JPA OneToMany级联操作详解之级联删除 - 好文

原文地址：https://blog.csdn.net/eddle/article/details/6919552
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<Auto> autos = new HashSet<Auto>();
    @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<Auto> getAutos() {
        return autos;
    }
    public void setAutos(Set<Auto> 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 |
+--------+---------+----------+----------+

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