Hibernate的使用技巧②

1.Hibernate统计记录的数量

Criteria c = session.createCriteria(User.class.getName());        c.setProjection(Projections.rowCount());        CheckRowNumResponseBean result = new CheckRowNumResponseBean();   result.setRowNum(Integer.parseInt(c.uniqueResult().toString()));   setResult(result);

 能使用的是c.list.size().返回的记录数，其实这样的话，hibernate是从数据库里面把所有满足条件的记录查询出来放到一个list里 面，然后再用list的length返回长度。这样不是此需求的初衷。因为这样会消耗性能。。。并且在数据过多的情况下面，会造成tomcat的 OutOFMemory错误

2.Hibernate封装vo

首先是    VO    实体

package com.ntej.exam.model.vo;   /** * 考试信息展示，显示在我的考试界面 和我的成绩界面 * */ public class ExamInfoVO {       private String E_ID;     private String E_Name;     private Integer E_Score;     private String MajorType;     private String MajorName;     private String ExamTitle;     private Integer QuestionNumber;     private Integer TotalTime;     private String ExamBeginDateTime;     private String ExamEndDateTime;     private Integer ExamCode;       public String getE_ID() {         return E_ID;     }       public void setE_ID(String e_ID) {         E_ID = e_ID;     }       public String getE_Name() {         return E_Name;     }       public void setE_Name(String e_Name) {         E_Name = e_Name;     }       public Integer getE_Score() {         return E_Score;     }       public void setE_Score(Integer e_Score) {         E_Score = e_Score;     }       public String getMajorType() {         return MajorType;     }       public void setMajorType(String majorType) {         MajorType = majorType;     }       public String getMajorName() {         return MajorName;     }       public void setMajorName(String majorName) {         MajorName = majorName;     }       public String getExamTitle() {         return ExamTitle;     }       public void setExamTitle(String examTitle) {         ExamTitle = examTitle;     }       public Integer getQuestionNumber() {         return QuestionNumber;     }       public void setQuestionNumber(Integer questionNumber) {         QuestionNumber = questionNumber;     }       public Integer getTotalTime() {         return TotalTime;     }       public void setTotalTime(Integer totalTime) {         TotalTime = totalTime;     }       public String getExamBeginDateTime() {         return ExamBeginDateTime;     }       public void setExamBeginDateTime(String examBeginDateTime) {         ExamBeginDateTime = examBeginDateTime;     }       public String getExamEndDateTime() {         return ExamEndDateTime;     }       public void setExamEndDateTime(String examEndDateTime) {         ExamEndDateTime = examEndDateTime;     }       public Integer getExamCode() {         return ExamCode;     }       public void setExamCode(Integer examCode) {         ExamCode = examCode;     }       @Override     public String toString() {         return "ExamInfoVO [E_ID=" + E_ID + ", E_Name=" + E_Name + ", E_Score="                 + E_Score + ", MajorType=" + MajorType + ", MajorName="                 + MajorName + ", ExamTitle=" + ExamTitle + ", QuestionNumber="                 + QuestionNumber + ", TotalTime=" + TotalTime                 + ", ExamBeginDateTime=" + ExamBeginDateTime                 + ", ExamEndDateTime=" + ExamEndDateTime + ", ExamCode="                 + ExamCode + "]";     }   }

查询方法

public List<ExamInfoVO> getExamInfoVosByemp(Emp emp){         try {              String queryString = "     SELECT e.E_ID,e.E_Name,s.E_Score,m.MajorType,m.MajorName,ei.ExamTitle,ei.QuestionNumber,ei.TotalTime,DATE_FORMAT(ei.ExamBeginDateTime,'%Y-%m-%d %H:%i:%s') as ExamBeginDateTime,DATE_FORMAT(ei.ExamEndDateTime,'%Y-%m-%d %H:%i:%s') as ExamEndDateTime, ei.ExamCode"                      + " FROM emp e,score s,major m,examinfo ei WHERE    e.E_ID=s.E_ID AND e.D_No=ei.D_No AND s.MajorID=m.MajorID AND s.ExamCode=ei.ExamCode AND e.E_ID='"+emp.getEId()+"';";      Query query= getSession().createSQLQuery(queryString);      List<ExamInfoVO> list = query.setResultTransformer(Transformers.aliasToBean(ExamInfoVO.class)).list();      return list;         } catch (RuntimeException re) {             throw re;         }     }

3.使用criteria

Hibernate中共提供了三种检索方式：HQL(Hibernate Query Language)、QBC(Query By Criteria)、QBE(Query By Example)。而今天的这个查询方法属于QBC，   List list=getHibernateSession().createCriteria(MeterUser.class).add(Restrictions.eq("name", "11")).addOrder(Order.desc("userid")).list();     Restrictions的用法： QBC常用限定方法 Restrictions.eq --> equal,等于. Restrictions.allEq --> 参数为Map对象,使用key/value进行多个等于的比对,相当于多个Restrictions.eq的效果 Restrictions.gt --> great-than > 大于 Restrictions.ge --> great-equal >= 大于等于 Restrictions.lt --> less-than, < 小于 Restrictions.le --> less-equal <= 小于等于 Restrictions.between --> 对应SQL的between子句 Restrictions.like --> 对应SQL的LIKE子句 Restrictions.in --> 对应SQL的in子句 Restrictions.and --> and 关系 Restrictions.or --> or 关系 Restrictions.isNull --> 判断属性是否为空,为空则返回true Restrictions.isNotNull --> 与isNull相反 Restrictions.sqlRestriction --> SQL限定的查询 Order.asc --> 根据传入的字段进行升序排序 Order.desc --> 根据传入的字段进行降序排序 MatchMode.EXACT --> 字符串精确匹配.相当于"like 'value'" MatchMode.ANYWHERE --> 字符串在中间匹配.相当于"like '%value%'" MatchMode.START --> 字符串在最前面的位置.相当于"like 'value%'" MatchMode.END --> 字符串在最后面的位置.相当于"like '%value'" 例子 查询年龄在20-30岁之间的所有学生对象 List list = session.createCriteria(Student.class)       .add(Restrictions.between("age",new Integer(20),new Integer(30)).list(); 查询学生姓名在AAA,BBB,CCC之间的学生对象 String[] names = {"AAA","BBB","CCC"}; List list = session.createCriteria(Student.class)       .add(Restrictions.in("name",names)).list(); 查询年龄为空的学生对象 List list = session.createCriteria(Student.class)       .add(Restrictions.isNull("age")).list(); 查询年龄等于20或者年龄为空的学生对象 List list = session.createCriteria(Student.class)       .add(Restrictions.or(Restrictions.eq("age",new Integer(20)),                  Restrictions.isNull("age")).list(); -------------------------------------------------------------------- 使用QBC实现动态查询 public List findStudents(String name,int age){ Criteria criteria = session.createCriteria(Student.class); if(name != null){   criteria.add(Restrictions.like("name",name,MatchMode.ANYWHERE)); } if(age != 0){   criteria.add(Restrictions.eq("age",new Integer(age))); } criteria.addOrder(Order.asc("name"));//根据名字升序排列 return criteria.list(); } ----------------------------------------------------------------------------------- 今天用了写hibernate高级查询时用了Restrictions(当然Expression也是可以以的)这个类.感觉不错. 下面的代码写的不易读.其实核心就是一句 Restrictions.or(Restrictions.like(),Restrictions.or(Restrictions.like,........)) 里面的or可以无限加的.还是比较好用 Session session = getHibernateTemplate().getSessionFactory()                 .openSession();         Criteria criteria = session.createCriteria(Film.class);         List<Film> list = criteria.add(             Restrictions.or(Restrictions.like("description", key,MatchMode.ANYWHERE),             Restrictions.or(Restrictions.like("name", key,MatchMode.ANYWHERE),                 Restrictions.or(    Restrictions.like("direct", key,MatchMode.ANYWHERE),                 Restrictions.or(Restrictions.like("mainplay",key,MatchMode.ANYWHERE),                         Restrictions.like("filearea", key,MatchMode.ANYWHERE)))))).list();         session.close();         return list; 窗体底端

4.使用随机数据库的数据放入list内

public List<Question> getQuestions(int majorid,int num){         String majorType=new MajorDAO().getMajorType(majorid);         System.out.println("getQuestions==="+majorType);         try {             if ("安全".equals(majorType)) {                 log.debug("安全题库查找。。");                 String hql="from Question q where q.majorId= 13 or q.majorId= "+majorid+"and q.isPass='是' order by rand()";                 Query query = getSession().createQuery(hql);                 query.setMaxResults(num);                 List<Question>list=query.list();                 return list;             }else {                 log.debug("质量题库查找。。");                 String hql="from Question q where q.majorId= "+majorid+" and q.isPass='是' order by rand() ";                 Query query = getSession().createQuery(hql);                 query.setMaxResults(num);                 List<Question>list=query.list();                 return list;             }         } catch (RuntimeException re) {             log.error("查找失败", re);             throw re;         }     }