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

    @Testpublic 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_110List<User> list = userMapper.selectAll();   // 分页拦截，ref: sign_m_210PageInfo<User> page = new PageInfo<User>(list);}}

hsqldb\mybatis-config-reasonable.xml

<configuration><plugins><plugin interceptor="com.github.pagehelper.PageInterceptor"><property name="reasonable" value="true"/></plugin></plugins>
</configuration>

UserMapper.xml

<mapper namespace="com.github.pagehelper.mapper.UserMapper"><select id="selectAll" resultType="User">select * from user order by id -- comment</select>
</mapper>

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@Overridepublic void afterAll() {// 这个方法即使不分页也会被执行，所以要判断 nullAbstractHelperDialect 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<Page> LOCAL_PAGE    = new ThreadLocal<Page>();protected static       boolean           DEFAULT_COUNT = true;// sign_m_110 开启分页public static <E> Page<E> startPage(int pageNum, int pageSize) {return startPage(pageNum, pageSize, DEFAULT_COUNT); // 默认进行 COUNT(0) 查询}public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count) {return startPage(pageNum, pageSize, count, null, null);}public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {Page<E> page = new Page<E>(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 dialectpublic 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_010dialect = tempDialect;}// sign_m_210 分页拦截@Overridepublic 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}}}
}