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Mybatis分页插件PageHelper手写实现示例

2022-08-15 10:45:37 来源：易采站长站 作者：

目录
引言编写我们的插件类上面有二个核心方法获取记录总数分页查询记录数如何获取前端传递过来的参数？总结

引言

PageHelper是一个非常好用的插件，以至于很想知道它底层是怎么实现的。至于MyBatis插件概念原理网上有很多，我不太喜欢去写一些概念性的东西，我比较喜欢自己动手实现的那种，话不多说，我们开干

搭建一个SpringBoot+MyBatis+MySql项目

编写我们的插件类

package com.example.demo.plugin;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Properties;
@Intercepts(
        {
                @Signature(type = Executor.class, 
                           method = "query", 
                           args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
                @Signature(type = Executor.class, 
                           method = "query", 
                           args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class,
                                   CacheKey.class, BoundSql.class}),
        }
)
public class MyPagePlugin implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Object[] args = invocation.getArgs();
        MappedStatement ms = (MappedStatement) args[0];
        Object parameter = args[1];
        RowBounds rowBounds = (RowBounds) args[2];
        ResultHandler resultHandler = (ResultHandler) args[3];
        Executor executor = (Executor) invocation.getTarget();
        CacheKey cacheKey;
        BoundSql boundSql;
        //由于逻辑关系，只会进入一次
        if (args.length == 4) {
            //4 个参数时
            boundSql = ms.getBoundSql(parameter);
            cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
        } else {
            //6 个参数时
            cacheKey = (CacheKey) args[4];
            boundSql = (BoundSql) args[5];
        }
        == 其实在这之上的代码都是拷贝PageHelper源码来的，下面才是重头戏，上面都是获取一些必要的参数==
        /**
        * 下面4行代码暂时只需要知道是用来传参数的就行，分页不是需要
        * 二个参数嘛 一个是当前页，一个是数量
        */
        Page page = ThreadLocalUtil.getPage();
        Map<String,Object> params = new HashMap<>();
        params.put("first_key",page.getPageNum());
        params.put("second_key",page.getPageSize());
        /**
        * 重点：获取数据库记录总数
        */
        //统计总数
        Long count = MyExecutorUtil.executeAutoCount(executor,ms,parameter,boundSql,rowBounds,resultHandler);
        /**
        * 重点：分页查询
        */
        List<Object> objects = MyExecutorUtil.pageQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey, params);
        return objects;
    }
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }
    @Override
    public void setProperties(Properties properties) {
    }
}

上面有二个核心方法

1：获取记录总数：

MyExecutorUtil.executeAutoCount(executor,ms,parameter,boundSql,rowBounds,resultHandler);

2：分页查询：

MyExecutorUtil.pageQuery(executor,>

获取记录总数

1：MyBatis源码底层会封装好我们在配置文件中写的每一条SQL语句，封装到一个SqlSource对象中去，在我们执行SQL的时候，会获取到这条SQL，然后封装到BoundSql这个对象中，所以在这里，我们既然能拿到BoundSql,那么也就意味着我们能拿到我们需要执行的那条Sql了

2：获取到了我们的sql之后，怎么查询总记录数呢？？

其实很简单，改Sql语句不就好了，Pagehelper底层也是这么做的，但是PageHelper底层比我这个版本的复杂太多了，但是我们无非就是将原先的SQL转换成 -> SELECT COUNT(0) FROM TABLE,就这样，但是我这个人比较懒，而且昨天看这个源码实现头疼，所以在这里直接写死了，但是问题不大哈

3：改好了Sql之后是不是就完成了呢？？

当然不是，改好了SQL当然是要去执行它了，如果就这样执行，还是会执行原先的SQL，但是你要知道这个有个很关键的东西，就是MappedStatement的id，如果不改这个id的话，即使你的Sql能够执行成功，那么返回的记录总数是个NULL，这里我想可能是因为ResultSetHandle的关系，因为这个id对应的还是我们之前的sql，也就是select * from student,那么必然有一个resultType的属性，也就是实体类映射，但是我们现在的sql是select count(0) from student,那么就对应不上了，也就是数据库查询出来的列与实体类对应不上，所以我们需要改变这个id。

//改变MapperStatement id的方法

      public static MappedStatement newCountMappedStatement(MappedStatement ms, String newMsId) {
              MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), newMsId, ms.getSqlSource(), ms.getSqlCommandType());
              builder.resource(ms.getResource());
              builder.fetchSize(ms.getFetchSize());
              builder.statementType(ms.getStatementType());
              builder.keyGenerator(ms.getKeyGenerator());
              if (ms.getKeyProperties() != null && ms.getKeyProperties().length != 0) {
                  StringBuilder keyProperties = new StringBuilder();
                  for (String keyProperty : ms.getKeyProperties()) {
                      keyProperties.append(keyProperty).append(",");
                  }
                  keyProperties.delete(keyProperties.length() - 1, keyProperties.length());
                  builder.keyProperty(keyProperties.toString());
              }
              builder.timeout(ms.getTimeout());
              builder.parameterMap(ms.getParameterMap());
              //count查询返回值int
              List<ResultMap> resultMaps = new ArrayList<ResultMap>();
              ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId(), Long.class, EMPTY_RESULTMAPPING).build();
              resultMaps.add(resultMap);
              builder.resultMaps(resultMaps);
              builder.resultSetType(ms.getResultSetType());
              builder.cache(ms.getCache());
              builder.flushCacheRequired(ms.isFlushCacheRequired());
              builder.useCache(ms.isUseCache());
              return builder.build();
          }

4：MyBatis的Sql是封装到BoundSql中去的，而原先的BoundSql中的Sql是我们配置文件中的，所以我们需要将select count(0) from student这条Sql语句封装到一个新的BoundSql中去

public static Long executeAutoCount(Executor executor, MappedStatement countMs,
                                       Object parameter, BoundSql boundSql,
                                       RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
       //改变MapperStatement中的ID属性
       String countMsId = countMs.getId() + "_COUNT";
       countMs = newCountMappedStatement(countMs,countMsId);
       //创建 count 查询的缓存 key
       CacheKey countKey = executor.createCacheKey(countMs, parameter, RowBounds.DEFAULT, boundSql);
       //调用获取count的sql
       String countSql = "select count(0) from student";
       //重新封装BoundSql对象
       BoundSql countBoundSql = new BoundSql(countMs.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
       //执行 count 查询
       List&lt;Long&gt; countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql);
       Long count = countResultList.get(0);
       return count;
   }

5：最后调用executor.query方法就可以得到我们的记录总数了

分页查询记录数

    1：其实分页查询与查询记录总数的原理是一样的2：首先获取原先的BoundSql中的Sql，也就是我们在配置文件中的Sql，比如>3：然后获取我们的分页参数，也就是前端传递过来的pageNum和pageSize二个参数4：修改Sql，select * from student limit ?,?5：这次我们就不用修改MapperStatement中的ID了，因为查询出来的都是studnet，所以映射不用修改6：重新实例化一个BoundSql对象，将新的Sql传递给它7：最后执行executor.query方法查询，得到结果

    如何获取前端传递过来的参数？

    1：我这里使用的是ThreadLocal，

    总结

      1：首选我们需要获取到原先需要执行的sql2：然后修改这条修改，分页查询无非就是总记录数，分页查询的记录数3：分别执行这二条Sql，得到我们想要的结果

      源码：

      就下面这5个类,其它的就是将SpringBoot+MyBatis+MySql日常配置就行了，然后再Controller记得传递一下参数

      package com.example.demo.plugin;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
@MapperScan({"com.example.demo.mapper"})
public class MapperConfig {
    @Bean
    public MyPagePlugin myPagePlugin() {
        return new MyPagePlugin();
    }
}

      package com.example.demo.plugin;
import com.example.demo.entity.Student;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.*;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
public class MyExecutorUtil {
    private static final List&lt;ResultMapping&gt; EMPTY_RESULTMAPPING = new ArrayList&lt;ResultMapping&gt;(0);
    public static  &lt;E&gt; List&lt;E&gt; pageQuery(Executor executor, MappedStatement ms, Object parameter,
                                         RowBounds rowBounds, ResultHandler resultHandler,
                                         BoundSql boundSql, CacheKey cacheKey,Map&lt;String,Object&gt; params) throws SQLException {
        executor.clearLocalCache();
        //生成分页的缓存 key
        CacheKey pageKey = cacheKey;
        //处理参数对象
        if(params.size() &lt; 0 || params.size() &gt; 2) {
            System.out.println("参数错误");
        }
        //获取sql
        String pageSql = getPageSql(params.size(),boundSql);
        List&lt;ParameterMapping&gt; mappingList = new ArrayList&lt;&gt;();
        mappingList.add(new ParameterMapping.Builder(ms.getConfiguration(), "first_key", Integer.class).build());
        mappingList.add(new ParameterMapping.Builder(ms.getConfiguration(), "second_key", Integer.class).build());
        //实例化新的BoundSql对象
        BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, mappingList, params);
        //执行分页查询
        return executor.query(ms, params, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
    }
    public static Long executeAutoCount(Executor executor, MappedStatement countMs,
                                        Object parameter, BoundSql boundSql,
                                        RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
        //改变MapperStatement中的ID属性
        String countMsId = countMs.getId() + "_COUNT";
        countMs = newCountMappedStatement(countMs,countMsId);
        //创建 count 查询的缓存 key
        CacheKey countKey = executor.createCacheKey(countMs, parameter, RowBounds.DEFAULT, boundSql);
        //调用获取count的sql
        String countSql = "select count(0) from student";
        //重新封装BoundSql对象
        BoundSql countBoundSql = new BoundSql(countMs.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
        //执行 count 查询
        List&lt;Long&gt; countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql);
        Long count = countResultList.get(0);
        return count;
    }
    public static MappedStatement newCountMappedStatement(MappedStatement ms, String newMsId) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), newMsId, ms.getSqlSource(), ms.getSqlCommandType());
        builder.resource(ms.getResource());
        builder.fetchSize(ms.getFetchSize());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        if (ms.getKeyProperties() != null &amp;&amp; ms.getKeyProperties().length != 0) {
            StringBuilder keyProperties = new StringBuilder();
            for (String keyProperty : ms.getKeyProperties()) {
                keyProperties.append(keyProperty).append(",");
            }
            keyProperties.delete(keyProperties.length() - 1, keyProperties.length());
            builder.keyProperty(keyProperties.toString());
        }
        builder.timeout(ms.getTimeout());
        builder.parameterMap(ms.getParameterMap());
        //count查询返回值int
        List&lt;ResultMap&gt; resultMaps = new ArrayList&lt;ResultMap&gt;();
        ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId(), Long.class, EMPTY_RESULTMAPPING).build();
        resultMaps.add(resultMap);
        builder.resultMaps(resultMaps);
        builder.resultSetType(ms.getResultSetType());
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());
        return builder.build();
    }
    private static String getPageSql(int paramsLength,BoundSql boundSql){
        StringBuilder str = new StringBuilder();
        str.append(boundSql.getSql());
        if(paramsLength == 1) {
            str.append(" LIMIT ?");
        }else {
            str.append(" LIMIT ?,? ");
        }
        return str.toString();
    }
}

          package com.example.demo.plugin;
  import org.apache.ibatis.cache.CacheKey;
  import org.apache.ibatis.executor.Executor;
  import org.apache.ibatis.mapping.BoundSql;
  import org.apache.ibatis.mapping.MappedStatement;
  import org.apache.ibatis.plugin.*;
  import org.apache.ibatis.session.ResultHandler;
  import org.apache.ibatis.session.RowBounds;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  import java.util.Properties;
  @Intercepts(
          {
                  @Signature(type = Executor.class,
                             method = "query",
                             args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
                  @Signature(type = Executor.class,
                             method = "query",
                             args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class,
                                     CacheKey.class, BoundSql.class}),
          }
  )
  public class MyPagePlugin implements Interceptor {
      @Override
      public Object intercept(Invocation invocation) throws Throwable {
          Object[] args = invocation.getArgs();
          MappedStatement ms = (MappedStatement) args[0];
          Object parameter = args[1];
          RowBounds rowBounds = (RowBounds) args[2];
          ResultHandler resultHandler = (ResultHandler) args[3];
          Executor executor = (Executor) invocation.getTarget();
          CacheKey cacheKey;
          BoundSql boundSql;
          //由于逻辑关系，只会进入一次
          if (args.length == 4) {
              //4 个参数时
              boundSql = ms.getBoundSql(parameter);
              cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
          } else {
              //6 个参数时
              cacheKey = (CacheKey) args[4];
              boundSql = (BoundSql) args[5];
          }
          Page page = ThreadLocalUtil.getPage();
          Map&lt;String,Object&gt; params = new HashMap&lt;&gt;();
          params.put("first_key",page.getPageNum());
          params.put("second_key",page.getPageSize());
          //统计总数
          Long count = MyExecutorUtil.executeAutoCount(executor,ms,parameter,boundSql,rowBounds,resultHandler);
          System.out.println("count = " + count);
          List&lt;Object&gt; objects = MyExecutorUtil.pageQuery(executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey, params);
          return objects;
      }
      @Override
      public Object plugin(Object target) {
          return Plugin.wrap(target, this);
      }
      @Override
      public void setProperties(Properties properties) {
      }
  }

        package com.example.demo.plugin;
public class Page {
    private Integer pageNum;
    private Integer pageSize;
    public Integer getPageNum() {
        return pageNum;
    }
    public void setPageNum(Integer pageNum) {
        this.pageNum = pageNum;
    }
    public Integer getPageSize() {
        return pageSize;
    }
    public void setPageSize(Integer pageSize) {
        this.pageSize = pageSize;
    }
}

         package com.example.demo.plugin;
  public class ThreadLocalUtil {
      public static ThreadLocal&lt;Page&gt; PAGE_INFO = new ThreadLocal&lt;&gt;();
      public static void startPage(int pageNum,int pageSize) {
          Page page = new Page();
          page.setPageNum(pageNum);
          page.setPageSize(pageSize);
          PAGE_INFO.set(page);
      }
      public static Page getPage() {
          return PAGE_INFO.get();
      }
  }

        @RestController
  public class StudnetController {
      @Autowired
      private StudentMapper studentMapper;
      @GetMapping("/getData")
      public Object getData() {
          ThreadLocalUtil.startPage(0,2);
          return studentMapper.getData();
      }
  }

      以上就是Mybatis分页插件PageHelper手写实现示例的详细内容，更多关于Mybatis分页插件PageHelper的资料请关注易采站长站其它相关文章！

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