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，
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 -------------------------------------------------------------------- Criteria criteria = session.createCriteria(Student.class); ----------------------------------------------------------------------------------- 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. 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; } } |
