有如下两个实体类 ， Student和Grade 为多对一关系。
Student.java
@Entity
public class Student {
private Integer id;
private String name;
private Grade grade;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@ManyToOne(cascade={CascadeType.PERSIST})
@JoinColumn(name="gradeid")
public Grade getGrade() {
return grade;
}
public void setGrade(Grade grade) {
this.grade = grade;
} }
Grade.java
@Entity
public class Grade { private Integer id;
private String name; @Id
@GeneratedValue(strategy=GenerationType.AUTO)
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
} }
可以看到Student类的grade属性已经设置了 @ManyToOne(cascade={CascadeType.PERSIST})
但只save Student对象时依然会报 org.hibernate.TransientObjectException 异常,如下：
public class Init {
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Grade g = new Grade();
g.setName("一班");
Student s = new Student();
s.setName("张三");
s.setGrade(g);
session.save(s);
session.getTransaction().commit();
}
}
运行结果：
Exception in thread "main" org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: accp.hib.Grade
解决办法如下：
方法一：改用persist方法来保存
把session.save(s); 更改为 session.persist(s);
再次保存成功。
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: insert into Grade (name, id) values (?, ?)
Hibernate: insert into Student (gradeid, name, id) values (?, ?, ?)
方法二：使用hibernate的自定义的级联，不要使用jpa的
把Student类grade的注解改为hibernate的注解
@ManyToOne(cascade={CascadeType.PERSIST})
@JoinColumn(name="gradeid")
改为:
@ManyToOne
@JoinColumn(name="gradeid")
@Cascade(value={CascadeType.SAVE_UPDATE})
注意，这里的CascadeType和@Cascade都是org.hibernate.annotations包下的，所以把
import javax.persistence.CascadeType;
改为：import org.hibernate.annotations.CascadeType;
并加上：import org.hibernate.annotations.Cascade;
再次保存成功。