JPA 实体管理器问题
我对 JPA/Hibernate 和 Java 都很陌生，并且在使用 EntityManager 类管理持久对象的基础知识方面遇到了一些麻烦。如果有人能向我解释一些非常基本的东西，我将非常感激，因为我无法从文档中弄清楚。
JSE 环境中的 JPA 2 / Hibernate / Postgresql。
* 下面的类定义*
以下内容按我的预期工作：
em.getTransaction().begin();
Car corolla = new Car();
Part clutch = new Part();
clutch.setCar( corolla );
Part bumper = new Part();
bumper.setCar( corolla );
em.persist( corolla );
em.persist( clutch );
em.persist( bumper );
em.getTransAction().commit();
但这不会删除从零件到数据库中汽车的链接：
tx.begin();
corolla.getParts().clear();
tx.commit();
为什么会出现这种情况？
提前致谢，如果这是一个愚蠢的问题，我很抱歉。
麦克风。
汽车类别：
@Entity
public class Car {
private Long id;
private Set<Part> parts;
....
public Car() { parts = new HashSet<Part>(); }
@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }
@OneToMany( mappedBy="car", cascade=CascadeType.ALL )
public Set<Part> getParts() { return this.parts; }
public void setParts( Set<Part> parts ) { this.parts = parts; }
....
}
零件类别：
@Entity
public class Part {
private Long id;
private Car car;
...
public Part() {};
@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }
@ManyToOne
@JoinColumn( name="car_id" )
public Car getCar() { return this.car; }
public void setCar( Car car ) { this.car = car; }
...
}
I'm quite new to both JPA/Hibernate and to Java, and am having some trouble with the basics of using the EntityManager class to manage persistent objects. I would appreciate it very much if someone would explain something very basic to me, because I can't figure it out from the documentation.
JPA 2 / Hibernate / Postgresql in a JSE environment.
* Class definitions below *
The following works as I would expect it to:
em.getTransaction().begin();
Car corolla = new Car();
Part clutch = new Part();
clutch.setCar( corolla );
Part bumper = new Part();
bumper.setCar( corolla );
em.persist( corolla );
em.persist( clutch );
em.persist( bumper );
em.getTransAction().commit();
But this does not remove the links from the parts to the cars in the database:
tx.begin();
corolla.getParts().clear();
tx.commit();
Why is this the case?
Thanks in advance, and sorry if this is a silly question.
Mike.
Car class:
@Entity
public class Car {
private Long id;
private Set<Part> parts;
....
public Car() { parts = new HashSet<Part>(); }
@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }
@OneToMany( mappedBy="car", cascade=CascadeType.ALL )
public Set<Part> getParts() { return this.parts; }
public void setParts( Set<Part> parts ) { this.parts = parts; }
....
}
Part class:
@Entity
public class Part {
private Long id;
private Car car;
...
public Part() {};
@Id
@GeneratedValue( generator="increment" )
@GenericGenerator( name="increment", strategy = "increment" )
public Long getId() { return id; }
private void setId( Long id ) { this.id = id; }
@ManyToOne
@JoinColumn( name="car_id" )
public Car getCar() { return this.car; }
public void setCar( Car car ) { this.car = car; }
...
}
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当双向关系的双方不一致时，拥有方（即没有
mappedBy的一方）的状态将被持久化到数据库中。因此，您需要一致地修改双方：When sides of bidirectional relationship disagree, a state of the owning side (i.e. a side without
mappedBy) is persisted to the database. So, you need to modify both sides consitently:它看起来不错，但是在从数据库中删除实体时，您应该使用：
It looks fine, however while removing an entity from your database you should be using: