MyBatis-PageHelper 源码解说

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• Github: MyBatis-PageHelper-源码解说

  ---

  总说明

  • 源码仓库： https://github.com/pagehelper/Mybatis-PageHelper
  • 克隆：git clone https://github.com/pagehelper/Mybatis-PageHelper.git
  • 切分支（tag）：git checkout master
  • JDK: 17

    ---

    单元测试

    • 参考：com.github.pagehelper.test.reasonable.PageTest

          @Test
          public void testMapperWithStartPage() {
              SqlSession sqlSession = MybatisReasonableHelper.getSqlSession();
              UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
              try {
                  // PageHelper.startPage(20, 50);    // 超出总页数时，会自动改为查询最后一页
                  PageHelper.startPage(4, 50);                // 开启分页，ref: sign_m_110
                  List list = userMapper.selectAll();   // 分页拦截，ref: sign_m_210
                  PageInfo page = new PageInfo(list);
              }
          }
      

      • hsqldb\mybatis-config-reasonable.xml
        • UserMapper.xml

              
                  select * from user order by id -- comment
              
          
          

          ---

          原理

          开启分页

          • com.github.pagehelper.PageHelper

            // sign_c_010
            public class PageHelper extends PageMethod implements Dialect, BoundSqlInterceptor.Chain {
                // sign_m_010  后处理
                @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) {
                        delegate.afterAll();
                        autoDialect.clearDelegate();
                    }
                    clearPage();    // 移除本地变量，ref: sign_m_011
                }
                // sign_m_011  移除本地变量
                public static void clearPage() {
                    LOCAL_PAGE.remove();
                }
            }
            

            • com.github.pagehelper.page.PageMethod

              public abstract class PageMethod {
                  protected static final ThreadLocal LOCAL_PAGE    = new ThreadLocal();
                  protected static       boolean           DEFAULT_COUNT = true;
                  // sign_m_110 开启分页
                  public static  Page startPage(int pageNum, int pageSize) {
                      return startPage(pageNum, pageSize, DEFAULT_COUNT); // 默认进行 COUNT(0) 查询
                  }
                  public static  Page startPage(int pageNum, int pageSize, boolean count) {
                      return startPage(pageNum, pageSize, count, null, null);
                  }
                  public static  Page startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
                      Page page = new Page(pageNum, pageSize, count);
                      page.setReasonable(reasonable);     // 分页合理化设置
                      page.setPageSizeZero(pageSizeZero); // true 且 pageSize = 0 时返回全部结果，false 时分页
                      ...
                      setLocalPage(page);
                      return page;
                  }
                  public static void setLocalPage(Page page) {
                      LOCAL_PAGE.set(page);
                  }
              }
              

              拦截器

              • com.github.pagehelper.PageInterceptor

                @Intercepts({
                    // 设置对目标接口和目标 (指定的方法名和参数类型) 方法进行拦截
                    @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 PageInterceptor implements Interceptor {
                    private volatile     Dialect    dialect;
                    private              String     default_dialect_class = "com.github.pagehelper.PageHelper";
                    @Override // 初始化 dialect
                    public void setProperties(Properties properties) {
                        ...
                        String dialectClass = ... default_dialect_class;    // 用默认的
                        Dialect tempDialect = ClassUtil.newInstance(dialectClass, properties);
                        tempDialect.setProperties(properties);
                        ...
                        // 初始化完成后再设置值，保证 dialect 完成初始化。
                        // 默认情况下，dialect 是 PageHelper 实例，ref: sign_c_010
                        dialect = tempDialect;
                    }
                    // sign_m_210 分页拦截
                    @Override
                    public Object intercept(Invocation invocation) throws Throwable {
                        try {
                            Object[] args = invocation.getArgs();
                            MappedStatement ms = (MappedStatement) args[0];
                            ...
                            if (args.length == 4) { // 4 个参数时
                                ...
                            } else {                // 6 个参数时
                                ...
                            }
                            ...
                            List resultList;
                            if (!dialect.skip(ms, parameter, rowBounds)) { // 需要分页
                                ...
                                // 判断是否需要进行 count 查询
                                if (dialect.beforeCount(ms, parameter, rowBounds)) {
                                    ... // 异步 Count 处理
                                    else {
                                        // 查询总数，将原接口改成 COUNT(0) 统计 SQL 再进行查询
                                        Long count = count(executor, ms, parameter, rowBounds, null, 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)) { // 将 count 设置到 Page 里面去
                                            // 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( // 将原 SQL 改成分页 SQL，再执行查询
                                    dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey
                                );
                                ... // 异步 Count 处理
                            } ... // else: 不分页，直接查询结果
                            return dialect.afterPage(resultList, parameter, rowBounds); // 将 list 添加到 Page，并返回 Page 对象
                        } finally {
                            if (dialect != null) {
                                dialect.afterAll(); // 清除线程变量。ref: sign_m_010
                            }
                        }
                    }
                }
                

                总结

                • 原理总体较简单
                • 只在一个拦截器里处理（统计与数据查询）
                • 最后返回 Page 封装集合类
                
                （图片来源网络，侵删）