Mybatis分页插件: pageHelper的使用及其原理解析

    <!-- https://mvnrepository.com/artifact/com.github.pagehelper/pagehelper-spring-boot-starter -->
        <dependency>
            <groupId>com.github.pagehelper</groupId>
            <artifactId>pagehelper-spring-boot-starter</artifactId>
            <version>1.2.12</version>
        </dependency>

       <!-- pageHelper -->
        <dependency>
          <groupId>com.github.pagehelper</groupId>
          <artifactId>pagehelper</artifactId>
          <version>5.1.10</version>
        </dependency>

    # mybatis 相关配置
    mybatis:
        configuration-properties:
        offsetAsPageNum: true
        rowBoundsWithCount: true
        reasonable: true
        mapper-locations: mybatis/mapper/*.xml

    # db 配置
    spring:
        datasource:
        driver-class-name: com.mysql.cj.jdbc.Driver
        username: root
        password: 123
        url: jdbc:mysql://localhost:3306/testdb?useUnicode=true&charactorEncoding=utf8&&serverTimezone=Asia/Shanghai

    //分页查询
    public PageResult pageQuery(CategoryPageQueryDTO categoryPageQueryDTO) {
        // 1、开启分页（Mybatis自动拼接sql）
        PageHelper.startPage(categoryPageQueryDTO.getPage(), categoryPageQueryDTO.getPageSize());
        //下一条sql进行分页，自动加入limit关键字分页（接受数据）
        Page page = categoryMapper.pageQuery(categoryPageQueryDTO);
        return new PageResult(page.getTotal(), page.getResult());
    }

  // com.github.pagehelper.autoconfigure.PageHelperAutoConfiguration#addPageInterceptor
    @PostConstruct
    public void addPageInterceptor() {
        // 初始化 com.github.pagehelper.PageInterceptor
        PageInterceptor interceptor = new PageInterceptor();
        Properties properties = new Properties();
        //先把一般方式配置的属性放进去
        properties.putAll(pageHelperProperties());
        //在把特殊配置放进去，由于close-conn 利用上面方式时，属性名就是 close-conn 而不是 closeConn，所以需要额外的一步
        properties.putAll(this.properties.getProperties());
        interceptor.setProperties(properties);
        for (SqlSessionFactory sqlSessionFactory : sqlSessionFactoryList) {
            // 添加inteceptor到 mybatis 中
            sqlSessionFactory.getConfiguration().addInterceptor(interceptor);
        }
    }
  // org.apache.ibatis.session.Configuration#addInterceptor
  public void addInterceptor(Interceptor interceptor) {
    interceptorChain.addInterceptor(interceptor);
  }

  // org.apache.ibatis.plugin.InterceptorChain#addInterceptor
  public void addInterceptor(Interceptor interceptor) {
    // 使用 ArrayList 保存intceptor
    interceptors.add(interceptor);
  }

// org.apache.ibatis.session.Configuration#newExecutor
  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    // 以interceptorChain包装 executor, 以便inteceptor发挥作用
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

  // org.apache.ibatis.plugin.InterceptorChain#pluginAll
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      // 使用plugin一层层包装 target, 具体实现为使用代理包装 target
      // 所以, interceptor 的使用顺序是按照添加的顺序来的, 并不能自行设置
      target = interceptor.plugin(target);
    }
    return target;
  }

    // com.github.pagehelper.PageInterceptor#plugin
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }
  // org.apache.ibatis.plugin.Plugin#wrap
  public static Object wrap(Object target, Interceptor interceptor) {
    // 获取注解中说明的方式列表 @Intercepts -> @Signature, 下面我们看 pageInterceptor的注解
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    // 过滤需要进行代理的接口, 而非全部代理
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      // 使用jdk方式生成动态代理
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          // 使用 Plugin 包装代理实现
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }
  // pageInterceptor的注解, 即定义要拦截的方法列表
@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}),
        }
)
  // 过滤代理的接口
  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<>();
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        // 只有设置了的接口才会被添加
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }

// org.apache.ibatis.plugin.Plugin#invoke
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        // 匹配的方法会被拦截, 即 query 方法
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
    // pageHelper 正式起作用的入口
    // com.github.pagehelper.PageInterceptor#intercept
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        try {
            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];
            }
            checkDialectExists();

            List resultList;
            //调用方法判断是否需要进行分页，如果不需要，直接返回结果
            if (!dialect.skip(ms, parameter, rowBounds)) {
                //判断是否需要进行 count 查询
                if (dialect.beforeCount(ms, parameter, rowBounds)) {
                    //查询总数
                    Long count = count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
                    //处理查询总数，返回 true 时继续分页查询，false 时直接返回
                    if (!dialect.afterCount(count, parameter, rowBounds)) {
                        //当查询总数为 0 时，直接返回空的结果
                        return dialect.afterPage(new ArrayList(), parameter, rowBounds);
                    }
                }
                resultList = ExecutorUtil.pageQuery(dialect, executor,
                        ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
            } else {
                //rowBounds用参数值，不使用分页插件处理时，仍然支持默认的内存分页
                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
            }
            return dialect.afterPage(resultList, parameter, rowBounds);
        } finally {
            if(dialect != null){
                dialect.afterAll();
            }
        }
    }

 /**
     * 跳过 count 和 分页查询
     *
     * @param ms              MappedStatement
     * @param parameterObject 方法参数
     * @param rowBounds       分页参数
     * @return true 跳过，返回默认查询结果，false 执行分页查询
     */
     // com.github.pagehelper.PageHelper#skip
    @Override
    public boolean skip(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
        if (ms.getId().endsWith(MSUtils.COUNT)) {
            throw new RuntimeException("在系统中发现了多个分页插件，请检查系统配置!");
        }
        // 如果 page 返回null, 则不需要进行分页, 即是否调用  PageHelper.start(pageNo, pageSize) 方法
        Page page = pageParams.getPage(parameterObject, rowBounds);
        if (page == null) {
            return true;
        } else {
            //设置默认的 count 列
            if (StringUtil.isEmpty(page.getCountColumn())) {
                page.setCountColumn(pageParams.getCountColumn());
            }
            autoDialect.initDelegateDialect(ms);
            return false;
        }
    }
    // com.github.pagehelper.page.PageAutoDialect#initDelegateDialect
    //多数据动态获取时，每次需要初始化
    public void initDelegateDialect(MappedStatement ms) {
        if (delegate == null) {
            if (autoDialect) {
                // 比如 MySqlDialect
                this.delegate = getDialect(ms);
            } else {
                dialectThreadLocal.set(getDialect(ms));
            }
        }
    }

    /**
     * 获取分页参数
     */
    // com.github.pagehelper.page.PageParams#getPage
    public Page getPage(Object parameterObject, RowBounds rowBounds) {
        Page page = PageHelper.getLocalPage();
        if (page == null) {
            if (rowBounds != RowBounds.DEFAULT) {
                if (offsetAsPageNum) {
                    page = new Page(rowBounds.getOffset(), rowBounds.getLimit(), rowBoundsWithCount);
                } else {
                    page = new Page(new int[]{rowBounds.getOffset(), rowBounds.getLimit()}, rowBoundsWithCount);
                    //offsetAsPageNum=false的时候，由于PageNum问题，不能使用reasonable，这里会强制为false
                    page.setReasonable(false);
                }
                if(rowBounds instanceof PageRowBounds){
                    PageRowBounds pageRowBounds = (PageRowBounds)rowBounds;
                    page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount());
                }
            } else if(parameterObject instanceof IPage || supportMethodsArguments){
                try {
                    page = PageObjectUtil.getPageFromObject(parameterObject, false);
                } catch (Exception e) {
                    return null;
                }
            }
            if(page == null){
                return null;
            }
            PageHelper.setLocalPage(page);
        }
        //分页合理化
        if (page.getReasonable() == null) {
            page.setReasonable(reasonable);
        }
        //当设置为true的时候，如果pagesize设置为0（或RowBounds的limit=0），就不执行分页，返回全部结果
        if (page.getPageSizeZero() == null) {
            page.setPageSizeZero(pageSizeZero);
        }
        return page;
    }

// com.github.pagehelper.PageHelper#beforeCount
    @Override
    public boolean beforeCount(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
        return autoDialect.getDelegate().beforeCount(ms, parameterObject, rowBounds);
    }

    // com.github.pagehelper.dialect.AbstractHelperDialect#beforeCount
    @Override
    public boolean beforeCount(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
        // 获取page参数信息, 该参数设置在 ThreadLocal 中
        Page page = getLocalPage();
        return !page.isOrderByOnly() && page.isCount();
    }
    // 如果需要进行count, 则需要自行组装count逻辑进行查询.
    // com.github.pagehelper.PageInterceptor#count
    private Long count(Executor executor, MappedStatement ms, Object parameter,
                       RowBounds rowBounds, ResultHandler resultHandler,
                       BoundSql boundSql) throws SQLException {
        // 在原有list 查询后添加  _COUNT 代表count查询id
        String countMsId = ms.getId() + countSuffix;
        Long count;
        //先判断是否存在手写的 count 查询
        MappedStatement countMs = ExecutorUtil.getExistedMappedStatement(ms.getConfiguration(), countMsId);
        if (countMs != null) {
            count = ExecutorUtil.executeManualCount(executor, countMs, parameter, boundSql, resultHandler);
        } else {
            countMs = msCountMap.get(countMsId);
            //自动创建
            if (countMs == null) {
                //根据当前的 ms 创建一个返回值为 Long 类型的 ms
                countMs = MSUtils.newCountMappedStatement(ms, countMsId);
                msCountMap.put(countMsId, countMs);
            }
            count = ExecutorUtil.executeAutoCount(dialect, executor, countMs, parameter, boundSql, rowBounds, resultHandler);
        }
        return count;
    }
    // 创建count ms
    // com.github.pagehelper.util.MSUtils#newCountMappedStatement(org.apache.ibatis.mapping.MappedStatement, java.lang.String)
    public static MappedStatement newCountMappedStatement(MappedStatement ms, String newMsId) {
        // 直接基于原有 sql 构建新的 MappedStatement
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), newMsId, ms.getSqlSource(), ms.getSqlCommandType());
        builder.resource(ms.getResource());
        // 注意此处并未使用到用户设置的分页参数 
        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();
    }

    /**
     * 执行自动生成的 count 查询
     */
     // com.github.pagehelper.util.ExecutorUtil#executeAutoCount
    public static Long executeAutoCount(Dialect dialect, Executor executor, MappedStatement countMs,
                                        Object parameter, BoundSql boundSql,
                                        RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
        Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
        //创建 count 查询的缓存 key
        CacheKey countKey = executor.createCacheKey(countMs, parameter, RowBounds.DEFAULT, boundSql);
        //调用方言获取 count sql
        String countSql = dialect.getCountSql(countMs, boundSql, parameter, rowBounds, countKey);
        //countKey.update(countSql);
        BoundSql countBoundSql = new BoundSql(countMs.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
        //当使用动态 SQL 时，可能会产生临时的参数，这些参数需要手动设置到新的 BoundSql 中
        for (String key : additionalParameters.keySet()) {
            countBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
        }
        //执行 count 查询
        Object countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql);
        Long count = (Long) ((List) countResultList).get(0);
        return count;
    }
    // com.github.pagehelper.PageHelper#getCountSql
    @Override
    public String getCountSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey countKey) {
        // 委托给各方言实现 sql 组装
        return autoDialect.getDelegate().getCountSql(ms, boundSql, parameterObject, rowBounds, countKey);
    }

    // com.github.pagehelper.dialect.AbstractHelperDialect#getCountSql
    @Override
    public String getCountSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey countKey) {
        Page<Object> page = getLocalPage();
        String countColumn = page.getCountColumn();
        if (StringUtil.isNotEmpty(countColumn)) {
            return countSqlParser.getSmartCountSql(boundSql.getSql(), countColumn);
        }
        return countSqlParser.getSmartCountSql(boundSql.getSql());
    }

    /**
     * 获取智能的countSql
     *
     * @param sql
     * @param name 列名，默认 0
     * @return
     */
     // com.github.pagehelper.parser.CountSqlParser#getSmartCountSql(java.lang.String, java.lang.String)
    public String getSmartCountSql(String sql, String name) {
        //解析SQL
        Statement stmt = null;
        //特殊sql不需要去掉order by时，使用注释前缀
        if(sql.indexOf(KEEP_ORDERBY) >= 0){
            return getSimpleCountSql(sql, name);
        }
        try {
            stmt = CCJSqlParserUtil.parse(sql);
        } catch (Throwable e) {
            //无法解析的用一般方法返回count语句
            return getSimpleCountSql(sql, name);
        }
        Select select = (Select) stmt;
        SelectBody selectBody = select.getSelectBody();
        try {
            //处理body-去order by
            processSelectBody(selectBody);
        } catch (Exception e) {
            //当 sql 包含 group by 时，不去除 order by
            return getSimpleCountSql(sql, name);
        }
        //处理with-去order by
        processWithItemsList(select.getWithItemsList());
        //处理为count查询
        sqlToCount(select, name);
        String result = select.toString();
        return result;
    }
    /**
     * 将sql转换为count查询
     *
     * @param select
     */
     // com.github.pagehelper.parser.CountSqlParser#sqlToCount
    public void sqlToCount(Select select, String name) {
        SelectBody selectBody = select.getSelectBody();
        // 是否能简化count查询
        List<SelectItem> COUNT_ITEM = new ArrayList<SelectItem>();
        // 如 select * from user 将会被转化为 select count(0) from user
        COUNT_ITEM.add(new SelectExpressionItem(new Column("count(" + name +")")));
        if (selectBody instanceof PlainSelect && isSimpleCount((PlainSelect) selectBody)) {
            // 简单sql直接转换select字段为 count(0) 即可, 而这个sql是否支持这种方式则得仔细验证
            ((PlainSelect) selectBody).setSelectItems(COUNT_ITEM);
        } else {
            // 如果对于复杂的sql查询, 则只能在现有sql外围加一个 select count(0) from (xxxxx) as table_count
            PlainSelect plainSelect = new PlainSelect();
            SubSelect subSelect = new SubSelect();
            subSelect.setSelectBody(selectBody);
            subSelect.setAlias(TABLE_ALIAS);
            // 将原sql作为临时表放入 plainSelect 中
            plainSelect.setFromItem(subSelect);
            plainSelect.setSelectItems(COUNT_ITEM);
            // 替换原有 select
            select.setSelectBody(plainSelect);
        }
    }
    /**
     * 是否可以用简单的count查询方式
     */
     // net.sf.jsqlparser.statement.select.PlainSelect
    public boolean isSimpleCount(PlainSelect select) {
        //包含group by的时候不可以
        if (select.getGroupBy() != null) {
            return false;
        }
        //包含distinct的时候不可以
        if (select.getDistinct() != null) {
            return false;
        }
        for (SelectItem item : select.getSelectItems()) {
            //select列中包含参数的时候不可以，否则会引起参数个数错误
            if (item.toString().contains("?")) {
                return false;
            }
            //如果查询列中包含函数，也不可以，函数可能会聚合列
            if (item instanceof SelectExpressionItem) {
                Expression expression = ((SelectExpressionItem) item).getExpression();
                if (expression instanceof Function) {
                    String name = ((Function) expression).getName();
                    if (name != null) {
                        String NAME = name.toUpperCase();
                        if(skipFunctions.contains(NAME)){
                            //go on
                        } else if(falseFunctions.contains(NAME)){
                            return false;
                        } else {
                            for (String aggregateFunction : AGGREGATE_FUNCTIONS) {
                                if(NAME.startsWith(aggregateFunction)){
                                    falseFunctions.add(NAME);
                                    return false;
                                }
                            }
                            skipFunctions.add(NAME);
                        }
                    }
                }
            }
        }
        return true;
    }

// com.github.pagehelper.dialect.AbstractHelperDialect#afterCount
    @Override
    public boolean afterCount(long count, Object parameterObject, RowBounds rowBounds) {
        Page page = getLocalPage();
        page.setTotal(count);
        if (rowBounds instanceof PageRowBounds) {
            ((PageRowBounds) rowBounds).setTotal(count);
        }
        //pageSize < 0 的时候，不执行分页查询
        //pageSize = 0 的时候，还需要执行后续查询，但是不会分页
        if (page.getPageSize() < 0) {
            return false;
        }
        // 还没到最后一页, 则需要进行分页查询
        return count > ((page.getPageNum() - 1) * page.getPageSize());
    }

    /**
     * 分页查询
     */
    public static  <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter,
                                 RowBounds rowBounds, ResultHandler resultHandler,
                                 BoundSql boundSql, CacheKey cacheKey) throws SQLException {
        //判断是否需要进行分页查询
        if (dialect.beforePage(ms, parameter, rowBounds)) {
            //生成分页的缓存 key
            CacheKey pageKey = cacheKey;
            //处理参数对象, 将会加入 pageStart, pageSize 等参数
            parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
            //调用方言获取分页 sql
            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);

            Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
            //设置动态参数
            for (String key : additionalParameters.keySet()) {
                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
            }
            //执行分页查询
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
        } else {
            //不执行分页的情况下，也不执行内存分页
            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
        }
    }
    // com.github.pagehelper.dialect.AbstractHelperDialect#processParameterObject
    @Override
    public Object processParameterObject(MappedStatement ms, Object parameterObject, BoundSql boundSql, CacheKey pageKey) {
        //处理参数
        Page page = getLocalPage();
        //如果只是 order by 就不必处理参数
        if (page.isOrderByOnly()) {
            return parameterObject;
        }
        Map<String, Object> paramMap = null;
        if (parameterObject == null) {
            paramMap = new HashMap<String, Object>();
        } else if (parameterObject instanceof Map) {
            //解决不可变Map的情况
            paramMap = new HashMap<String, Object>();
            paramMap.putAll((Map) parameterObject);
        } else {
            paramMap = new HashMap<String, Object>();
            //动态sql时的判断条件不会出现在ParameterMapping中，但是必须有，所以这里需要收集所有的getter属性
            //TypeHandlerRegistry可以直接处理的会作为一个直接使用的对象进行处理
            boolean hasTypeHandler = ms.getConfiguration().getTypeHandlerRegistry().hasTypeHandler(parameterObject.getClass());
            MetaObject metaObject = MetaObjectUtil.forObject(parameterObject);
            //需要针对注解形式的MyProviderSqlSource保存原值
            if (!hasTypeHandler) {
                for (String name : metaObject.getGetterNames()) {
                    paramMap.put(name, metaObject.getValue(name));
                }
            }
            //下面这段方法，主要解决一个常见类型的参数时的问题
            if (boundSql.getParameterMappings() != null && boundSql.getParameterMappings().size() > 0) {
                for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
                    String name = parameterMapping.getProperty();
                    if (!name.equals(PAGEPARAMETER_FIRST)
                            && !name.equals(PAGEPARAMETER_SECOND)
                            && paramMap.get(name) == null) {
                        if (hasTypeHandler
                                || parameterMapping.getJavaType().equals(parameterObject.getClass())) {
                            paramMap.put(name, parameterObject);
                            break;
                        }
                    }
                }
            }
        }
        return processPageParameter(ms, paramMap, page, boundSql, pageKey);
    }

    // 加入 page 参数
    // com.github.pagehelper.dialect.helper.MySqlDialect#processPageParameter
    @Override
    public Object processPageParameter(MappedStatement ms, Map<String, Object> paramMap, Page page, BoundSql boundSql, CacheKey pageKey) {
        // First_PageHelper, Second_PageHelper
        paramMap.put(PAGEPARAMETER_FIRST, page.getStartRow());
        paramMap.put(PAGEPARAMETER_SECOND, page.getPageSize());
        //处理pageKey
        pageKey.update(page.getStartRow());
        pageKey.update(page.getPageSize());
        //处理参数配置
        if (boundSql.getParameterMappings() != null) {
            List<ParameterMapping> newParameterMappings = new ArrayList<ParameterMapping>(boundSql.getParameterMappings());
            if (page.getStartRow() == 0) {
                newParameterMappings.add(new ParameterMapping.Builder(ms.getConfiguration(), PAGEPARAMETER_SECOND, Integer.class).build());
            } else {
                newParameterMappings.add(new ParameterMapping.Builder(ms.getConfiguration(), PAGEPARAMETER_FIRST, Integer.class).build());
                newParameterMappings.add(new ParameterMapping.Builder(ms.getConfiguration(), PAGEPARAMETER_SECOND, Integer.class).build());
            }
            MetaObject metaObject = MetaObjectUtil.forObject(boundSql);
            metaObject.setValue("parameterMappings", newParameterMappings);
        }
        return paramMap;
    }
    // 组装分页sql
    // com.github.pagehelper.dialect.AbstractHelperDialect#getPageSql
    @Override
    public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
        String sql = boundSql.getSql();
        Page page = getLocalPage();
        //支持 order by
        String orderBy = page.getOrderBy();
        if (StringUtil.isNotEmpty(orderBy)) {
            pageKey.update(orderBy);
            sql = OrderByParser.converToOrderBySql(sql, orderBy);
        }
        if (page.isOrderByOnly()) {
            return sql;
        }
        return getPageSql(sql, page, pageKey);
    }
    // com.github.pagehelper.dialect.helper.MySqlDialect#getPageSql
    @Override
    public String getPageSql(String sql, Page page, CacheKey pageKey) {
        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
        sqlBuilder.append(sql);
        // 分页sql拼接, limit xxx
        if (page.getStartRow() == 0) {
            sqlBuilder.append(" LIMIT ? ");
        } else {
            sqlBuilder.append(" LIMIT ?, ? ");
        }
        return sqlBuilder.toString();
    }

 // com.github.pagehelper.PageHelper#afterPage
    @Override
    public Object afterPage(List pageList, Object parameterObject, RowBounds rowBounds) {
        //这个方法即使不分页也会被执行，所以要判断 null
        AbstractHelperDialect delegate = autoDialect.getDelegate();
        if (delegate != null) {
            return delegate.afterPage(pageList, parameterObject, rowBounds);
        }
        return pageList;
    }

    // com.github.pagehelper.dialect.AbstractHelperDialect#afterPage
    @Override
    public Object afterPage(List pageList, Object parameterObject, RowBounds rowBounds) {
        // 取出本线程的page变量, 放入list
        Page page = getLocalPage();
        if (page == null) {
            return pageList;
        }
        page.addAll(pageList);
        // count 值临时变换, 用于应对没有进行count的场景, 使外部表现一致
        if (!page.isCount()) {
            page.setTotal(-1);
        } else if ((page.getPageSizeZero() != null && page.getPageSizeZero()) && page.getPageSize() == 0) {
            page.setTotal(pageList.size());
        } else if(page.isOrderByOnly()){
            page.setTotal(pageList.size());
        }
        return page;
    }

// com.github.pagehelper.dialect.helper.OracleDialect
public class OracleDialect extends AbstractHelperDialect {

    @Override
    public Object processPageParameter(MappedStatement ms, Map<String, Object> paramMap, Page page, BoundSql boundSql, CacheKey pageKey) {
        paramMap.put(PAGEPARAMETER_FIRST, page.getEndRow());
        paramMap.put(PAGEPARAMETER_SECOND, page.getStartRow());
        //处理pageKey
        pageKey.update(page.getEndRow());
        pageKey.update(page.getStartRow());
        //处理参数配置
        handleParameter(boundSql, ms);
        return paramMap;
    }
    // 获取带分页的sql
    @Override
    public String getPageSql(String sql, Page page, CacheKey pageKey) {
        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 120);
        // 很明显, oracle 和 mysql 的分页实现是不一样的, oracle 使用 row_id 实现, 而 mysql 使用 limit 实现 
        sqlBuilder.append("SELECT * FROM ( ");
        sqlBuilder.append(" SELECT TMP_PAGE.*, ROWNUM ROW_ID FROM ( ");
        sqlBuilder.append(sql);
        sqlBuilder.append(" ) TMP_PAGE)");
        sqlBuilder.append(" WHERE ROW_ID <= ? AND ROW_ID > ?");
        return sqlBuilder.toString();
    }

}

 // 实际上在每次运行完成pageInterceptor之后，都会在finnaly中进行一次清理工作
        try {
            // do page things
        } finally {
            // afterAll 即为清理任务
            if(dialect != null){
                dialect.afterAll();
            }
        }
    // com.github.pagehelper.PageHelper#afterAll
    @Override
    public void afterAll() {
        //这个方法即使不分页也会被执行，所以要判断 null
        AbstractHelperDialect delegate = autoDialect.getDelegate();
        if (delegate != null) {
            // 默认为空
            delegate.afterAll();
            // delegate 移除，这里也是使用 ThreadLocal 实现，直接remove即可
            autoDialect.clearDelegate();
        }
        // 清理 page对象，下次不再有该设置，也就是说 page 设置是一次性的
        clearPage();
    }
    // com.github.pagehelper.page.PageMethod#clearPage
    public static void clearPage() {
        LOCAL_PAGE.remove();
    }
    // 下次再进行分页时，重新调用  PageHelper.startPage(x, x, x); 即可