Java中解決 cannot be cast to java.io.Serializable的問題

1.問題描述：
有時候在 Entity 中join 另一個 Entity例如:

public class Animal extends DTO{
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", nullable = false, updatable = false, unique = true)
    private Long id;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.EAGER, targetEntity = People.class)
    @JoinColumn(name = "id", referencedColumnName = "id", nullable = true, insertable = false, updatable = false)
    private People people;
}

public class People{
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", nullable = false, updatable = false, unique = true)
}

運行的時候會報錯

cannot be cast to java.io.Serializable

2.解決在 Entity中加入 implements Serializable

public class People implements Serializable{
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", nullable = false, updatable = false, unique = true)
}

原因：Hibernate 有二級緩存， 緩存會將對象寫進硬盤。就必須序列化。以及兼容對象在網絡鐘的傳輸。