jpa OneToMany级联操作详解之级联删除

 下面给出了一个级联删除的demo，其中Garage与Auto是一个一对多的关系。
在级联删除的关键地方，就是在父栏添加 CascadeType.REMOVE标注。

Garage.java
/** 
 * many to one 一对多关联
 */
Java代码 
package com.jpa.bean1;   
import java.util.HashSet;  
import java.util.Set;   
import javax.persistence.CascadeType;  
import javax.persistence.Column;  
import javax.persistence.Entity;  
import javax.persistence.FetchType;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.OneToMany;  
@Entity 
public class Garage {   
    private Integer gid;  
    private String garagenum;  
    private Set autos = new HashSet();         
    @Id @GeneratedValue 
    public Integer getGid() {  
        return gid;  
    }  
    public void setGid(Integer gid) {  
        this.gid = gid;  
    }  
    @Column(length=20)  
    public String getGaragenum() {  
        return garagenum;  
    }  
    public void setGaragenum(String garagenum) {  
        this.garagenum = garagenum;  
    }  
    //CascadeType属性有四个值，其中REMOVE属性是实现级联删除，要实现级联删除
    //在父栏必需添加CascadeType.REMOVE标注，这是级联删除的关键
    @OneToMany(cascade={CascadeType.REMOVE},mappedBy="garage")  
    public Set getAutos() {  
        return autos;  
    }  
    public void setAutos(Set autos) {  
        this.autos = autos;  
    }  
    public void addGarageAuto(Auto auto) {  
        auto.setGarage(this);  
        this.autos.add(auto);  
    }   
} 
Auto.java
/** 
 * one to many 多对一关联 
 */
Java代码 
package com.jpa.bean1;   
import javax.persistence.CascadeType;  
import javax.persistence.Entity;  
import javax.persistence.GeneratedValue;  
import javax.persistence.Id;  
import javax.persistence.JoinColumn;  
import javax.persistence.ManyToOne;  
@Entity 
public class Auto {   
    private Integer autoId;  
    private String autotype;  
    private String autonum;  
    private Garage garage;   
    @Id @GeneratedValue 
    public Integer getAutoId() {  
        return autoId;  
    }  
    public void setAutoId(Integer autoId) {  
        this.autoId = autoId;  
    }  
    public String getAutotype() {  
        return autotype;  
    }  
    public void setAutotype(String autotype) {  
        this.autotype = autotype;  
    }  
    public String getAutonum() {  
        return autonum;  
    }  
    public void setAutonum(String autonum) {  
        this.autonum = autonum;  
    }  
    @ManyToOne()  
    @JoinColumn(name="garageid")  
    public Garage getGarage() {  
        return garage;  
    }  
    public void setGarage(Garage garage) {  
        this.garage = garage;  
    }  
} 
级联删除操作前的持久化数据
mysql> select * from garage;
+-----+-----------+
| gid | garagenum |
+-----+-----------+
| 1 | room1 |
| 2 | room2 |
| 3 | room3 |
+-----+-----------+

mysql> select * from auto;
+--------+---------+----------+----------+
| autoId | autonum | autotype | garageid |
+--------+---------+----------+----------+
| 1 | hk2222 | car | 1 |
| 2 | bj0000 | car | 1 |
| 3 | jn1d31 | bus | 3 |
| 4 | sh3243 | car | 3 |
+--------+---------+----------+----------+ 
junit测试方法delete()

Java代码 
@Test public void delete() {  
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");  
    EntityManager em = factory.createEntityManager();  
    em.getTransaction().begin();  
    Garage garage = em.find(Garage.class, 3);  
    em.remove(garage);  
    em.getTransaction().commit();  
    em.close();  
    factory.close();  
} 

 @Test public void delete() {
  EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");
  EntityManager em = factory.createEntityManager();
  em.getTransaction().begin();
  Garage garage = em.find(Garage.class, 3);
  em.remove(garage);
  em.getTransaction().commit();
  em.close();
  factory.close();
 }
调用delete方法是实现级联删除，此时表garage中的gid为3的字段被全部删除，同时也级联删除了与garage相关联表auto中garageid为3的字段
级联删除后的持久化数据
mysql> select * from auto;  
+--------+---------+----------+----------+  
| autoId | autonum | autotype | garageid |  
+--------+---------+----------+----------+  
|      1 | hk2222  | car      |        1 |  
|      2 | bj0000  | car      |        1 |  
+--------+---------+----------+----------+ 

mysql> select * from auto;
+--------+---------+----------+----------+
| autoId | autonum | autotype | garageid |
+--------+---------+----------+----------+
|      1 | hk2222  | car      |        1 |
|      2 | bj0000  | car      |        1 |
+--------+---------+----------+----------+