PageHelper原理

PageHelper是什么

基于mybatis的一款分页插件，仅用少量代码即可实现分页功能

查询数量sql怎么生成的

修改了原生的sqlSource，产生了count的MappedStatement

    public static MappedStatement newCountMappedStatement(MappedStatement ms) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId() + "_COUNT", 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(), int.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();
    }

分页sql怎么产生的

对于不同的数据源做不同的处理，对mysql而言，就是产生了对应的limit语句

public class MysqlParser extends AbstractParser {
    @Override
    public String getPageSql(String sql) {
        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
        sqlBuilder.append(sql);
        sqlBuilder.append(" limit ?,?");
        return sqlBuilder.toString();
    }

    @Override
    public Map<String, Object> setPageParameter(MappedStatement ms, Object parameterObject, BoundSql boundSql, Page<?> page) {
        Map<String, Object> paramMap = super.setPageParameter(ms, parameterObject, boundSql, page);
        paramMap.put(PAGEPARAMETER_FIRST, page.getStartRow());
        paramMap.put(PAGEPARAMETER_SECOND, page.getPageSize());
        return paramMap;
    }
}

分页参数怎么保存的

通过startPage方法，将分页参数放到ThreadLocal中

    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);
        page.setPageSizeZero(pageSizeZero);
        //当已经执行过orderBy的时候
        Page<E> oldPage = SqlUtil.getLocalPage();
        if (oldPage != null && oldPage.isOrderByOnly()) {
            page.setOrderBy(oldPage.getOrderBy());
        }
        SqlUtil.setLocalPage(page);
        return page;
    }

插件如何融于mybatis中

通过注册mybatis插件，PageHelper实现了Interceptor接口，核心的处理逻辑都在intercept方法中

    /**
     * Mybatis拦截器方法
     *
     * @param invocation 拦截器入参
     * @return 返回执行结果
     * @throws Throwable 抛出异常
     */
    public Object intercept(Invocation invocation) throws Throwable {
        if (autoRuntimeDialect) {
            SqlUtil sqlUtil = getSqlUtil(invocation);
            return sqlUtil.processPage(invocation);
        } else {
            if (autoDialect) {
                initSqlUtil(invocation);
            }
            return sqlUtil.processPage(invocation);
        }
    }

sqlSource的动态判断

    public void processMappedStatement(MappedStatement ms) throws Throwable {
        SqlSource sqlSource = ms.getSqlSource();
        MetaObject msObject = SystemMetaObject.forObject(ms);
        SqlSource pageSqlSource;
        if (sqlSource instanceof StaticSqlSource) {
            pageSqlSource = new PageStaticSqlSource((StaticSqlSource) sqlSource);
        } else if (sqlSource instanceof RawSqlSource) {
            pageSqlSource = new PageRawSqlSource((RawSqlSource) sqlSource);
        } else if (sqlSource instanceof ProviderSqlSource) {
            pageSqlSource = new PageProviderSqlSource((ProviderSqlSource) sqlSource);
        } else if (sqlSource instanceof DynamicSqlSource) {
            pageSqlSource = new PageDynamicSqlSource((DynamicSqlSource) sqlSource);
        } else {
            throw new RuntimeException("无法处理该类型[" + sqlSource.getClass() + "]的SqlSource");
        }
        msObject.setValue("sqlSource", pageSqlSource);
        //由于count查询需要修改返回值，因此这里要创建一个Count查询的MS
        msCountMap.put(ms.getId(), MSUtils.newCountMappedStatement(ms));
    }

parser的动态判断

从sql链接中获取方言，例如mysql，通过mysql判断

    public static Parser newParser(Dialect dialect) {
        Parser parser = null;
        switch (dialect) {
            case mysql:
            case mariadb:
            case sqlite:
                parser = new MysqlParser();
                break;
            case oracle:
                parser = new OracleParser();
                break;
            case hsqldb:
                parser = new HsqldbParser();
                break;
            case sqlserver:
                parser = new SqlServerParser();
                break;
            case sqlserver2012:
                parser = new SqlServer2012Dialect();
                break;
            case db2:
                parser = new Db2Parser();
                break;
            case postgresql:
                parser = new PostgreSQLParser();
                break;
            case informix:
                parser = new InformixParser();
                break;
            case h2:
                parser = new H2Parser();
                break;
            default:
                throw new RuntimeException("分页插件" + dialect + "方言错误!");
        }
        return parser;
    }

参考

• https://github.com/pagehelper/Mybatis-PageHelper/blob/master/README_zh.md