JPA教程 - JPA多對一映射示例

以下代碼顯示了如何進行多對一映射。

它創(chuàng)建兩個實體，人員和部門。一個部門可以有很多人。

在Person類的部門字段中，它用@ManyToOne注釋標記部門。

@Entity
public class Person {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
   
  private String name;
  
  @ManyToOne (cascade=CascadeType.ALL)
  private Department department;

例子

下面的代碼來自Person.java。

package cn.w3cschool.common;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.CascadeType;

@Entity
public class Person {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
   
  private String name;
  
  @ManyToOne (cascade=CascadeType.ALL)
  private Department department;
  
  public Person() {}

  public Person(String name) {
    this.name = name;
  }


  public Department getDepartment() {
    return department;
  }

  public void setDepartment(Department department) {
    this.department = department;
  }

  public Long getId() {
    return id;
  }
  public void setId(Long id) {
    this.id = id;
  }
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }

  @Override
  public String toString() {
    return "Person [id=" + id + ", name=" + name + "]";
  }

}

下面的代碼來自PersonDaoImpl.java。

package cn.w3cschool.common;


import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test(){
    Department d = new Department();
    d.setName("Design");

    Person p1 = new Person("Tom");
    p1.setDepartment(d);

    Person p2 = new Person("Jack");
    p2.setDepartment(d);
    
    d.getPersons().add(p1);
    d.getPersons().add(p2);
    
    em.persist(p1);
    em.persist(p2);
    em.persist(d);

  }
  @PersistenceContext
  private EntityManager em;
}

下面的代碼來自Department.java。

package cn.w3cschool.common;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
  
  private String name;
  
  @OneToMany(mappedBy="department",cascade=CascadeType.ALL)
  private Set<Person> persons = new HashSet<Person>();
  
  public Set<Person> getPersons() {
    return persons;
  }

  public void setPersons(Set<Person> persons) {
    this.persons = persons;
  }

  public long getId() {
    return id;
  }

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

  public String getName() {
    return name;
  }

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

  @Override
  public String toString() {
    return "Department [id=" + id + ", name=" + name + ", persons=" + persons
        + "]";
  }

}

以下是數據庫轉儲。

Table Name: DEPARTMENT
 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Design





Table Name: PERSON
 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Jack

    Column Name: DEPARTMENT_ID,
    Column Type: BIGINT:
    Column Value: 1

 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 2

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Tom

    Column Name: DEPARTMENT_ID,
    Column Type: BIGINT:
    Column Value: 1