我是JPA的新手，并向http://uaihebert.com/jpa-manytomany-unidirectional-and-bidirectional/学习
我创建了两个表，一个是Person，另一个是Notebook。如果没有记错的话，如果代码成功运行，将创建person_has_notebook。 （如果我不熟悉JPA，请更正我）。
人
@Entity
@Table(name = "Person")
@XmlRootElement
@NamedQueries({
@NamedQuery(name = "Person.findAll", query = "SELECT p FROM Person p")
, @NamedQuery(name = "Person.findById", query = "SELECT p FROM Person p WHERE p.id = :id")
, @NamedQuery(name = "Person.findByName", query = "SELECT p FROM Person p WHERE p.name = :name")
, @NamedQuery(name = "Person.findByCompName", query = "SELECT p FROM Person p WHERE p.compName = :compName")})
public class Person implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "id")
private Integer id;
@Basic(optional = false)
@Column(name = "Name")
private String name;
@Basic(optional = false)
@Column(name = "CompName")
private String compName;
@ManyToMany
@JoinTable(name="person_has_notebooks", joinColumns={@JoinColumn(name="person_id")}, inverseJoinColumns={@JoinColumn(name="notebook_id")})
private List notebooks;
public List getNotebooks() {
return notebooks;
}
public void setNotebooks(List notebooks) {
this.notebooks = notebooks;
}
public Person() {
}
public Person(Integer id) {
this.id = id;
}
public Person(Integer id, String name, String compName) {
this.id = id;
this.name = name;
this.compName = compName;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getCompName() {
return compName;
}
public void setCompName(String compName) {
this.compName = compName;
}
@Override
public int hashCode() {
int hash = 0;
hash += (id != null ? id.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 Person)) {
return false;
}
Person other = (Person) object;
if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
return false;
}
return true;
}
@Override
public String toString() {
return "entity.Person[ id=" + id + " ]";
}
}
笔记本
@Entity
@Table(name = "NoteBook")
@XmlRootElement
@NamedQueries({
@NamedQuery(name = "NoteBook.findAll", query = "SELECT n FROM NoteBook n")
, @NamedQuery(name = "NoteBook.findById", query = "SELECT n FROM NoteBook n WHERE n.id = :id")
, @NamedQuery(name = "NoteBook.findBySerialName", query = "SELECT n FROM NoteBook n WHERE n.serialName = :serialName")
, @NamedQuery(name = "NoteBook.findByRam", query = "SELECT n FROM NoteBook n WHERE n.ram = :ram")
, @NamedQuery(name = "NoteBook.findByHd", query = "SELECT n FROM NoteBook n WHERE n.hd = :hd")})
public class NoteBook implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "id")
private Integer id;
@Basic(optional = false)
@Column(name = "SerialName")
private String serialName;
@Basic(optional = false)
@Column(name = "Ram")
private int ram;
@Basic(optional = false)
@Column(name = "HD")
private int hd;
@ManyToMany(mappedBy="notebooks")
private List person;
public List getPersons() {
return person;
}
public void setPersons(List persons) {
this.person = persons;
}
public NoteBook() {
}
public NoteBook(Integer id) {
this.id = id;
}
public NoteBook(Integer id, String serialName, int ram, int hd) {
this.id = id;
this.serialName = serialName;
this.ram = ram;
this.hd = hd;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getSerialName() {
return serialName;
}
public void setSerialName(String serialName) {
this.serialName = serialName;
}
public int getRam() {
return ram;
}
public void setRam(int ram) {
this.ram = ram;
}
public int getHd() {
return hd;
}
public void setHd(int hd) {
this.hd = hd;
}
@Override
public int hashCode() {
int hash = 0;
hash += (id != null ? id.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 NoteBook)) {
return false;
}
NoteBook other = (NoteBook) object;
if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
return false;
}
return true;
}
@Override
public String toString() {
return "entity.NoteBook[ id=" + id + " ]";
}
}
多对多1
public class ManyToMany1 {
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("ManyToMany1PU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
NoteBook noteA = new NoteBook();
noteA.setSerialNumber("A0123");
NoteBook noteB = new NoteBook();
noteB.setSerialNumber("B0123");
NoteBook noteC = new NoteBook();
noteC.setSerialNumber("C0123");
List notebooks = new ArrayList();
notebooks.add(noteA);
notebooks.add(noteB);
notebooks.add(noteC);
Person person = new Person();
person.setName("Zorro");
person.setNotebooks(notebooks);
em.persist(person);
em.getTransaction().commit();
}
}
错误
Exception Description: Predeployment of PersistenceUnit [ManyToMany1PU] failed.
Internal Exception: Exception [EclipseLink-7214] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.ValidationException
Exception Description: The target entity of the relationship attribute [notebooks] on the class [class entity.Person] cannot be determined. When not using generics, ensure the target entity is defined on the relationship mapping.
at org.eclipse.persistence.exceptions.PersistenceUnitLoadingException.exceptionSearchingForPersistenceResources(PersistenceUnitLoadingException.java:127)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(PersistenceProvider.java:107)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactory(PersistenceProvider.java:177)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:79)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:54)
at manytomany1.ManyToMany1.main(ManyToMany1.java:27)
我已经将
mappedBy分配给了较弱的一面，但是为什么仍然会出现此类错误？ 最佳答案
为什么不按照消息告诉您的方式使用泛型？
@ManyToMany
@JoinTable(name="person_has_notebooks", joinColumns={@JoinColumn(name="person_id")}, inverseJoinColumns={@JoinColumn(name="notebook_id")})
private List<NoteBook> notebooks;
和
@ManyToMany(mappedBy="notebooks")
private List<Person> person;
关于java - 不使用泛型时，请确保在关系映射上定义了目标实体，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/41810463/