Hibernate查询特定几个字段
1. 问题场景
考试系统开发中，类似考题、答题卡等对象，可能包含如“题目内容”、“答案及评语”等信息量非常大的字段，而实际使用中，列表显示不需要查询所有的对象属性，这时若把所有属性全部取出来太耗费资源，因此查询特定的几个字段就可以。
2. 实现方法
说明：实现方法可能有很多，这里只列出亲测有效的两种：
1.通过hql语句实现，分别获取字段数据并手动组装成对象
Question.java
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package com.geariot.platform.exam.entities;
import java.util.Date;
import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table;
import org.hibernate.annotations.Cache; import org.hibernate.annotations.CacheConcurrencyStrategy; import org.hibernate.annotations.GenericGenerator;
@Entity @Table(name="question") @Cache(usage = CacheConcurrencyStrategy.READ_WRITE, region="javaClassName") public class Question {
private String id; private int level; //难度 private int type; //类别，单选题0,多选题1,填空题2,问答题3 private String typeName; //类别名称 private String abs;//题目缩略内容 private String content;//题目内容 private String subject;//科目名称 private String subjectId;//科目Id private Date createTime;//创建时间 private String answer; //标准答案
@Id @Column(length=32) @GeneratedValue(generator="system-uuid") @GenericGenerator(name="system-uuid", strategy="uuid") public String getId() { return id; } public void setId(String id) { this.id = id; }
public int getLevel() { return level; } public void setLevel(int level) { this.level = level; } public int getType() { return type; } public void setType(int type) { this.type = type; }
@Column(name = "type_name") public String getTypeName() { return typeName; } public void setTypeName(String typeName) { this.typeName = typeName; } public String getContent() { return content; } public void setContent(String content) { this.content = content; } public String getSubject() { return subject; } public void setSubject(String subject) { this.subject = subject; }
@Column(name = "subject_id") public String getSubjectId() { return subjectId; } public void setSubjectId(String subjectId) { this.subjectId = subjectId; }
@Column(name = "create_time") public Date getCreateTime() { return createTime; } public void setCreateTime(Date createTime) { this.createTime = createTime; } public String getAnswer() { return answer; } public void setAnswer(String answer) { this.answer = answer; }
public String getAbs() { return abs; } public void setAbs(String abs) { this.abs = abs; }
@Override public String toString() { return "Question [id=" + id + ", level=" + level + ", type=" + type + ", typeName=" + typeName + ", abs=" + abs + ", content=" + content + ", subject=" + subject + ", subjectId=" + subjectId + ", createTime=" + createTime + ", answer=" + answer + "]"; } } |
QuestionDaoImpl.java（附复合查询）
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public List<Question> getQuestions(Question q, int start, int number) { StringBuffer basicQueryStr ; basicQueryStr = new StringBuffer("select id, level, type, typeName, abs, subject from Question"); QuestionAndQueryCreator creator = new QuestionAndQueryCreator(q); String creatorStr = creator.createStatement(); Query query = QueryUtils.createLocalQuery(getSession(),basicQueryStr, creatorStr,ORDER_CON.DESC_ORDER); query.setCacheable(true); query.setFirstResult(start);//设置起始行 query.setMaxResults(number);//每页条数
List<Question> list = new ArrayList<Question>(); List<Object[]> resultList = query.list(); for(Object[] obj:resultList) { Question que = new Question();
que.setId((String)obj[0]); que.setLevel((int)obj[1]); que.setType((int)obj[2]); que.setTypeName((String)obj[3]); que.setAbs((String)obj[4]); que.setSubject((String)obj[5]);
list.add(que); } return query.list(); } |
浏览器调用接口（省略）返回json结果：
{"data":[["abc321",0,1,"多选","题目","历史"]],"code":"0","msg":"成功","size":1} |
发现返回的”data”对象少了属性名，只有属性值，这样不太复合要求。
2. 通过HQL语句new POJO()实现
Question.java
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package com.geariot.platform.exam.entities;
import java.util.Date;
import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table;
import org.hibernate.annotations.Cache; import org.hibernate.annotations.CacheConcurrencyStrategy; import org.hibernate.annotations.GenericGenerator;
@Entity @Table(name="question") @Cache(usage = CacheConcurrencyStrategy.READ_WRITE, region="javaClassName") public class Question {
//因为"select new Question(id, level, type, typeName, abs, subject) from Question" //所以必须创建带有待查询参数的构造方法和空的构造方法 public Question(String id, int level, int type, String typeName, String abs, String subject) { this.id = id; this.level = level; this.type = type; this.typeName = typeName; this.abs = abs; this.subject = subject; } public Question() {
} private String id; private int level; //难度 private int type; //类别，单选题0,多选题1,填空题2,问答题3 private String typeName; //类别名称 private String abs;//题目缩略内容 private String content;//题目内容 private String subject;//科目名称 private String subjectId;//科目Id private Date createTime;//创建时间 private String answer; //标准答案
@Id @Column(length=32) @GeneratedValue(generator="system-uuid") @GenericGenerator(name="system-uuid", strategy="uuid") public String getId() { return id; } public void setId(String id) { this.id = id; }
public int getLevel() { return level; } public void setLevel(int level) { this.level = level; } public int getType() { return type; } public void setType(int type) { this.type = type; }
@Column(name = "type_name") public String getTypeName() { return typeName; } public void setTypeName(String typeName) { this.typeName = typeName; } public String getContent() { return content; } public void setContent(String content) { this.content = content; } public String getSubject() { return subject; } public void setSubject(String subject) { this.subject = subject; }
@Column(name = "subject_id") public String getSubjectId() { return subjectId; } public void setSubjectId(String subjectId) { this.subjectId = subjectId; }
@Column(name = "create_time") public Date getCreateTime() { return createTime; } public void setCreateTime(Date createTime) { this.createTime = createTime; } public String getAnswer() { return answer; } public void setAnswer(String answer) { this.answer = answer; }
public String getAbs() { return abs; } public void setAbs(String abs) { this.abs = abs; }
@Override public String toString() { return "Question [id=" + id + ", level=" + level + ", type=" + type + ", typeName=" + typeName + ", abs=" + abs + ", content=" + content + ", subject=" + subject + ", subjectId=" + subjectId + ", createTime=" + createTime + ", answer=" + answer + "]"; }
} |
QuestionDaoImpl.java（附复合查询）
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public List<Question> getQuestions(Question q, int start, int number) { StringBuffer basicQueryStr ; basicQueryStr = new StringBuffer("select new Question(id, level, type, typeName, abs, subject) from Question"); QuestionAndQueryCreator creator = new QuestionAndQueryCreator(q); String creatorStr = creator.createStatement(); Query query = QueryUtils.createLocalQuery(getSession(),basicQueryStr, creatorStr,ORDER_CON.DESC_ORDER); query.setCacheable(true); query.setFirstResult(start);//设置起始行 query.setMaxResults(number);//每页条数
return query.list(); } |
接口调用（省略）json返回结果
{"data":[{"typeName":"多选","id":"abc321","abs":"题目","level":0,"subject":"历史","type":1}],"code":"0","msg":"成功","size":1} |