解决hibernate更新中间表时，会先删除再更新的问题

问题：如题，hibernate在更新中间表时，会先删除旧的数据再插入新数据的问题。
情景：比如现在数据库有一张user表，一张role表，一张user_role中间表

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• User实体类

@Data
@Entity
@Table(name = "user")
public class User implements Serializable {
    private static final long serialVersionUID = -6325275400468409210L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Expose
    private long id;
    @JsonIgnore
    @Column
    private String uid;
    @Column
    @Expose
    private String phone;
    @Column
    @Expose
    private String email;
    @Column
    @JsonIgnore
    private String password;
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    @Expose
    private List<Role> roles;

• 测试：当user有角色“ROLE_USER_NORMAL”的前提下，想给user在添加一个角色“ROLE_USER_ADMIN”，这是会出现user_role表中id为1的数据被删除，然后添加了两条新数据，id分别为2和3，其中id为2的数据和被删除的id为1的数据相同。

@Test
public void testAddRoleForUser() throws Exception {
    User user = userRepository.findByPhone("183xxxxxxxx");
    Role role = roleRepository.findByName("ROLE_USER_ADMIN");
    user.getRoles().add(role);
    userRepository.save(user);
}

• 解决：
  1、将List<Role> roles改为Set<Role> roles。
  2、重写主表实体类的equals()和hasCode()方法。

@Data
@Entity
@Table(name = "user")
public class User implements Serializable {
    private static final long serialVersionUID = -6325275400468409210L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Expose
    private long id;
    @JsonIgnore
    @Column
    private String uid;
    @Column
    @Expose
    private String phone;
    @Column
    @Expose
    private String email;
    @Column
    @JsonIgnore
    private String password;
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    @Expose
    private Set<Role> roles;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof User)) return false;
        return false;
    }

    @Override
    public int hashCode() {
        int result = 20;
        result = (int) (result * 31 + id);
        result = result * 31 + uid.hashCode();
        result = result * 31 + phone.hashCode();
        result = result * 31 + email.hashCode();
        result = result * 31 + password.hashCode();
        return result;
    }

此时再测试时，将不会删除id为1的数据，而是直接添加id为2的新数据。
再来测试一下从两个角色中删除“ROLE_USER_ADMIN”这个角色。

@Test
public void testDeleteRoleFromUser() throws Exception {
    User user = userRepository.findByPhone("18349178884");
    Set<Role> roles = user.getRoles();
    Iterator<Role> it = roles.iterator();
    //roles.removeIf(role1 -> role1.getName().equals("ROLE_USER_ADMIN")); 等价于下面的while循环
    while (it.hasNext()) {
        Role role = it.next();
        if (role.getName().equals("ROLE_USER_ADMIN")) {
            it.remove();
        }
    }
    log.info("roles: {}", roles);
    userRepository.save(user);
}

成功！！！