MyBatis 物理分页

MyBatis使用RowBounds实现的分页是逻辑分页,也就是先把数据记录全部查询出来,然在再根据offset和limit截断记录返回

为了在数据库层面上实现物理分页,又不改变原来MyBatis的函数逻辑,可以编写plugin截获MyBatis Executor的statementhandler,重写SQL来执行查询

参考资料: http://blog.csdn.net/hupanfeng/article/details/9265341

下面的插件代码只针对MySQL

plugin代码

package plugin;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Properties;

import org.apache.ibatis.executor.parameter.ParameterHandler;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.Invocation;
import org.apache.ibatis.plugin.Plugin;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.factory.DefaultObjectFactory;
import org.apache.ibatis.reflection.factory.ObjectFactory;
import org.apache.ibatis.reflection.wrapper.DefaultObjectWrapperFactory;
import org.apache.ibatis.reflection.wrapper.ObjectWrapperFactory;
import org.apache.ibatis.scripting.defaults.DefaultParameterHandler;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.RowBounds;

/**
 * 通过拦截<code>StatementHandler</code>的<code>prepare</code>方法，重写sql语句实现物理分页。
 * 老规矩，签名里要拦截的类型只能是接口。
 *
 */
@Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class})})
public class PaginationInterceptor implements Interceptor {
    private static final Log logger = LogFactory.getLog(PaginationInterceptor.class);
    private static final ObjectFactory DEFAULT_OBJECT_FACTORY = new DefaultObjectFactory();
    private static final ObjectWrapperFactory DEFAULT_OBJECT_WRAPPER_FACTORY = new DefaultObjectWrapperFactory();
    private static String DEFAULT_PAGE_SQL_ID = ".*Page$"; // 需要拦截的ID(正则匹配)

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        MetaObject metaStatementHandler = MetaObject.forObject(statementHandler, DEFAULT_OBJECT_FACTORY,
                DEFAULT_OBJECT_WRAPPER_FACTORY);
        RowBounds rowBounds = (RowBounds) metaStatementHandler.getValue("delegate.rowBounds");
        // 分离代理对象链(由于目标类可能被多个拦截器拦截，从而形成多次代理，通过下面的两次循环可以分离出最原始的的目标类)
        while (metaStatementHandler.hasGetter("h")) {
            Object object = metaStatementHandler.getValue("h");
            metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }
        // 分离最后一个代理对象的目标类
        while (metaStatementHandler.hasGetter("target")) {
            Object object = metaStatementHandler.getValue("target");
            metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
        }

        // property在mybatis settings文件内配置
        Configuration configuration = (Configuration) metaStatementHandler.getValue("delegate.configuration");

        // 设置pageSqlId
        String pageSqlId = configuration.getVariables().getProperty("pageSqlId");
        if (null == pageSqlId || "".equals(pageSqlId)) {
            logger.warn("Property pageSqlId is not setted,use default '.*Page$' ");
            pageSqlId = DEFAULT_PAGE_SQL_ID;
        }

        MappedStatement mappedStatement = (MappedStatement)
                metaStatementHandler.getValue("delegate.mappedStatement");
        // 只重写需要分页的sql语句。通过MappedStatement的ID匹配，默认重写以Page结尾的MappedStatement的sql
        if (mappedStatement.getId().matches(pageSqlId)) {
            BoundSql boundSql = (BoundSql) metaStatementHandler.getValue("delegate.boundSql");
            Object parameterObject = boundSql.getParameterObject();
            if (parameterObject == null) {
                throw new NullPointerException("parameterObject is null!");
            } else {
                String sql = boundSql.getSql();
                // 重写sql
                String pageSql = sql + " LIMIT " + rowBounds.getOffset() + "," + rowBounds.getLimit();
                metaStatementHandler.setValue("delegate.boundSql.sql", pageSql);
                // 采用物理分页后，就不需要mybatis的内存分页了，所以重置下面的两个参数
                metaStatementHandler.setValue("delegate.rowBounds.offset", RowBounds.NO_ROW_OFFSET);
                metaStatementHandler.setValue("delegate.rowBounds.limit", RowBounds.NO_ROW_LIMIT);
            }
        }
        // 将执行权交给下一个拦截器
        return invocation.proceed();
    }

    @Override
    public Object plugin(Object target) {
        // 当目标类是StatementHandler类型时，才包装目标类，否者直接返回目标本身,减少目标被代理的次数
        if (target instanceof StatementHandler) {
            return Plugin.wrap(target, this);
        } else {
            return target;
        }
    }

    @Override
    public void setProperties(Properties properties) {
        //To change body of implemented methods use File | Settings | File Templates.
    }

}

配置plugin

    <plugins>
        <plugin interceptor="plugin.PaginationInterceptor" />
    </plugins>

查询SQL

<!-- 测试分页查询 -->
<select id="selectUserByPage" resultMap="dao.base.userResultMap">
        <![CDATA[
        SELECT * FROM user
        ]]>
</select>

调用示例

@Override
    public List<User> selectUserByPage(int offset, int limit) {
        RowBounds rowBounds = new RowBounds(offset, limit);
        return getSqlSession().selectList("dao.userdao.selectUserByPage", new Object(), rowBounds);
    }

另外,结合Spring MVC,编写翻页和生成页码代码

页码类

package util;

/**
 * Created with IntelliJ IDEA.
 * User: zhenwei.liu
 * Date: 13-8-7
 * Time: 上午10:29
 * To change this template use File | Settings | File Templates.
 */
public class Pagination {
    private String url; // 页码url
    private int pageSize = 10;  // 每页显示记录数
    private int currentPage = 1;    // 当前页码
    private int maxPage = Integer.MAX_VALUE;    // 最大页数

    // 获取offset
    public int getOffset() {
        return (currentPage - 1) * pageSize;
    }

    // 获取limit
    public int getLimit() {
        return getPageSize();
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public int getPageSize() {
        return pageSize;
    }

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

    public int getCurrentPage() {
        return currentPage;
    }

    public void setCurrentPage(int currentPage) {
        if (currentPage < 1)
            currentPage = 1;
        if (currentPage > maxPage)
            currentPage = maxPage;
        this.currentPage = currentPage;
    }

    public int getMaxPage() {
        return maxPage;
    }

    public void setMaxPage(int maxPage) {
        this.maxPage = maxPage;
    }
}

为了计算最大页码,需要知道数据表的总记录数,查询SQL如下

<!-- 记录总数 -->
    <select id="countUser" resultType="Integer">
        <![CDATA[
        SELECT COUNT(*) FROM user
        ]]>
    </select>

@Override
    public Integer countTable() {
        return getSqlSession().selectOne("dao.userdao.countUser");
    }

Controller中的使用

@RequestMapping("/getUserByPage")
    public String getUserByPage(@RequestParam
                                    int page, Model model) {
        pagination.setCurrentPage(page);
        pagination.setUrl(getCurrentUrl());
        pagination.setMaxPage(userDao.countTable() / pagination.getPageSize() + 1);
        List<User> userList = userDao.selectUserByPage(
                pagination.getOffset(), pagination.getLimit());
        model.addAttribute(pagination);
        model.addAttribute(userList);
        return "index";
    }