java - 在 Hibernate 中添加 Employee 时出现 NullPointerException

Question

我有一个 Hibernate 实体“Intranet”，其中包含以下代码:

// Imports removed

@Entity
@Table(name = "intranets", uniqueConstraints = @UniqueConstraint(columnNames = "company_name"))
@Cacheable
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
public class Intranet {

    @Id
    @Column(name = "INTRANET_ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int intranetId;

    @Column(name = "owner_id")
    private int ownerId;

    @Column(name = "setup_done")
    private boolean isSetUp = false;

    @Column(name = "company_name")
    private String companyName;

    @ElementCollection
    @CollectionTable(name = "employees")
    @Column(name = "employee")
    private List<Integer> employeeIds;


    public int getIntranetId() {

        return intranetId;
    }

    public void setIntranetId(int intranetId) {

        this.intranetId = intranetId;
    }

    public boolean isSetUp() {

        return isSetUp;
    }

    public void setSetUp(boolean isSetUp) {

        this.isSetUp = isSetUp;
    }

    public String getCompanyName() {

        return companyName;
    }

    public void setCompanyName(String companyName) {

        this.companyName = companyName;
    }

    public int getOwnerId() {

        return ownerId;
    }

    public void setOwnerId(int ownerId) {

        this.ownerId = ownerId;
    }

    public List<Integer> getEmployeeIds() {

        return employeeIds;
    }

    public void setEmployeeIds(List<Integer> employeeIds) {

        this.employeeIds = employeeIds;
    }

    public void addEmployee(int id) {
        this.employeeIds.add(id);
    }

    public void removeEmployee(int id) {
        this.employeeIds.remove(new Integer(id));
    }
}

如您所见，我添加了两个方法，它们并不直接是 getter 和 setter，但它们就像方便的方法，可以轻松访问集合并添加/删除员工。

当我在程序中调用“addEmployee()”时，我收到一个简单的 NullPointerException。问题基本上是如何将集合初始化为不具有null，因为我读到Hibernate使用集合的内部实现，而不是简单地采用ArrayList.

感谢您提前提供帮助。

Answer 1

你有一个NullPointerException，因为你必须自己初始化列表。您使用哪种实现并不重要。这将解决您的问题:

@ElementCollection
@Column(name = "employee")
private List<Integer> employeeIds = new ArrayList<>();