在实施@OneToMany双向关系时，有很多方法会搞砸。这里说明一下最佳实践方式：
关键点：
- 始终从父级到子级实现级联
- 在父类上使用mappedBy
- 在父类上使用orphanRemoval以删除父类不再引用的子类
- 在父类上使用helper方法可以使关联的两端保持同步
- 始终使用延迟提取
- 使用自然/业务key或使用实体标识符并覆盖equals()，hashCode()如此处所示
源代码可以在这里找到 。父类：
@Entity
public class Tournament implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(cascade = CascadeType.ALL,
mappedBy = "tournament", orphanRemoval = true)
private List<TennisPlayer> tennisPlayers = new ArrayList<>();
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子类：
@Entity
public class TennisPlayer implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "tournament_id")
private Tournament tournament;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Tournament getTournament() {
return tournament;
}
public void setTournament(Tournament tournament) {
this.tournament = tournament;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof TennisPlayer)) {
return false;
}
return id != null && id.equals(((TennisPlayer) obj).id);
}
@Override
public int hashCode() {
return 2018;
}
}
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