Question

我正在尝试使用Hibernate在 Oracle数据库中存储几个对象。但是，我似乎无法正确地建立映射关系。就目前而言，我收到了 NullPointerException 。

最后，每个 Client 应包含 ID，姓氏，电子邮件和一组预订。

每个航班应包含航班号，出发时间和出发机场。

预订应具有 ID，客户，航班，预订日期和预订座位数。

目前，我只是在一个客户端上对其进行测试，以免使其过于复杂，但我感到相当困惑。

这是我到目前为止所做的：

主要

import java.sql.Date;
import java.sql.Time;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class JPAApplication {
    private EntityManagerFactory entityManagerFactory;

    public JPAApplication() {
        Logger.getLogger("org.hibernate").setLevel(Level.ALL);
        entityManagerFactory = Persistence.createEntityManagerFactory("DB1");
    }

    public void testFlights() {
        EntityManager em = entityManagerFactory.createEntityManager();
        em.getTransaction().begin();
        Client firstClient = new Client("Kiro", "Betona", "kirobetona@gmail.com");      
        em.persist(firstClient);
    }

    public static void main(String[] args) {
        JPAApplication app = new JPAApplication();
        app.testFlights();
    }
}

客户

import java.util.*;
import javax.persistence.*;
import javax.validation.constraints.NotNull;

@Entity
public class Client {

    private int id; 

    private String firstName;   

    private String lastName;

    private String email;

    private Set<Booking> reservations; 

    Client(String firstName, String lastName, String email) {       
            this.firstName = firstName;
            this.lastName = lastName;
            this.email = email;
    }

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    public int getId() {
        return id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public void setId(int id) {
        this.id = id;
    }

    @OneToMany(targetEntity = Booking.class, mappedBy="client", fetch=FetchType.EAGER)
    public Set<Booking> getReservations() {
        return reservations;
    }

    public void setReservations(Set<Booking> reservations) {
        this.reservations = reservations;
    }
}

预订

import java.util.Date;
import javax.persistence.*;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
import javax.validation.constraints.NotNull;

@Entity
public class Booking {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    private Client client;

    private Flight flight;

    private int bookedSeats = 1;

    @Temporal(TemporalType.DATE)
    private Date bookingDate;

    Booking(Client client, Flight flight, Date bookingDate) {       
            this.client = client;
            this.flight = flight;
            this.bookingDate = bookingDate;         
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @ManyToOne
    @JoinColumn(name="id")
    public Client getClient() {
        return client;
    }

    @OneToOne
    @PrimaryKeyJoinColumn(name="flight")
    public Flight getFlight() {
        return flight;
    }

    public int getBookedSeats() {
        return bookedSeats;
    }

    public void setBookedSeats(int bookedSeats) {
        this.bookedSeats = bookedSeats;
    }

    public Date getBookingDate() {
        return bookingDate;
    }

    public void setBookingDate(Date bookingDate) {
        this.bookingDate = bookingDate;
    }

    public void setClient(Client client) {
        this.client = client;
    }

    public void setFlight(Flight flight) {
        this.flight = flight;
    }
}

飞行

import java.sql.Time;
import javax.persistence.*;
import javax.validation.constraints.NotNull;

@Entity
public class Flight {

    private String flightNumber;

    private Time startingTime;

    private String startingAirport;

    Flight(String flightNumber, Time startingTime, String startingAirport) {        
            this.flightNumber = flightNumber;
            this.startingTime = startingTime;
            this.startingAirport = startingAirport;     
    }

    @Id
    @OneToOne(targetEntity = Booking.class, mappedBy="flight", fetch=FetchType.EAGER)
    @PrimaryKeyJoinColumn(name="flight")
    public String getFlightNumber() {
        return flightNumber;
    }

    public Time getStartingTime() {
        return startingTime;
    }

    public void setStartingTime(Time startingTime) {
        this.startingTime = startingTime;
    }

    public String getStartingAirport() {
        return startingAirport;
    }

    public void setStartingAirport(String startingAirport) {
        this.startingAirport = startingAirport;
    }

    public void setFlightNumber(String flightNumber) {
        this.flightNumber = flightNumber;
    }

}

Answer 1

当您在Booking中使用@JoinColumn批注时，我会感到有些困惑：您要让Booking使用名为“ id”的列作为客户端的外键，但是与Booking主键冲突。

您可以改为将列名称分开，如下所示：

@JoinColumn（name =“ clientId”）

另外，对于Hibernate实例化Entity对象，实体should have a no-arg constructor。在示例代码中，默认的no-arg构造函数已被覆盖。

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