Question

嗨，我从志愿者手中获取技能时遇到了问题。出于某种原因，我在使用这种方法时没有得到列表

public Volunteer getVolunteer(int id){

 Volunteer vo;

 Query q;

 q = em.createNamedQuery("Volunteer.findById").setParameter("id", id);
 vo = (Volunteer) q.getSingleResult();

 for(Skill s: vo.getSkills()){
  System.out.println(s);
 }

 return vo;

}

列表为空，因此提取似乎不起作用。

我正在使用JPA Eclipselink和Glassfish

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技能实体：

@Entity
@Table(name="skill")
@NamedQueries({
    @NamedQuery(name = "Skill.findAll", query = "SELECT s FROM Skill s"),
    @NamedQuery(name = "Skill.findById", query = "SELECT s FROM Skill s WHERE s.id = :id"),
    @NamedQuery(name = "Skill.findBySkillDescription", query = "SELECT s FROM Skill s WHERE s.skillDescription = :skillDescription")})
public class Skill implements Serializable {
 @Override
 public String toString() {
  return "Skill [id=" + id + ", skillDescription=" + skillDescription
    + "]";
 }

 private static final long serialVersionUID = 1L;

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 @Column(unique=true, nullable=false)
 private int id;

 @Column(name="skill_description", length=130)
 private String skillDescription;

 //bi-directional many-to-many association to Volunteer
    @ManyToMany(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
 @JoinTable(
  name="skill_volunteer"
  , joinColumns={
   @JoinColumn(name="skill_id", nullable=false)
   }
  , inverseJoinColumns={
   @JoinColumn(name="volunteer_id", nullable=false)
   }
  )
 private List<Volunteer> volunteers;

    public Skill() {
    }

 public int getId() {
  return this.id;
 }

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

 public String getSkillDescription() {
  return this.skillDescription;
 }

 public void setSkillDescription(String skillDescription) {
  this.skillDescription = skillDescription;
 }

 public List<Volunteer> getVolunteers() {
  return this.volunteers;
 }

 public void setVolunteers(List<Volunteer> volunteers) {
  this.volunteers = volunteers;
 }

}

志愿者实体：

@Entity
@Table(name="volunteer")
@NamedQueries({
    @NamedQuery(name = "Volunteer.findAll", query = "SELECT v FROM Volunteer v"),
    @NamedQuery(name = "Volunteer.findById", query = "SELECT v FROM Volunteer v WHERE v.id = :id"),
    @NamedQuery(name = "Volunteer.findByPhone", query = "SELECT v FROM Volunteer v WHERE v.phone = :phone"),
    @NamedQuery(name = "Volunteer.findByEmail", query = "SELECT v FROM Volunteer v WHERE v.email = :email"),
    @NamedQuery(name = "Volunteer.findByFirstName", query = "SELECT v FROM Volunteer v WHERE v.firstName = :firstName"),
    @NamedQuery(name = "Volunteer.findByLastName", query = "SELECT v FROM Volunteer v WHERE v.lastName = :lastName")})
public class Volunteer implements Serializable {
 @Override
 public String toString() {
  return "Volunteer [id=" + id + ", email=" + email + ", firstName="
    + firstName + ", lastName=" + lastName + ", phone=" + phone
    + "]";
 }

 private static final long serialVersionUID = 1L;

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 @Column(unique=true, nullable=false)
 private int id;

 @Column(length=255)
 private String email;

 @Column(name="first_name", length=255)
 private String firstName;

 @Column(name="last_name", length=255)
 private String lastName;

 @Column(length=255)
 private String phone;

 //bi-directional many-to-many association to Event
 @ManyToMany(mappedBy="volunteers")
 private List<Event> events;

 //bi-directional many-to-many association to Skill
 @ManyToMany(mappedBy="volunteers", cascade=CascadeType.ALL, fetch=FetchType.EAGER)
 private List<Skill> skills;

    public Volunteer() {
    }

 public int getId() {
  return this.id;
 }

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

 public String getEmail() {
  return this.email;
 }

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

 public String getFirstName() {
  return this.firstName;
 }

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

 public String getLastName() {
  return this.lastName;
 }

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

 public String getPhone() {
  return this.phone;
 }

 public void setPhone(String phone) {
  this.phone = phone;
 }

 public List<Event> getEvents() {
  return this.events;
 }

 public void setEvents(List<Event> events) {
  this.events = events;
 }

 public List<Skill> getSkills() {
  return this.skills;
 }

 public void setSkills(List<Skill> skills) {
  this.skills = skills;
 }

}

Answer 1

列表为空，因此提取似乎不起作用。

获取一个空列表不能得出结论“抓取”不起作用以及您是否使用LAZY或EAGER获取类型，如果有任何志愿者，您应该获得技能记录。

由于映射看起来是正确的，我首先查看生成的SQL并对数据库运行以确认

生成的SQL是预期结果
数据正确无误。

为此，请在persistence.xml：

  <property name="eclipselink.logging.level" value="FINEST" />
  <property name="eclipselink.logging.level.sql" value="FINEST" />

然后找到findById和getSkills的SQL查询，并针对数据库运行它们。

并且请使用获得的给定id的结果更新问题（生成的SQL和确认数据都正常）。

Answer 2

试着使用em.flush（），这对我有用;

Query q = em.createNamedQuery("Volunteer.findById").setParameter("id", id);
vo = (Volunteer) q.getSingleResult();
em.flush();
for(Skill s: vo.getSkills()){
    System.out.println(s);  
}

Answer 3

双向关系最常见的问题是当用户不维护双方时 - JPA实体是普通的java对象，而JPA不像EJB 2.0那样为你修复关系。将志愿者添加到技能时，还必须将技能添加到志愿者，以便缓存与您在数据库中强制执行的更改保持同步。

ManyToMany JPA获取

3 个答案: