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jpa 多对多 JPA 多对多连接表示例

2023-06-14 01:20 Java教程

jpa 多对多

JPA 多对多（Many-to-Many）是一种常见的数据库关系，它表示两个实体之间存在多对多的关系。在 JPA 中，可以使用 @ManyToMany 注解来映射多对多的关系。

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @ManyToMany(mappedBy = "students")
    private List courses;

    // getters and setters ...
}

上面的代码段中，我们使用了 @ManyToMany 注解来映射 Student 和 Course 之间的多对多关系。@ManyToMany 注解有一个 mappedBy 属性，它指向 Course 类中 students 属性。这样，JPA 就可以根据这个属性来识别出 Student 和 Course 之间的关系。

@Entity
public class Course {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})  // 这里加上 cascade 级联操作属性 
    @JoinTable(name = "student_course", joinColumns = {@JoinColumn(name = "course_id")}, inverseJoinColumns = {@JoinColumn(name = "student_id")})  // 这里加上 JoinTable 来映射中间表 
    private List students;

   // getters and setters ...  										   }

在上面的代码段中，我们使用了 cascade 属性来声明当保存、更新、删除 Course 实体时会同时保存、更新、删除相应的 Student 实体。此外，我们还使用了 JoinTable 来映射中间表 student_course （即 student 表和 course 表之间的关联表）。

JPA 多对多连接表示例

JPA教程 - JPA 多对多连接表示例

以下代码显示如何为多对多映射设置连接表。

例子

下面的代码来自PersonDaoImpl.java。

package cn..common;

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

import org.springframework.transaction.annotation.Transactional;

@Transactional
public 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..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;

@Entity
public 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..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;

@Entity
public 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
        + "]";
  }

}

下载 ManyToMany_Join_Table.zip

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

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