因为在使用Hibernate的Annotation时遇到坑,坑了一晚上时间,所以写一篇文章记一下经验
如果并没有对Hibernate入门,还请在课室或者在技研中心,接入学校的教学网络,进入服务器smb://10.15.231.233/Video (在技研的网络里叫OPENSUSE-SERVER)。在 数据库->ORM->Hibernate 目录内查看视频教程了解
我的项目使用的是Hibernate 4.3.11.Final,是一个Maven工程。所用到的辅助工具有fastjson,用来把类直接输出成json查看,还有junit4,测试用。
依赖列表(pom.xml)节选
org.hibernate
hibernate-core
4.3.11.Final
org.slf4j
slf4j-simple
1.7.18
mysql
mysql-connector-java
5.1.38
junit
junit
4.8.2
test
com.alibaba
fastjson
1.2.8
我的工程是一个有关于评分的系统。在里面有两个类,一个是评分模板MarkingTemplate,一个是评分项目MarkingItem。
MarkingTemplate在数据库中的字段如下
字段名 | 数据类型 | 备注 |
---|---|---|
id | INT | 数据编号(自动增长) |
title | nvarchar(50) | 模板标题 |
commit | nvarchar(50) | 模板备注 |
实体类字段如下:
private int id;
private String title;
private String commit;
private int value;
private MarkingTemplate markingTemplate;
MarkingItem在数据库里的字段如下
字段名 | 数据类型 | 备注 |
---|---|---|
id | INT | 数据编号(自增长) |
title | nvarchar(50) | 评分项目标题 |
commit | nvarchar(50) | 评分项目备注 |
value | INT | 项目分值(只有整数) |
belong_to_template | INT | 所属模板的编号(外键链接至contest_templates) |
实体类字段如下:
private int id;
private String title;
private String commit;
private List markingItems;
很明显,我数据库里只有MarkingItem到MarkingTemplate的关联,没有明显的反过来的关联。看视频教程的话,都是用的实体类配置文件实现这些关系,但是我想尝试使用Annotation(注解)来实现这个关系,因为本身项目简单,而且注解很简洁很方便。
MarkingTemplate类源码和注解
@Entity
@Table(name = "marking_templates")
public class MarkingTemplate {
private int id;
private String title;
private String commit;
private List markingItems;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getCommit() {
return commit;
}
public void setCommit(String commit) {
this.commit = commit;
}
@OneToMany(mappedBy = "markingTemplate")
public List getMarkingItems() {
return markingItems;
}
public void setMarkingItems(List markingItems) {
this.markingItems = markingItems;
}
}
MarkingItem源码和注解
@Entity
@Table(name = "marking_items")
public class MarkingItem {
private int id;
private String title;
private String commit;
private int value;
private MarkingTemplate markingTemplate;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getCommit() {
return commit;
}
public void setCommit(String commit) {
this.commit = commit;
}
public int getValue() {
return value;
}
public void setValue(int value) {
this.value = value;
}
@ManyToOne(targetEntity = MarkingTemplate.class)
@JoinColumn(name = "belong_to_template")
public MarkingTemplate getMarkingTemplate() {
return markingTemplate;
}
public void setMarkingTemplate(MarkingTemplate markingTemplate) {
this.markingTemplate = markingTemplate;
}
}
如果要实现双向的关联,那么需要在两边的类关联的字段所对应的属性声明上添加注解。
先解释一下MarkingItem的getMarkingTemplate()
@ManyToOne(targetEntity = MarkingTemplate.class)
@JoinColumn(name = "belong_to_template")
public MarkingTemplate getMarkingTemplate() {
return markingTemplate;
}
从数据库里提取belong_to_template字段的数值然后再利用其查询即可得出MarkingTemplate的数值。@ManyToOne
表示了一个多对一的关系。模板内项目对模板是多对一关系。targetEntity标明这个字段是关联到哪个类,通过这个,Hibernate可以很方便地把这个查出来的号码把MarkingItems所属的MarkingTemplate实体化出来。@JoinColumn
表明插入列的字段名称,这个可以省去,我添加这个的原因是因为类中的属性名和数据库内的字段名不相符。
再解释一下MarkingTemplate的getMarkingItems()
@OneToMany(mappedBy = "markingTemplate")
public List getMarkingItems() {
return markingItems;
}
@OneToMany
注解声明了一个一对多关系。从数据库字段表可以看出,模板对模板内项目是一对多关系。一个模板拥有多个模板内项目,这些项目在类里被存放在一个List里。而在数据库里,是每个项目都有一个字段标明它们属于哪个模板。mappedBy参数表明这个被在Many一方的类的那个属性给映射了。
当查出实体数据之后,Hibernate默认是对关联属性的数据进行延迟加载,就是等到调用get方法的时候再去生成SQL语句查询这个类。
测试类里我利用fastjson直接把数据可视化了。
获取MarkingTemplate
@Test
public void TestGetMarkingTemplate(){
System.out.println(JSON.toJSONString(session.get(MarkingTemplate.class,1)));
}
输出结果
{
"commit": "通用模板",
"id": 1,
"markingItems": [
{
"commit": "结合竞赛主题,作品选题具有实用性、合理性、创新性",
"id": 6,
"markingTemplate": {
"$ref": "$"
},
"title": "选题",
"value": 10
},
{
"commit": "界面友好,完成速度快,操作简洁明了",
"id": 7,
"markingTemplate": {
"$ref": "$"
},
"title": "界面",
"value": 20
},
{
"commit": "采用的软件开发方法与技术是否先进及与项目相宜",
"id": 8,
"markingTemplate": {
"$ref": "$"
},
"title": "技术",
"value": 10
},
{
"commit": "功能强大,具有实用性",
"id": 9,
"markingTemplate": {
"$ref": "$"
},
"title": "功能",
"value": 40
},
{
"commit": "作品的市场潜力与价值",
"id": 10,
"markingTemplate": {
"$ref": "$"
},
"title": "市场",
"value": 10
},
{
"commit": "对问题的实际理解程度",
"id": 11,
"markingTemplate": {
"$ref": "$"
},
"title": "答辩",
"value": 10
}
],
"title": "模板1"
}
因为fastjson在生成json string的时候调用了里面的get方法,所以这里没体现hibernate的延迟加载。
获取MarkingItem
@Test
public void TestGetMarkingItems(){
List markingItems = session.createQuery("from MarkingItem").setFirstResult(0).setMaxResults(9).list();
for(Object o: markingItems){
System.out.println(JSON.toJSONString(o));
}
}
输出结果
{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"$ref":"$"},{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"界面","value":20},{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"技术","value":10},{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"功能","value":40},{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"市场","value":10},{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"答辩","value":10}],"title":"模板1"},"title":"选题","value":10}
{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"选题","value":10},{"$ref":"$"},{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"技术","value":10},{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"功能","value":40},{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"市场","value":10},{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"答辩","value":10}],"title":"模板1"},"title":"界面","value":20}
{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"选题","value":10},{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"界面","value":20},{"$ref":"$"},{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"功能","value":40},{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"市场","value":10},{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"答辩","value":10}],"title":"模板1"},"title":"技术","value":10}
{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"选题","value":10},{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"界面","value":20},{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"技术","value":10},{"$ref":"$"},{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"市场","value":10},{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"答辩","value":10}],"title":"模板1"},"title":"功能","value":40}
{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"选题","value":10},{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"界面","value":20},{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"技术","value":10},{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"功能","value":40},{"$ref":"$"},{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"答辩","value":10}],"title":"模板1"},"title":"市场","value":10}
{"commit":"对问题的实际理解程度","id":11,"markingTemplate":{"commit":"通用模板","id":1,"markingItems":[{"commit":"结合竞赛主题,作品选题具有实用性、合理性、创新性","id":6,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"选题","value":10},{"commit":"界面友好,完成速度快,操作简洁明了","id":7,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"界面","value":20},{"commit":"采用的软件开发方法与技术是否先进及与项目相宜","id":8,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"技术","value":10},{"commit":"功能强大,具有实用性","id":9,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"功能","value":40},{"commit":"作品的市场潜力与价值","id":10,"markingTemplate":{"$ref":"$.markingTemplate"},"title":"市场","value":10},{"$ref":"$"}],"title":"模板1"},"title":"答辩","value":10}
嗯。。。这里把模板也加载出来了。