jpa一对一关联查询 JPA 一对一连接列示例

jpa一对一关联查询

JPA一对一关联查询是指在两个实体之间建立一对一的关联，这种关联可以用来表示两个实体之间的父子关系，也可以用来表示多对一的关系。JPA中的一对一关联查询可以使用@OneToOne注释来实现，这样就可以在实体之间建立一对一的关联。

JPA中的一对一关联查询有两种方式：单向和双向。单向是指在实体A中有@OneToOne注释，而在实体B中没有@OneToOne注释；双向是指在实体A和实体B中都有@OneToOne注释。

// 单向 
@Entity 
public class A { 

    @Id 
    private Long id; 

    @OneToOne(fetch = FetchType.LAZY) 
    @JoinColumn(name = "b_id") 
    private B b; 

    // getter and setter ... 
} 

   // 双向  
@Entity  
public class A {  

    @Id  
    private Long id;  

    @OneToOne(mappedBy = "a", fetch = FetchType.LAZY)  
    private B b;  

    // getter and setter ...  
}  

 @Entity  
public class B {  

    @Id  
    private Long id;  

    @OneToOne(fetch = FetchType.LAZY)  
    @JoinColumn(name = "a_id")     // 此处需要加上JoinColumn注释，表明此处是外键列     // 此处需要加上JoinColumn注释，表明此处是外键列     // 此处需要加上JoinColumn注释，表明此处是外键列     // 此处需要加上JoinColumn注释，表明此处是外键列     // 此处需要加上JoinColumn注释，表明此处是外键列     // 此处需要加上JoinColumn注释，
JPA 一对一连接列示例
JPA教程 -  JPA一对一连接列示例

以下部分显示如何在JPA中为一对一映射设置连接列。
我们使用 @JoinColumn 注释设置连接列。
  @OneToOne
  @JoinColumn(name="DEPT_ID") 
  private Department department;
例子
下面的代码来自Department.java。
package cn..common;

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

@Entity
public class Department {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
  
  private String name;
  
  @OneToOne(mappedBy="department")
  private Person person;
  
  public Person getPerson() {
    return person;
  }

  public void setPerson(Person person) {
    this.person = person;
  }

  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 + ", person=" + person
        + "]";
  }
}
下面的代码来自Person.java。
package cn..common;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;
@Entity
public class Person {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
   
  private String name;
  @OneToOne
  @JoinColumn(name="DEPT_ID") 
  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 + "]";
  }

}
下面的代码来自PersonDaoImpl.java。
package cn..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(){
    Person p1 = new Person("Tom");
    
    p1.setName("Tom");
    
    Department d = new Department();
    d.setName("Design");
    p1.setDepartment(d);
    d.setPerson(p1);
    
    em.persist(p1);
    em.persist(d);
    
    Query query = em.createQuery("SELECT e FROM Person e");
    List<Person> list = (List<Person>) query.getResultList();
    System.out.println(list.get(0).getDepartment());
    
  }
  @PersistenceContext
  private EntityManager em;
}
下载 OneToOne_Map_Retrieve.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





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

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

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