多的一方:不写任何关联关系
package com.eneitys;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name="tb_student")
public class Student {
private int id;
private String stuName;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getStuName() {
return stuName;
}
public void setStuName(String stuName) {
this.stuName = stuName;
}
}
一的一方:添加多的一方的Set集合
package com.eneitys;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
@Entity
@Table(name="tb_group")
public class Group {
private int id;
private String grpName;
private Set<Student> set=new HashSet<Student>();
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getGrpName() {
return grpName;
}
public void setGrpName(String grpName) {
this.grpName = grpName;
}
@OneToMany
@JoinColumn(name="group_id")
public Set<Student> getSet() {
return set;
}
public void setSet(Set<Student> set) {
this.set = set;
}
}
总结:
@OneToMany在默认情况下,Hibernate会把一对多当成多对多的一种特殊情况!结果就是会帮我们生成一张中间表,所以注意要写@JoinColumn(name="")注解,因为在数据库中,不管是一对多还是多对一,都是在多的一方添加一的一个外键,所以这个jioncolumn的name值一般写成一的外键字段名