一个通用的面向对象的查询接口

现在做持久化基本上都会用到ORM，现在最通用的解决方案当然就是Hibernate以及由之引申出来的EJB3的持久化方案，我们在做DAO层的时候经常是直接写HQL，并不是说它不好，而是不够通用。例如本来有个功能是查询某种类型的User，但是需要额外的过滤和排序，那么就需要拼HQL的，不太好维护，所以有一个面向对象的查询接口还是比较好。
下面就是这个雏形，是我现在的项目中用到的，抛砖引玉吧。
下面是全部的类和主要的方法，其他的方法都忽略了，可以到Google Code下载全部的源代码。

/**
 * Search info needed by client, only provide additional conditions and sorts.
 */
public class SearchInfo implements Serializable
{
    /**
     * Additional conditions for some query info provided by service.
     */
    private List<Condition> conditions = new ArrayList<Condition>();
    /**
     * Sort info.
     */
    private List<Sort> sorts = new ArrayList<Sort>();
    /**
     * Page size.
     */
    private int pageSize = Constants.DEFAULT_PAGE_SIZE;
    /**
     * First return row.
     */
    private int firstRow;
    /**
     * Whether need return total rows.
     */
    private boolean getTotalRows;

/**
 * All query info needed to construct a QL string.
 */
public class QueryInfo extends SearchInfo implements Serializable
{
    public static final String COUNT_COLUMN = "totalrows";
    /**
     * Select fields.
     */
    private List<Field> selects = new ArrayList<Field>();
    /**
     * From tagets.
     */
    private List<From> froms = new ArrayList<From>();
    /**
     * Group by fields.
     */
    private List<Field> groupBys = new ArrayList<Field>();
    /**
     * Having conditions.
     */
    private List<Condition> havings = new ArrayList<Condition>();
    /**
     * Native hints.
     */
    private List<String> hints = new ArrayList<String>();
    /**
     * Fetch size.
     */
    private int fetchSize;

/**
 * Search result by search info.
 */
public class SearchResult
{
    /**
     * Total rows matched, if search info do not let get total rows, it will return -1.
     */
    private int totalRows=-1;
    /**
     * Indicate if any more matched result can be returned
     */
    private boolean hasMore;
    /**
     * Page result only.
     */
    private List pageResult = new ArrayList();

/**
 * Search condition, used in where and having part of QL.
 */
public class Condition implements Serializable
{
    /**
     * Condtion field, can be null, for example, for not, and and or Condition.
     */
    private Field field;
    /**
     * Operator.
     */
    private Operator operator;
    /**
     * Condition value.
     */
    private Object value;

/**
 * Query field, used in select, condition, sort, having and sort part of QL.
 */
public class Field implements Serializable
{
    /**
     * From target of field.
     */
    private From from;
    /**
     * Field name.
     */
    private String name;
    /**
     * Aggregation function.
     */
    private Function function;
    /**
     * Filed alias, especially needed when Function is not null.
     */
    private String alias;

/**
 * Query target, used in from part of QL, Field also need reference to it.
 */
public class From implements Serializable
{
    public static final Class DIRECT_TABLE_CLASS= DirectTable.class;
    private static final String DIRECT_TABLE_SEPERATOR=" ";
    /**
     * Model class type. If is direct table
     */
    private Class target;
    /**
     * Alias for model class or table.
     */
    private String alias;

/**
 * Aggregation function.
 */
public enum Function implements Serializable
{
    count,sum,max,min,avg
}

/**
 * Operator for condition
 */
public enum Operator implements Serializable
{
    equal("="), notEqual("!="), like("like"), isNull("is null"),
    isNotNull("is not null"), greater(">"), greaterOrEqual(">="),
    less("<"), lessOrEqual("<="), in("in"), and("and"),
    or("or"), not("not");
    private final String value;

    Operator(String value)
    {
        this.value = value;
    }

    public String toString()
    {
        return value;
    }
}

/**
 * Sort info, used in sort part of QL.
 */
public class Sort implements Serializable
{
    /**
     * Sort field.
     */
    private Field field;
    /**
     * Sort order.
     */
    private boolean asc;