JPA 一对多映射是 JPA 中最常用的映射方式，它表示一个实体对象可以关联到多个其他实体对象。在 JPA 中，一对多映射可以通过 @OneToMany 注释来实现，该注释可以应用于实体类的属性上，表示该属性是一个集合，其中包含了多个关联的实体。
要正确配置一对多映射，需要在拥有 @OneToMany 注释的属性上添加 @ManyToOne 注释。@ManyToOne 注释表示该属性是一个引用，指向另一个实体。这样，JPA 将能够正确地将这些实体之间的关联建立起来。
@Entity
public class Employee {
@Id
private Long id;
private String name;
@OneToMany(mappedBy = "employee")
private List<Task> tasks;
// getters and setters...
}
@Entity
public class Task {
@Id
private Long id;
private String description;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "employee_id")
private Employee employee;
// getters and setters...
}以下代码显示如何创建多对多映射。
它描述了人与部门之间的关系。一个部门可以有很多人，一个人可以为许多部门工作。
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.ManyToMany;
@Entity
public class Person {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private long id;
private String name;
@ManyToMany
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;
}
下载 ManyToMany_Map.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_DEPARTMENT
Row:
Column Name: PERSON_ID,
Column Type: BIGINT:
Column Value: 1
Column Name: DEPARTMENTS_ID,
Column Type: BIGINT:
Column Value: 1
Row:
Column Name: PERSON_ID,
Column Type: BIGINT:
Column Value: 1
Column Name: DEPARTMENTS_ID,
Column Type: BIGINT:
Column Value: 2
Row:
Column Name: PERSON_ID,
Column Type: BIGINT:
Column Value: 2
Column Name: DEPARTMENTS_ID,
Column Type: BIGINT:
Column Value: 1
Row:
Column Name: PERSON_ID,
Column Type: BIGINT:
Column Value: 2
Column Name: DEPARTMENTS_ID,
Column Type: BIGINT:
Column Value: 2
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