JPA查询不返回任何结果

Question

我正在尝试使用下面的代码在数据存储区中搜索一些类型为“Book”的实体。但是它返回空List.I得到一个空的结果列表，其中size = -1。我在哪里出错？

@ApiMethod(name = "searchBook")
public List<Book> searchBook(@Named("bookName") String bookName,@Named("languageId")String languageId, @Named("subjectId")String subjectId) {
    EntityManager mgr = getEntityManager();
    List<Book> bookList=new ArrayList<Book>();
    String queryString="";

    queryString="SELECT x FROM Book x WHERE ";

   //If i dont want to set any parameter for some field, I send BLANK as the parameter..
    if(bookName!=null && !bookName.matches("BLANK")){
        queryString=queryString+"x.bookName =:bookName"+" AND ";
    }
    if(languageId!=null && !languageId.matches("BLANK")){
        queryString=queryString+"x.languageId =:languageId"+" AND ";

    }

    if(subjectId!=null && !subjectId.matches("BLANK")){
        queryString=queryString+"x.categoryId =:subjectId"+" AND ";

    }

    //Removing last word "AND" and whitespaces from the end of queryString
    queryString=queryString.substring(0,queryString.length()-5);
    Query q = mgr.createQuery(queryString);

    //setting query parameters..
    if(bookName!=null && !bookName.matches("BLANK")){
        q.setParameter("bookName", bookName);

    }
    if(languageId!=null && !languageId.matches("BLANK")){
        q.setParameter("languageId", languageId);
    }

    if(subjectId!=null && !subjectId.matches("BLANK")){
        q.setParameter("subjectId", subjectId);
    }

    //executing the query...
    try {
            mgr.getTransaction().begin();
            bookList = (List<Book>) q.getResultList();
            mgr.getTransaction().commit();
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }finally{
        mgr.close();
    }

    return bookList;
}

在我的情况下，我通过仅为subjectId设置一个参数进行了测试。设置参数有问题吗？这是在查询中设置动态参数数量的正确方法吗？