MyBatis实现物理分页的实例

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

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";
  }

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