惠州什么是JPA多对多连接表？_北大青鸟IT学校

你知道什么是JPA多对多连接表吗？下面惠州北大青鸟老师就给大家科普一下什么是JPA多对多连接表。

以下代码显示如何为多对多映射设置连接表。

实例

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

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

import org.springframework.transaction.annotation.Transactional;

@Transactionalpublic class PersonDaoImpl {  public void test(){
   Department d1 = new Department();
   d1.setName("Design");

   Department d2 = new Department();
   d2.setName("Testing");
   
   Person p1 = new Person("Tom");
   p1.getDepartments().add(d1);
   p1.getDepartments().add(d2);
   
   Person p2 = new Person("Jack");
   p2.getDepartments().add(d1);
   p2.getDepartments().add(d2);
   
   d1.getPersons().add(p1);
   d1.getPersons().add(p2);
   
   
   em.persist(p1);
   em.persist(p2);
   em.persist(d1);
   em.persist(d2);
 }
 @PersistenceContext  private EntityManager em;
}

下面的代码来自Person.java。

package cn.w3cschool.common;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;

@Entitypublic class Person {
 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)  private long id;  
 private String name;
 
 @ManyToMany
 @JoinTable(name="PERSON_DEPT",
 joinColumns=@JoinColumn(name="PERSON_ID"),
 inverseJoinColumns=@JoinColumn(name="DEPT_ID"))  private Set<Department> departments = new HashSet<Department>();  
 public Person() {}  public Person(String name) {
   this.name = name;
 }
 public Set<Department> getDepartments() {
   return departments;
 }  public void setDepartments(Set<Department> departments) {
   this.departments = departments;
 }  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 + ", departments="
       + departments + "]";
 }


}

下面的代码来自Department.java。

package cn.w3cschool.common;

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

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entitypublic class Department implements java.io.Serializable{
 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)  private long id;  
 private String name;
 
 @ManyToMany  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
       + "]";
 }

}

以下是数据库转储。

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

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

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

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





Table Name: DEPARTMENT_PERSON
Row:
   Column Name: DEPARTMENT_ID,
   Column Type: BIGINT:
   Column Value: 1

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

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

   Column Name: PERSONS_ID,
   Column Type: BIGINT:
   Column Value: 2





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

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

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

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





Table Name: PERSON_DEPT
Row:
   Column Name: PERSON_ID,
   Column Type: BIGINT:
   Column Value: 1

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

Row:
   Column Name: PERSON_ID,
   Column Type: BIGINT:
   Column Value: 1

   Column Name: DEPT_ID,
   Column Type: BIGINT:
   Column Value: 2

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

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

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

   Column Name: DEPT_ID,
   Column Type: BIGINT:
   Column Value: 2

什么是JPA多对多连接表？相信大家应该知道了吧！想了解更多关于JAVA的知识吗？可以来惠州北大青鸟新方舟校区了解一下。

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