Grails插件MyBatis分页设计 grailsmybatispluginpagination

新的MyBatis比老的Ibatis有了个新的东东Plugin，在配置文件中

<configuration>
    <settings>
        <setting name="cacheEnabled" value="true"/>
    </settings>

    <plugins>
        <plugin interceptor="store.MySqlPaginationPlugin"/>
    </plugins>
</configuration>

有了这个插件设计，就不用在那么麻烦的按照老的Ibatis的方式设计分页了，只需按照MyBatis要求的插件方式来拦截sql修改成分页形式了
先弄个基类，共享代码，支持所有的数据库，将不同的东西分离出来

Intercepts({@Signature(type = Executor.class, method = "query",
        args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public abstract class PaginationBasePlugin implements Interceptor {

    public Object intercept(Invocation invocation) throws Throwable {
        Object[] queryArgs = invocation.getArgs();
        MappedStatement ms = (MappedStatement) queryArgs[0];
        BoundSql boundSql = ms.getBoundSql(queryArgs[1]);
        String sql = boundSql.getSql().trim();

        Object args = queryArgs[1];

        sql = paginationSql(sql,args); //通过子类实现
        BoundSql newBoundSql = new BoundSql(ms.getConfiguration(), sql, boundSql.getParameterMappings(), boundSql.getParameterObject());

        MappedStatement newMs = copyFromMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));
        queryArgs[0] = newMs;
        return invocation.proceed();
    }
    protected abstract String paginationSql(String sql,Object args); //子类实现
    //see: MapperBuilderAssistant
    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.fetchSize(ms.getFetchSize());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        builder.keyProperty(ms.getKeyProperty());

        //setStatementTimeout()
        builder.timeout(ms.getTimeout());

        //setStatementResultMap()
        builder.parameterMap(ms.getParameterMap());

        //setStatementResultMap()
        builder.resultMaps(ms.getResultMaps());
        builder.resultSetType(ms.getResultSetType());

        //setStatementCache()
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());

        return builder.build();
    }

    public static class BoundSqlSqlSource implements SqlSource {

        private BoundSql boundSql;

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

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

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

    public void setProperties(Properties properties) {
        //suffix = properties.get("dirname").toString();
    }
}

以MySql数据库分页举例，做一个MySqlPaginationPlugin

@Intercepts({@Signature(type = Executor.class, method = "query",
        args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public class MySqlPaginationPlugin extends PaginationBasePlugin {
    @Override
    protected String paginationSql(String sql,Object args) {
        if(args instanceof Map){
            if(((Map) args).containsKey("pagination")){
                Map map = (Map) args;
                RowBounds rowBounds = (RowBounds) map.get("pagination");
                sql = sql+" limit " + rowBounds.getOffset()+ ","+rowBounds.getLimit();  //分页了
            }
        }
        return sql;
    }
}

MySql的分页比较简单，其他数据库的分页要求对sql参数进行分离和复杂点的修改，这些玩意大家自己就会倒腾了，网上有很多代码，再次不弄了

用法如下，从代码中可以看出，只要Xml Mapper的Sql语句输入参数为map类型，并且里面含有pagination键值就可以了，否则不分页

class ClientGateway {
    static forceAsListOps = ["selectClient"];

    def queryClient(RowBounds rowBounds) {
        HashMap map = new HashMap()
        map.put("pagination", rowBounds);  //关键标志
        sqlMapper.selectList("selectClient", map);
    }
}

pagination的值为一个RowBounds类型，这个是MyBatis里面带的东西，我就直接拿来用了，反正就那两个属性offset，limit
这样同一个sql语句既可以支持分页也可以不支持分页，避免了写两套相同的sql，不好维护

<mapper namespace="client">
    <select id="selectClient" parameterType="map" resultType="client">
        select * from client
    </select>   
</mapper>

那么其中这个sql语句的count又如何不写两套sql弄出来呢？
同样道理，自己在map里面再加个特殊的标记，就可以一个sql语句也支持count了
综上，通过MyBatis的插件拦截sql是一个sql语句既可以直接执行，也可以支持分页和获取所有行数。提高了可维护性。
代码中的pagination字符串自己弄成常量吧，不满意的地方可以自己修改，思路就是这样了，我使着挺好用。