java - 访问 jpa 实体关系时出现 NullPointerException

Question

当我尝试为关系调用 getter 时，出现空指针异常。据我了解，容器将在实体仍受管理时使用适当的列表对象填充此字段。

模式是预先存在的，所以这是一个自下而上的映射。

这是一对多关系的“一方”实体:

@Entity
@Table(name=CollectorHeader.TABLE_NAME)
public class CollectorHeader implements Serializable {
    ...
    @Id
    @Column(name = "COLLECTORHEADERID")
    private long id;

    @OneToMany(mappedBy="collectorHeader", fetch=FetchType.LAZY)
    private List<CollectorDetail> collectorDetails;
    ...
}

这是我的“多”面实体:

@Entity
@Table(name = CollectorDetail.TABLE_NAME)
public class CollectorDetail implements Serializable {
    ...
    @Id
    @Column(name = "COLLECTORDETAILID", unique = true)
    long id;
    ...
    @NotNull 
    @ManyToOne
    @JoinColumn(name = "COLLECTORHEADERID")
    private CollectorHeader collectorHeader;
    ...
    public CollectorDetail(long id, @NotNull CollectorHeader collectorHeader, 
                           long provenanceLinkPk, @NotNull String provenanceLinkClass) {
        setId(id);
        setCollectorHeader(collectorHeader);
        setProvenanceLinkPk(provenanceLinkPk);
        setProvenanceLinkClass(provenanceLinkClass);
    }
}

这就是我所说的关系:

public CollectorDetail createCollectorDetail(long collectorHeaderId, long     provenanceLinkPk, @NotNull String provenanceLinkClass) throws SystemException {
    CollectorHeader collectorHeader = em.find(CollectorHeader.class, collectorHeaderId);
    if(collectorHeader == null) {
        String error = "There is no Collector Header with the id: '" + collectorHeaderId + "'";
        log.error(error);
        throw new SystemException(error);
    }
    CollectorDetail collectorDetail =
        new CollectorDetail(NextNumberFactory.getInstance().getNextNumberLong("cocollectordetail")
                , collectorHeader
                , provenanceLinkPk
                , provenanceLinkClass);
    collectorHeader.getCollectorDetails().add(collectorDetail); //NULLPOINTEREXCEPTION
    em.merge(collectorDetail);
    em.merge(collectorHeader);
    return collectorDetail;
}

SQL 模式:

CREATE TABLE COCOLLECTORHEADER (
    COLLECTORHEADERID   DECIMAL(20,0) NOT NULL PRIMARY KEY,
    COLLECTEDTIMESTAMP TIMESTAMP, -- the date when the information was collected
    PROCESSEDTIMESTAMP TIMESTAMP, -- the date when the information was processed
    FILEFORMAT VARCHAR(16) NOT NULL,
    SEQUENCENUMBER INTEGER
);

CREATE TABLE COCOLLECTORDETAIL (
    COLLECTORDETAILID   DECIMAL(20,0) NOT NULL PRIMARY KEY,
    COLLECTORHEADERID   DECIMAL(20,0) NOT NULL
        REFERENCES COCOLLECTORHEADER(COLLECTORHEADERID),
    PROVENANCELINKPK        DECIMAL(20,0) NOT NULL,
    PROVENANCELINKCLASS VARCHAR(128) NOT NULL
);

如有任何帮助，我们将不胜感激。

Answer 1

使用有效的集合对象初始化 collectorDetails:

private List<CollectorDetail> collectorDetails = new ArrayList<CollectorDetail>();

同时检查 Is it good practice to initialize fields inside a JPA entity getter?