于Mybatis Plugin机制实现简单的分页机制

分页其实很简单。
可以手写的。
直接贴源代码好了。
注:有仿照MybatisPlus的机制,仅供学些。

第一步:写一个拦截器

import com.baomidou.mybatisplus.annotation.DbType;
import com.baomidou.mybatisplus.extension.toolkit.JdbcUtils;
import org.apache.ibatis.binding.MapperMethod;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlCommandType;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.SystemMetaObject;
import plus.pagination.DialectBuilder;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.*;

/**
 * 参考了MP的分页机制，适合初学者学习入门使用
 * 适合初学者学习
 * Created by Administrator on 2019/8/10.
 */
@Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})})
public class CustPaginationInterceptor implements Interceptor {

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        MetaObject metaObject = SystemMetaObject.forObject(invocation.getTarget());
        MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("delegate.mappedStatement");
        if(mappedStatement.getSqlCommandType()== SqlCommandType.SELECT){
                BoundSql boundSql = (BoundSql) metaObject.getValue("delegate.boundSql");
               Object paramObj = boundSql.getParameterObject();
               Object castedObject= getPageParam(boundSql);
                if(null==castedObject){
                    return  invocation.proceed();
                }
                Page page= (Page)castedObject;
                String originalSql=boundSql.getSql();
                String pageSql= DialectBuilder.pageAbleForH2Select(originalSql, page);
                metaObject.setValue("delegate.boundSql.sql",pageSql);
              //查询总条数
                Connection connection= (Connection) invocation.getArgs()[0];
                DbType dbType = JdbcUtils.getDbType(connection.getMetaData().getURL());
                Statement statement= connection.createStatement();
                //java 8+ try Resource机制会自动关闭资源
                try (ResultSet resultSet = statement.executeQuery(String.format("select count(1) from  (%s)",originalSql))) {
                    if (resultSet.next()) {
                        Long total = resultSet.getLong(1);
                        page.setTotal(total);
                    }
                }

         }

        return invocation.proceed();
    }

        private Page getPageParam(BoundSql boundSql){
           Object paramObject= boundSql.getParameterObject();
           if(paramObject instanceof MapperMethod.ParamMap||paramObject instanceof MapperMethod.ParamMap){
               MapperMethod.ParamMap paramMap= (MapperMethod.ParamMap) paramObject;
            Set keySet=   paramMap.keySet();
            Iterator it=keySet.iterator();
            while (it.hasNext()){
                if(paramMap.get(it.next()) instanceof  Page){
                    return (Page)paramMap.get(it.next()) ;
        
                }
            }
           }
            if(boundSql.getParameterObject() instanceof  Page){
                return (Page) boundSql.getParameterObject();
            }
            return null;
        }

    @Override
    public Object plugin(Object target) {
        if (target instanceof StatementHandler) {
            return Plugin.wrap(target, this);
        }
        return target;
    }

    @Override
    public void setProperties(Properties properties) {

      for (Map.Entry<Object, Object> entry:  properties.entrySet()){
          System.out.println(entry.getKey()+"="+entry.getValue());
      }
    }
}

package plus.pagination;

import com.baomidou.mybatisplus.core.toolkit.Assert;
import plus.Page;
package plus.pagination;

import com.baomidou.mybatisplus.core.toolkit.Assert;
import plus.Page;

/**
 * Created by Administrator on 2019/8/11.
 */
public class DialectBuilder {

    public static String pageAbleForH2Select(String originalSql, Page page) {
        Assert.notNull(page, "the page Object must not be null");
        if (page.getCurrentPage() > 1) {
            return String.format("select * from ( %s) temp limit %d offset %d", originalSql, page.getPageSize() * page.getPageSize(), (page.getCurrentPage() - 1) * page.getPageSize());
        }
        return String.format("select * from %s limit %d", page.getPageSize());
    }
}

--Page对象
package plus;

import java.util.List;

/**
 * Created by Administrator on 2019/8/10.
 */

public class Page {

    private List records;

    private  Integer pageSize;

    private Long total;

    private String orderByColumn;

    private  Integer currentPage;

    public List getRecords() {
        return records;
    }

    public void setRecords(List records) {
        this.records = records;
    }

    public Integer getPageSize() {
        return pageSize;
    }

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

    public Long getTotal() {
        return total;
    }

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

    public String getOrderByColumn() {
        return orderByColumn;
    }

    public void setOrderByColumn(String orderByColumn) {
        this.orderByColumn = orderByColumn;
    }

    public Integer getCurrentPage() {
        return currentPage;
    }

    public void setCurrentPage(Integer currentPage) {
        this.currentPage = currentPage;
    }

}

第二步:将拦截器添加至Mybatis的 configuration.addInterceptor(plugin)中去，可以配置或者代码中去实现，太简单了，所以忽视。

到这里已经完成了分页了，下面给出在DAO层如何使用的示例。

@Mapper
public interface AccountMapper extends BaseMapper<Account>{

  @Select("select * from t_account")
  List<Account> selectAllAccountInfo(Page page,String param);
}

如何使用

public List<Account> getReadAccountInfo(Page page ){

    return  userMapper.selectAllUserInfo(page,"");
}

仓库地址：https://github.com/Leeyongke/read-write
谢谢观看。