java - 如何修复正确添加到设置 Hibernate 实体

Question

我读了一篇关于正确重定义 equals/hashCode 的文章: https://vladmihalcea.com/how-to-implement-equals-and-hashcode-using-the-jpa-entity-identifier/

执行这些覆盖是为了不丢失已写入集合的记录。

代码:

@Entity
public class Client {

    @Id
    @Column
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column
    private String name;

    public Client() {
    }

    public Client(String name) {
        this.name = name;
    }

    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 boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Client client = (Client) o;
        return Objects.equals(id, client.id) &&
                Objects.equals(name, client.name);
    }

    public int hashCode() {
        return 31;
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder("Client{");
        sb.append("id=").append(id);
        sb.append(", name='").append(name).append('\'');
        sb.append('}');
        return sb.toString();
    }
}

然后我测试我的类以确保它正常工作:

@Transactional
public class ClientTest {

    @PersistenceContext
    protected EntityManager em;

    @Test
    public void storeToSetBeforeMerge_ShouldBeContains() {
        Set<Client> map = new HashSet<>();
        Client client1 = new Client("John");
        Client client2 = new Client("Mike");
        map.add(client1);
        map.add(client2);

        Client merge1 = em.merge(client1);
        Client merge2 = em.merge(client2);

        assertTrue(map.contains(merge1));   // not true!
        assertTrue(map.contains(merge2));   // not true!
    }
}

我的问题是为什么条件不满足。毕竟，我已经指出 hashCode 返回相同的值:31。我做错了什么？

我无法理解这个决定的意义。如果这个解决方案不能解决问题，我无法从Set中找到我需要的元素

Answer 1

您没有像文章中那样在 merge() 之前调用 persist() 。文章作者在第一条评论中对此进行了解释。

Merge is for integrating changes on detached entities, which have been persisted previously.

新实体的生命周期从persist()开始。然后在具有 ID 的分离实体上调用 merge() ，条件将得到满足。