爲什麼ejb中的外鍵聲明爲對象（實體）？

Question

我正在開發一個使用EJB，JPA和netbeans的Java Web EE應用程序。我已經創建了一個名爲SQL用戶爲註冊和登錄，並命名爲prescripts其中有3個外鍵指的用戶的主鍵idusers（文檔ID，pharmid，patid）其它表的表。 我也命名爲用戶淨豆實體bean，並命名爲UsersFacade會話bean和prescripts實體bean創建：prescripts和會話bean：PrescriptsFacade。

我的問題是這樣的：

爲什麼在用戶EJB的所有變量（列）被聲明爲他們（字符串字符串，整數整數等），並在prescripts聲明爲用戶？

//users.java 
@Id 
@GeneratedValue(strategy = GenerationType.IDENTITY) 
@Basic(optional = false) 
@Column(name = "idusers") 
private Integer idusers; 
@Column(name = "user_type") 
private String userType; 
@Column(name = "name") 
private String name; 
@Column(name = "nickname") 
private String nickname; 

//prescripts.java 
@Id 
@GeneratedValue(strategy = GenerationType.IDENTITY) 
@Basic(optional = false) 
@Column(name = "idprescripts") 
private Integer idprescripts; 
@Column(name = "data") 
private String data; 
@JoinColumn(name = "pharm_id", referencedColumnName = "idusers") 
@ManyToOne 
private Users users; 
@JoinColumn(name = "doc_id", referencedColumnName = "idusers") 
@ManyToOne 
private Users users1; 
@JoinColumn(name = "pat_id", referencedColumnName = "idusers") 
@ManyToOne 
private Users users2; 

對於用戶來說我用這個代碼在一個servlet在SQL基地中插入一行：

Users currentUsers; 
currentUsers = new Users(); 
String Type = request.getParameter("user_type"); 
String Name = request.getParameter("name"); 
String Nickname = request.getParameter("nickname"); 
currentUsers.setUserType(Type); 
currentUsers.setName(Name); 
currentUsers.setNickname(Nickname); 
UsersFacade.create(currentUsers); 

我應該怎樣在prescripts這種方式插入行？

這不起作用（它顯示錯誤空指針異常）：

currentPresc = new Prescripts(); 
String PatID = request.getParameter("pat_id"); 
String DocID = request.getParameter("doc_id"); 
String PharmID = request.getParameter("pharm_id"); 
String Data = request.getParameter("data"); 
int patid = Integer.parseInt(PatID); 
int docid = Integer.parseInt(DocID); 
int pharmid = Integer.parseInt(PharmID); 
currentUsers = UsersFacade.find(patid); 
currentPresc.setUsers(currentUsers); 
currentUsers = UsersFacade.find(docid); 
currentPresc.setUsers1(currentUsers); 
currentUsers = UsersFacade.find(pharmid); 
currentPresc.setUsers2(currentUsers); 
currentPresc.setData(Data); 
PrescriptsFacade.create(currentPresc); 

我跳過了set和get方法和簡化的原因一些變量。請任何幫助真的非常感謝，我很stucked與現在這個2周：'（

---

我送你的用戶和prescripts全類：

Prescripts.java

package entities; 

import java.io.Serializable; 
import javax.persistence.Basic; 
import javax.persistence.Column; 
import javax.persistence.Entity; 
import javax.persistence.GeneratedValue; 
import javax.persistence.GenerationType; 
import javax.persistence.Id; 
import javax.persistence.JoinColumn; 
import javax.persistence.ManyToOne; 
import javax.persistence.NamedQueries; 
import javax.persistence.NamedQuery; 
import javax.persistence.Table; 


@Entity 
@Table(name = "prescripts") 
@NamedQueries({ 
    @NamedQuery(name = "Prescripts.findAll", query = "SELECT p FROM Prescripts p"), 
    @NamedQuery(name = "Prescripts.findByIdprescripts", query = "SELECT p FROM Prescripts p WHERE p.idprescripts = :idprescripts"), 
    @NamedQuery(name = "Prescripts.findByData", query = "SELECT p FROM Prescripts p WHERE p.presc = :presc")}) 
public class Prescripts implements Serializable { 
    private static final long serialVersionUID = 1L; 
    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Basic(optional = false) 
    @Column(name = "idprescripts") 
    private Integer idprescripts; 

    @JoinColumn(name = "doc_id", referencedColumnName = "idusers") 
    @ManyToOne 
    private Users doc_id; 

    @JoinColumn(name = "pat_id", referencedColumnName = "idusers") 
    @ManyToOne 
    private Users pat_id; 

    @JoinColumn(name = "pharm_id", referencedColumnName = "idusers") 
    @ManyToOne 
    private Users pharm_id; 
    @Column(name = "presc") 
    private String presc; 

    public Prescripts() { 
    } 

    public Prescripts(Users pat_id, Users pharm_id, Users doc_id, String presc) { 

     this.pharm_id = pharm_id; 
     this.doc_id = doc_id; 
     this.pat_id = pat_id; 
     this.presc = presc; 


    } 

    public Integer getIdprescripts() { 
     return idprescripts; 
    } 

    public void setIdprescripts(Integer idprescripts) { 
     this.idprescripts = idprescripts; 
    } 

    public String getPresc() { 
     return presc; 
    } 

    public void setPresc(String presc) { 
     this.presc = presc; 
    } 


    public Users getPharmId() { 
     return pharm_id; 
    } 

    public void setPharmId(Users pharm_id) { 
     this.pharm_id = pharm_id; 
    } 


    public Users getDocId() { 
     return doc_id; 
    } 

    public void setDocId(Users doc_id) { 
     this.doc_id = doc_id; 
    } 

    public Users getPatId() { 
     return pat_id; 
    } 

    public void setPatId(Users pat_id) { 
     this.pat_id = pat_id; 
    } 

    @Override 
    public int hashCode() { 
     int hash = 0; 
     hash += (idprescripts != null ? idprescripts.hashCode() : 0); 
     return hash; 
    } 

    @Override 
    public boolean equals(Object object) { 
     // TODO: Warning - this method won't work in the case the id fields are not set 
     if (!(object instanceof Prescripts)) { 
      return false; 
     } 
     Prescripts other = (Prescripts) object; 
     if ((this.idprescripts == null && other.idprescripts != null) || (this.idprescripts != null && !this.idprescripts.equals(other.idprescripts))) { 
      return false; 
     } 
     return true; 
    } 

    @Override 
    public String toString() { 
     return "entities.Prescripts[idprescripts=" + idprescripts + "]"; 
    } 

} 

Users.java

package entities; 

import java.io.Serializable; 
import java.util.Collection; 
import java.util.List; 
import javax.persistence.*; 
import javax.xml.bind.annotation.XmlTransient; 


@Entity 
@Table(name = "users") 
@NamedQueries({ 
    @NamedQuery(name = "Users.findAll", query = "SELECT u FROM Users u"), 
    @NamedQuery(name = "Users.findByIdusers", query = "SELECT u FROM Users u WHERE u.idusers = :idusers"), 
    @NamedQuery(name = "Users.findByUserType", query = "SELECT u FROM Users u WHERE u.userType = :userType"), 
    @NamedQuery(name = "Users.findByNickname", query = "SELECT u FROM Users u WHERE u.nickname = :nickname"), 
    @NamedQuery(name = "Users.findByName", query = "SELECT u FROM Users u WHERE u.name = :name"), 
    @NamedQuery(name = "Users.findByPassword", query = "SELECT u FROM Users u WHERE u.password = :password"), 
    @NamedQuery(name = "Users.findByEmail", query = "SELECT u FROM Users u WHERE u.email = :email"), 
    @NamedQuery(name = "Users.findByCity", query = "SELECT u FROM Users u WHERE u.city = :city"), 
    @NamedQuery(name = "Users.findByStreet", query = "SELECT u FROM Users u WHERE u.street = :street"), 
    @NamedQuery(name = "Users.findByAt", query = "SELECT u FROM Users u WHERE u.at = :at"), 
    @NamedQuery(name = "Users.findByAmka", query = "SELECT u FROM Users u WHERE u.amka = :amka"), 
    @NamedQuery(name = "Users.findByAfm", query = "SELECT u FROM Users u WHERE u.afm = :afm"), 
    @NamedQuery(name = "Users.findByVerify", query = "SELECT u FROM Users u WHERE u.verify = :verify")}) 
public class Users implements Serializable { 
    @OneToMany(mappedBy = "pat_id") 
    private List<Prescripts> prescriptsList; 
    @OneToMany(mappedBy = "doc_id") 
    private List<Prescripts> prescriptsList1; 
    @OneToMany(mappedBy = "pharm_id") 
    private List<Prescripts> prescriptsList2; 



    private static final long serialVersionUID = 1L; 
    @Id 

    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Basic(optional = false) 
    @Column(name = "idusers") 
    private Integer idusers; 

    @Column(name = "user_type") 
    private String userType; 
    @Column(name = "name") 
    private String name; 
    @Column(name = "nickname") 
    private String nickname; 
    @Column(name = "password") 
    private String password; 
    @Column(name = "email") 
    private String email; 
    @Column(name = "city") 
    private String city; 
    @Column(name = "street") 
    private String street; 
    @Column(name = "AT") 
    private String at; 
    @Column(name = "AMKA") 
    private String amka; 
    @Column(name = "AFM") 
    private String afm; 
    @Column(name = "verify") 
    private Boolean verify; 

    public Users() { 
    } 

     public Users(String userType,String name,String nickname, String password, String email, String city, String street, String at, 
      String amka, String afm, Boolean verify) 
    { 
     this.userType= userType; 
     this.name= name; 
     this.nickname= nickname; 
     this.password= password; 
     this.email = email; 
     this.city = city; 
     this.street = street; 
     this.at = at; 
     this.amka = amka; 
     this.afm = afm; 
     this.verify=verify; 

    } 

    public Integer getIdusers() { 
     return idusers; 
    } 

    public void setIdusers(Integer idusers) { 
     this.idusers = idusers; 
    } 

    public String getUserType() { 
     return userType; 
    } 

    public void setUserType(String userType) { 
     this.userType = userType; 
    } 

    public String getName() { 
     return name; 
    } 

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

    public String getNickname() { 
     return nickname; 
    } 

    public void setNickname(String nickname) { 
     this.nickname = nickname; 
    } 

    public String getPassword() { 
     return password; 
    } 

    public void setPassword(String password) { 
     this.password = password; 
    } 

    public String getEmail() { 
     return email; 
    } 

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

    public String getCity() { 
     return city; 
    } 

    public void setCity(String city) { 
     this.city = city; 
    } 

    public String getStreet() { 
     return street; 
    } 

    public void setStreet(String street) { 
     this.street = street; 
    } 

    public String getAt() { 
     return at; 
    } 

    public void setAt(String at) { 
     this.at = at; 
    } 

    public String getAmka() { 
     return amka; 
    } 

    public void setAmka(String amka) { 
     this.amka = amka; 
    } 

    public String getAfm() { 
     return afm; 
    } 

    public void setAfm(String afm) { 
     this.afm = afm; 
    } 

    public Boolean getVerify() { 
     return verify; 
    } 

    public void setVerify(Boolean verify) { 
     this.verify = verify; 
    } 

    @Override 
    public int hashCode() { 
     int hash = 0; 
     hash += (idusers != null ? idusers.hashCode() : 0); 
     return hash; 
    } 

    @Override 
    public boolean equals(Object object) { 
     // TODO: Warning - this method won't work in the case the id fields are not set 
     if (!(object instanceof Users)) { 
      return false; 
     } 
     Users other = (Users) object; 
     if ((this.idusers == null && other.idusers != null) || (this.idusers != null && !this.idusers.equals(other.idusers))) { 
      return false; 
     } 
     return true; 
    } 

    @Override 
    public String toString() { 
     return "entities.Users[idusers=" + idusers + "]"; 
    } 



    public List<Prescripts> getPrescriptsList() { 
     return prescriptsList; 
    } 

    public void setPrescriptsList(List<Prescripts> prescriptsList) { 
     this.prescriptsList = prescriptsList; 
    } 

    public List<Prescripts> getPrescriptsList1() { 
     return prescriptsList1; 
    } 

    public void setPrescriptsList1(List<Prescripts> prescriptsList1) { 
     this.prescriptsList1 = prescriptsList1; 
    } 

    public List<Prescripts> getPrescriptsList2() { 
     return prescriptsList2; 
    } 

    public void setPrescriptsList2(List<Prescripts> prescriptsList2) { 
     this.prescriptsList2 = prescriptsList2; 
    } 





} 

Answer 1

主要問題似乎是聲明User-class中的變量作爲Integer，但Prescript中的「user_id-variable」聲明爲User。

這是一個簡單的方式EJB3的作品。你在思考sql和關係數據庫。您將表的主鍵（例如Users.idusers）定義爲列，但使用自然對象定義對其他對象（準確地說，實體）的引用。因此，Prescripts.users被聲明爲一個用戶對象。 EJB的平臺會照顧這個轉變成一個數據庫列（在這種情況下，很可能命名爲users_idusers），用正確的類型（在這種情況下，整數），但這是照顧的，你不應該需要關心這一點。

你應該去通過EJB3教程或兩個 - 有很多這樣的例子，並確保您完成教程。你似乎錯過了一些基礎知識。還要注意你的代碼可能更簡單。通常不需要「@Column」-annotations，通常不需要mapped_by，列名同上等。還可以使用單數名稱（用戶而不是用戶）。主鍵的通用標準簡單地爲@Id Long id，這使得記住所有實體的主鍵名稱變得容易（但有些更喜歡不同的名稱）。

要回答你的實際的問題，我們需要更多的信息，包括哪些是TestServlet.java線233.你的代碼似乎或多或少是正確的，但它是別人難以驗證。從EntityManger/facade找到用戶對象，然後將其設置到Prescipts對象中是正確的方法。

Answer 2

在java中，實體之間的關係（在實體 - 關係模型）被表示爲，（一到一個）引用的對象（一個一對多）

A到對象的引用或列表關係數據庫實現實體（表中的行）之間的關係作爲外鍵，它與另一個表中的行的主鍵匹配。

（單詞「關係」的「關係型數據庫」並實際上只是意味着在一個表中的列（元組）是相互關聯的......外鍵的東西是加法。）

在設計數據庫表之前，我一般都認爲聰明才能開始使用適當的實體關係商店模型...