jpa级联操作详解1-级联保存(CascadeType.PERSIST)

不论是对于jpa或者是hibernate来说字段的数据库映射都不是难点，而是很多初学者都对jpa级联操作等一系列的东西不大明白，在这一系列的文章中我通过简单的java实体对象来与大家共同理解jpa（hibernate做实现产品）的级联操作等难点知识，希望能够共同提高。为了保证简单易懂，本系列文章避免光讲理论知识，而忽视实际动手，在下面的例子中都有简单易懂的例子，为了加深理解大家也可以在自己的机器上调试。同时为了方便理解本系列文章采用对比讲解，能让人一目了然。同时欢迎大家共同探讨，一起完善这教程
jpa级联操作详解1(cascade) 之 cascade={CascadeType.PERSIST}
onetomany 一对多关联 实体bean：汽车和车库

（一）Java代码  收藏代码package com.hibernate.jpa.bean1;  import javax.persistence.Entity;  import javax.persistence.GeneratedValue;  import javax.persistence.Id;  import javax.persistence.JoinColumn;  import javax.persistence.ManyToOne;  @Entity  public class Auto {      /**      * one to many 一对多关联      */      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;      }  }  ------车库  package com.hibernate.jpa.bean1;  import java.util.HashSet;  import java.util.Set;  import javax.persistence.Column;  import javax.persistence.Entity;  import javax.persistence.GeneratedValue;  import javax.persistence.Id;  import javax.persistence.OneToMany;  @Entity  public class Garage {      /**      * many to one 多对一      */      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;      }      @OneToMany(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);      }  }   ---------junit保存方法      @Test public void save() {          EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");          EntityManager em = factory.createEntityManager();          em.getTransaction().begin();          Garage garage = new Garage();          garage.setGaragenum("room1");          Auto auto1 = new Auto();          auto1.setAutonum("bj0000");          auto1.setAutotype("car");          Auto auto2 = new Auto();          auto2.setAutonum("bj1231");          auto2.setAutotype("bus");          garage.addGarageAuto(auto1);          garage.addGarageAuto(auto2);          em.persist(garage);          em.getTransaction().commit();          em.close();          factory.close();      }  

运行以上save()方法之后，数据库中只对应的表，但是只有garage表中被存入了数据，而auto表中没有被存入数据，仅仅是生成了表而已。
数据库中的表数据：
+—–+———–+
| gid | garagenum |
+—–+———–+
| 1 | room1 |
+—–+———–+

mysql> select * from auto;
Empty set (0.00 sec)
这儿可以注意到虽然生成了auto数据库表，但是无法存储有关auto的数据，因为没有先保存auto或设置级联保存
观察发出的sql语句:
Hibernate: insert into Garage (garagenum) values (?)

（二）先保存auto
将junit测试类中的save方法改为

@Test public void save() {EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");EntityManager em = factory.createEntityManager();em.getTransaction().begin();Garage garage = new Garage();garage.setGaragenum("room1");Auto auto1 = new Auto();auto1.setAutonum("bj0000");auto1.setAutotype("car");Auto auto2 = new Auto();auto2.setAutonum("bj1231");auto2.setAutotype("bus");garage.addGarageAuto(auto1);garage.addGarageAuto(auto2);em.persist(auto1);em.persist(auto2);em.persist(garage);em.getTransaction().commit();em.close();factory.close();}

观察发出的sql语句：
Hibernate: insert into Auto (autonum, autotype, garageid) values (?, ?, ?)
Hibernate: insert into Auto (autonum, autotype, garageid) values (?, ?, ?)
Hibernate: insert into Garage (garagenum) values (?)
Hibernate: update Auto set autonum=?, autotype=?, garageid=? where autoId=?
Hibernate: update Auto set autonum=?, autotype=?, garageid=? where autoId=?
当然也生成了对应的数据记录，但是对数据库进行了5次操作

mysql> select * from garage;+-----+-----------+| gid | garagenum |+-----+-----------+| 1 | room1 |+-----+-----------+mysql> select * from auto;+--------+---------+----------+----------+| autoId | autonum | autotype | garageid |+--------+---------+----------+----------+| 1 | bj0000 | car | 1 || 2 | bj1231 | bus | 1 |+--------+---------+----------+----------+-----------------------------------------------------------------------

（三）设置cascade={CascadeType.PERSIST}
当把
@OneToMany(mappedBy=”garage”)
public Set getAutos() {
return autos;
}
改为：
@OneToMany(cascade={CascadeType.PERSIST} ,mappedBy=”garage”)
public Set getAutos() {
return autos;
}
即多添加了一行cascade={CascadeType.PERSIST} 申明级联级联保存
删除前面生成的数据库表garage 和 auto
再次运行save()方法
这是我们看到数据库中都有对应的记录
+—–+———–+
| gid | garagenum |
+—–+———–+
| 1 | room1 |
+—–+———–+

+——–+———+———-+———-+
| autoId | autonum | autotype | garageid |
+——–+———+———-+———-+
| 1 | bj0000 | car | 1 |
| 2 | bj1231 | bus | 1 |
+——–+———+———-+———-+
观察发出的sql语句:
Hibernate: insert into Garage (garagenum) values (?)
Hibernate: insert into Auto (autonum, autotype, garageid) values (?, ?, ?)
Hibernate: insert into Auto (autonum, autotype, garageid) values (?, ?, ?)