如何使用 JPA Hibernate 将 EntityManager 注入 DAO 或 Servlet

Question

当我在 Servlet 中调用 onGet 方法时，出现 NullPointerException。我检查了它，DAO 中的方法已正确实现。我应该在哪里注入 entityManagaer 对象以及如何注入？在道？还是在 Servlet 中？我试图用注释来做，但它不起作用。

在方法 onGet 中，我从 ReservationDAO class 调用方法 getAllReservations() 并且该方法导致 Nullpointerexception。我想从我的数据库中检索数据。这个方法应该 return 我想要得到的来自 dB 的所有数据。以及如何解决？

数据库存储两个表，其中包含两个 Reservation 对象和两个 Person 对象

我的代码如下：

ReservationServlet class

package servlets;

import DAO.ReservationDAO;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import model.Reservation;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

@WebServlet("reservation")
public class ReservationServlet extends HttpServlet {

ReservationDAO reservationDAO = new ReservationDAO();
Gson gson = new GsonBuilder().setPrettyPrinting().create();

@Override
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    List<Reservation> reservationsList = reservationDAO.getAllReservations();
    System.out.println("reservationsList size: " + reservationsList.size());
    //String json = gson.toJson(reservationsList);

    response.getWriter().write(reservationsList.get(0).getPerson().getName());
}
}

ReservationDAO class

package DAO;

import model.Reservation;

import javax.persistence.EntityManager;
import javax.persistence.*;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;   
import javax.persistence.criteria.Root;
import javax.transaction.Transactional;
import java.util.List;

public class ReservationDAO {

@PersistenceUnit(unitName="restapi")
EntityManagerFactory emf = Persistence.createEntityManagerFactory("restapi");
@PersistenceContext(unitName="restapi")
EntityManager em;

@Transactional
public List<Reservation> getAllReservations() {
    CriteriaBuilder builder = em.getCriteriaBuilder();
    CriteriaQuery<Reservation> criteriaQuery = builder.createQuery(Reservation.class);

    Root<Reservation> employee = criteriaQuery.from(Reservation.class);
    criteriaQuery.select(employee);

    TypedQuery<Reservation> query = em.createQuery(criteriaQuery);
    return query.getResultList();
}

public Reservation getReservationById(EntityManager em, int id) {
    Query query = em.createQuery("FROM Reservation WHERE id =" + id);
    return (Reservation) query.getSingleResult();
}


public void updateReservation(EntityManager em, int id, Reservation reservation) {
    Query query = em.createQuery("UPDATE Reservation SET table_number = :tn, start_time = :st, end_time = :et WHERE id = :id");
    query.setParameter("tn", reservation.getTableNumber()).setParameter("st", reservation.getStartTime())
            .setParameter("et", reservation.getEndTime()).setParameter("id", reservation.getId()).executeUpdate();
}

public void removeReservation(EntityManager em, int id) {
    Query query = em.createQuery("DELETE FROM Reservation WHERE id = :id");
    query.setParameter("id", id).executeUpdate();
}

public void createReservation(EntityManager em, Reservation reservation) {
    em.persist(reservation);
}
}

预订模式class

package model;

import javax.persistence.*;

@Entity
public class Reservation {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;

@Column(name = "table_number", columnDefinition = "VARCHAR(30) NOT NULL")
private int tableNumber;

@Column(name = "start_time" ,columnDefinition = "VARCHAR(20) NOT NULL")
private String startTime;

@Column(name = "end_time", columnDefinition = "VARCHAR(20) NOT NULL")
private String endTime;

@ManyToOne
private Person person;

public Reservation() {

}

public Reservation(int tableNumber, String startTime, String endTime, Person person) {
    this.tableNumber = tableNumber;
    this.startTime = startTime;
    this.endTime = endTime;
    this.person = person;
}

public int getTableNumber() {
    return tableNumber;
}

public void setTableNumber(int tableNumber) {
    this.tableNumber = tableNumber;
}

public String getStartTime() {
    return startTime;
}

public void setStartTime(String startTime) {
    this.startTime = startTime;
}

public String getEndTime() {
    return endTime;
}

public void setEndTime(String endTime) {
    this.endTime = endTime;
}

public long getId() {
    return id;
}

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

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

public Person getPerson() {
    return person;
}

public void setPerson(Person person) {
    this.person = person;
}  
}

人物模型class

package model;

import javax.persistence.*;
import java.util.Collection;
import java.util.List;
import java.util.Set;

@Entity
public class Person {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;

@Column(name = "full_name")
private String name;

@Column(name = "phone_number")
private String phoneNumber;

private String email;

@OneToMany(mappedBy = "person")
private List<Reservation> reservationList;

public Person() {

}

public Person(String name, String phoneNumber, String email, List<Reservation> reservationList) {
    this.name = name;
    this.phoneNumber = phoneNumber;
    this.email = email;
    this.reservationList = reservationList;
}

public long getId() {
    return id;
}

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

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getPhoneNumber() {
    return phoneNumber;
}

public void setPhoneNumber(String phoneNumber) {
    this.phoneNumber = phoneNumber;
}

public String getEmail() {
    return email;
}

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

public List<Reservation> getReservationList() {
    return reservationList;
}

public void setReservationList(List<Reservation> reservationList) {
    this.reservationList = reservationList;
}
}

App class - 我用 Hibernate 运行应用程序

import DAO.ReservationDAO;
import model.Person;
import model.Reservation;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import java.util.ArrayList;
import java.util.List;

public class App {

public void populateDb(EntityManager em) {
    EntityTransaction transaction = em.getTransaction();

    List<Reservation> reservationList = new ArrayList<>();

    Person person1 = new Person("John Smith", "458-321-842", "sgfhvdsg@gmail.com", reservationList);
    Reservation reservation1 = new Reservation(37, "12.37", "13.45", person1);
    reservationList.add(reservation1);

    Person person2 = new Person("Gonzalez", "123-456-789", "dsafi@gmail.com", reservationList);
    Reservation reservation2 = new Reservation(24, "14.23", "16.22", person2);
    reservationList.add(reservation2);


    transaction.begin();

    em.persist(person1);
    em.persist(reservation1);

    em.persist(person2);
    em.persist(reservation2);

    transaction.commit();
}

public static void main(String[] args) {
    App app = new App();
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("restapi");
    EntityManager em = emf.createEntityManager();

    app.populateDb(em);

    ReservationDAO reservationDAO = new ReservationDAO();
    List<Reservation> allReservations = reservationDAO.getAllReservations();

    System.out.println("allReservations size: " + allReservations.size());
    em.clear();

    em.close();
    emf.close();
}

}

HTTP ERROR 500

Problem accessing /reservation. Reason:

    Server Error

Caused by:

java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
    at java.util.ArrayList.rangeCheck(ArrayList.java:657)
    at java.util.ArrayList.get(ArrayList.java:433)
    at servlets.ReservationServlet.doGet(ReservationServlet.java:30)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)

Answer 1

在 doget 方法中 List<Reservation> reservationsList 你得到的是 null。你能做什么

if (  reservationsList !=null && !reservationsList.isEmpty())
{


  System.out.println("reservationsList size: " + reservationsList.size());
 //String json =gson.toJson(reservationsList); 

response.getWriter().write(reservationsList.get(0).getPerson().getName()); } 
else {
    response.getWriter().write("record not found")

}

如何使用 JPA Hibernate 将 EntityManager 注入 DAO 或 Servlet

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