java - JPA 持久化具有一对多关系的实体

Question

配置

• EcliplseLink 2.3.2
• JPA 2.0
• 实体是使用 Entity Classes from Database... 向导从 netbeans 的数据库模式自动创建的。
• Controller 类是使用 JPA Controller Classes from Entity Classes... 向导从 netbeans 自动创建的

问题的简短版本

在经典场景中，两个表具有一对多关系。我先创建父实体，然后创建子实体，然后将子实体附加到父实体的集合中。当我创建( Controller 方法)父实体时，我希望子实体被创建并与父实体相关联。为什么没有发生？

长版

父类

@Entity
@XmlRootElement
public class Device implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    private Integer id;
    @Column(unique=true)
    private String name;
    @Temporal(TemporalType.TIMESTAMP)
    private Date updated;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "deviceId")
    private Collection<NetworkInterface> networkInterfaceCollection;

    public Device() {
    }

    public Device(String name) {
        this.name = name;
        updated = new Date();
    }

    // setters and getters...

    @XmlTransient
    public Collection<NetworkInterface> getNetworkInterfaceCollection() {
        return networkInterfaceCollection;
    }

    public void setNetworkInterfaceCollection(Collection<NetworkInterface> networkInterfaceCollection) {
        this.networkInterfaceCollection = networkInterfaceCollection;
    }

    public void addNetworkInterface(NetworkInterface net) {
        this.networkInterfaceCollection.add(net);
    }

    public void removeNetworkInterface(NetworkInterface net) {
        this.networkInterfaceCollection.remove(net);
    }
    // other methods
}

子类

@Entity
@Table(name = "NETWORK_INTERFACE")
@XmlRootElement
public class NetworkInterface implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    private Integer id;
    private String name;
    @Temporal(TemporalType.TIMESTAMP)
    private Date updated;
    @JoinColumn(name = "DEVICE_ID", referencedColumnName = "ID")
    @ManyToOne(optional = false)
    private Device deviceId;

    public NetworkInterface() {
    }

    public NetworkInterface(String name) {
        this.name = name;
        this.updated = new Date();
    }

    // setter and getter methods...

    public Device getDeviceId() {
        return deviceId;
    }

    public void setDeviceId(Device deviceId) {
        this.deviceId = deviceId;
    }
}

主类

public class Main {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("wifi-dbPU");
        DeviceJpaController deviceController = new DeviceJpaController(emf);
        NetworkInterfaceJpaController netController = new NetworkInterfaceJpaController(emf);

        Device device = new Device("laptop");
        NetworkInterface net = new NetworkInterface("eth0");

        device.getNetworkInterfaceCollection().add(net);
        deviceController.create(device);
    }
}

此类在行中抛出 NullPointerException:device.getNetworkInterfaceCollection().add(net);

系统知道有一个新实体 device 并且它的集合中有一个元素 net。我希望它在 db 中写入 device，获取设备的 id，将其附加到 net 并将其写入 db。

而不是这个，我发现这些是我必须做的步骤:

deviceController.create(device);
net.setDeviceId(device);
device.getNetworkInterfaceCollection().add(net);
netController.create(net);

当父类知道它是子类并且应该为我创建它时，为什么我必须创建子类？

DeviceJpaController 的创建方法(抱歉，字段中的名称很长，它们是自动生成的)。

public EntityManager getEntityManager() {
    return emf.createEntityManager();
}

public void create(Device device) {
    if (device.getNetworkInterfaceCollection() == null) {
        device.setNetworkInterfaceCollection(new ArrayList<NetworkInterface>());
    }
    EntityManager em = null;
    try {
        em = getEntityManager();
        em.getTransaction().begin();
        Collection<NetworkInterface> attachedNetworkInterfaceCollection = new ArrayList<NetworkInterface>();
        for (NetworkInterface networkInterfaceCollectionNetworkInterfaceToAttach : device.getNetworkInterfaceCollection()) {
            networkInterfaceCollectionNetworkInterfaceToAttach = em.getReference(networkInterfaceCollectionNetworkInterfaceToAttach.getClass(), networkInterfaceCollectionNetworkInterfaceToAttach.getId());
            attachedNetworkInterfaceCollection.add(networkInterfaceCollectionNetworkInterfaceToAttach);
        }
        device.setNetworkInterfaceCollection(attachedNetworkInterfaceCollection);
        em.persist(device);
        for (NetworkInterface networkInterfaceCollectionNetworkInterface : device.getNetworkInterfaceCollection()) {
            Device oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface = networkInterfaceCollectionNetworkInterface.getDeviceId();
            networkInterfaceCollectionNetworkInterface.setDeviceId(device);
            networkInterfaceCollectionNetworkInterface = em.merge(networkInterfaceCollectionNetworkInterface);
            if (oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface != null) {
                oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface.getNetworkInterfaceCollection().remove(networkInterfaceCollectionNetworkInterface);
                oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface = em.merge(oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface);
            }
        }
        em.getTransaction().commit();
    } finally {
        if (em != null) {
            em.close();
        }
    }
}

Answer 1

我终于理解了持久化一对多实体背后的逻辑。过程是:

1. 创建父类
2. 坚持下去
3. 创建子类
4. 将 child 与 parent 联系起来
5. 持久化子集合(更新父集合)

附代码:

public class Main {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("wifi-dbPU");
        DeviceJpaController deviceController = new DeviceJpaController(emf);
        NetworkInterfaceJpaController netController = new NetworkInterfaceJpaController(emf);

        Device device = new Device("laptop");                 // 1
        deviceController.create(device);                      // 2

        NetworkInterface net = new NetworkInterface("eth0");  // 3
        net.setDeviceId(device.getId());                      // 4
        netController.create(net);                            // 5 
        // The parent collection is updated by the above create     
    }
}

现在，我可以找到一个设备(例如带有 id 的设备)并且我可以让它的所有子设备使用

Collection<NetworkInterface> netCollection = device.getNetworkInterfaceCollection()

在我在问题中发布的设备实体类中，不需要方法 addNetworkInterface 和 removeNetwokrInterface。