mybatis使用拦截器OffsetLimitInterceptor实现分页的步骤，并指定数据库方言dialectClass

使用MyBatis实现分页，可以使用拦截器OffsetLimitInterceptor来实现。下面是实现分页的步骤：

1. 定义一个实现了Interceptor接口的分页拦截器类OffsetLimitInterceptor。

2. 在intercept()方法中，获取MappedStatement对象，检查该对象中是否有分页信息。

3. 如果存在分页信息，获取分页参数，根据数据库方言dialectClass来生成对应的分页SQL。

4. 将生成的SQL设置到BoundSql对象中，替换原有的SQL语句。

5. 调用invocation.proceed()方法执行SQL语句。

6. 在执行完SQL语句后，获取分页结果，将其封装到Page对象中，并返回该对象。

以下是指定数据库方言dialectClass的示例代码：

public class OffsetLimitInterceptor implements Interceptor {

    private String dialectClass;

    public Object intercept(Invocation invocation) throws Throwable {
        // 获取MappedStatement对象和分页参数
        MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
        Object parameter = invocation.getArgs()[1];
        BoundSql boundSql = mappedStatement.getBoundSql(parameter);
        String sql = boundSql.getSql();
        Object parameterObject = boundSql.getParameterObject();
        Page page = getPage(parameterObject);

        // 如果存在分页信息，根据dialectClass生成对应的分页SQL
        if (page != null) {
            String pageSql = generatePageSql(sql, page);
            // 将生成的SQL设置到BoundSql对象中，替换原有的SQL语句
            BoundSql newBoundSql = new BoundSql(mappedStatement.getConfiguration(), pageSql,
                    boundSql.getParameterMappings(), boundSql.getParameterObject());
            MappedStatement newMappedStatement = copyFromMappedStatement(mappedStatement,
                    new BoundSqlSqlSource(newBoundSql));
            invocation.getArgs()[0] = newMappedStatement;
        }

        // 执行原有的SQL语句
        Object result = invocation.proceed();

        // 封装分页结果
        if (page != null) {
            page.setResult((List) result);
            return page;
        } else {
            return result;
        }
    }

    // 根据数据库方言dialectClass生成对应的分页SQL
    private String generatePageSql(String sql, Page page) {
        if (dialectClass.equals("mysql")) {
            return sql + " limit " + page.getStartRow() + "," + page.getPageSize();
        } else if (dialectClass.equals("oracle")) {
            return "select * from (select rownum rn, t.* from (" + sql + ") t where rownum <= "
                    + page.getEndRow() + ") where rn >= " + page.getStartRow();
        } else {
            throw new RuntimeException("Unsupported dialectClass:" + dialectClass);
        }
    }

    private Page getPage(Object parameterObject) {
        if (parameterObject instanceof Page) {
            return (Page) parameterObject;
        } else {
            return null;
        }
    }

    private MappedStatement copyFromMappedStatement(MappedStatement ms, SqlSource newSqlSource) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(),
                ms.getId(), newSqlSource, ms.getSqlCommandType());
        builder.resource(ms.getResource());
        builder.parameterMap(ms.getParameterMap());
        builder.resultMaps(ms.getResultMaps());
        builder.fetchSize(ms.getFetchSize());
        builder.timeout(ms.getTimeout());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        builder.keyProperty(StringUtils.join(ms.getKeyProperties(), ","));
        builder.keyColumn(StringUtils.join(ms.getKeyColumns(), ","));
        builder.databaseId(ms.getDatabaseId());
        builder.lang(ms.getLang());
        builder.resultOrdered(ms.isResultOrdered());
        builder.resultSets(StringUtils.join(ms.getResultSets(), ","));
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());
        return builder.build();
    }

    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    public void setProperties(Properties properties) {
        this.dialectClass = properties.getProperty("dialectClass");
    }

    private static class BoundSqlSqlSource implements SqlSource {

        private BoundSql boundSql;

        public BoundSqlSqlSource(BoundSql boundSql) {
            this.boundSql = boundSql;
        }

        public BoundSql getBoundSql(Object parameterObject) {
            return boundSql;
        }
    }
}

在MyBatis的配置文件中，配置该拦截器并指定数据库方言dialectClass：

<plugins>
    <plugin interceptor="com.example.OffsetLimitInterceptor">
        <property name="dialectClass" value="mysql"/>
    </plugin>
</plugins>