@OneToMany mappedBy映射到_____

Question

什么mappedBy地图来？或者,相反,它应该映射到什么？

headers下面的字段@Entity Foo按照@OneToMany 文档进行映射？然后Foo将是一个包装javax.mail.Header？

package net.bounceme.dur.usenet.model;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.mail.Header;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.persistence.*;

@Entity
public class Articles implements Serializable {

    private static final long serialVersionUID = 1L;
    private static final Logger LOG = Logger.getLogger(Articles.class.getName());
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column
    private String subject;

    @OneToMany(mappedBy="foo")  //a wrapper for Header is needed?
    private List<Header> headers = new ArrayList<>();

    public Articles() {
    }

    public Articles(Message message) {
        try {
            subject = message.getSubject();
        } catch (MessagingException ex) {
            Logger.getLogger(Articles.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Articles)) {
            return false;
        }
        Articles other = (Articles) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return subject;
    }

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }
}

Answer 1

是的,需要javax.mail.Header的包装器,通常你不能直接持久化任意类,特别是那些不是Serializable的类.它们也不能成为设计实体之间关系的列表元素.

mappedBy的值是拥有双向关系一侧的字段的名称.为了举例,我们假设Article实体与Foo实体之间存在一对多的关系:

@OneToMany(mappedBy="article")
private List<Foo> headers;

现在我们知道这个关系必须有另一端,并且它是属性,它位于Foo实体,确实有Article作为类型并被命名为article:

@Entity
public class Foo {
...
   @ManyToOne
    Article article;
}

当双向关系持久保存到数据库时,使用所有方(在本例中为Foo中的文章).在JPA 2.0规范中,这用以下词语告知:

管理实体之间的双向关系将基于关系所属方所持有的引用而持久化.开发人员有责任保持内存引用保持在拥有方,而保持在反方保持的内容引用在更改时保持一致.在单向一对一和一对多关系的情况下,开发人员有责任确保遵守关系的语义.