hibernate annotation 之 一对多单向外键关联

转自：hibernate annotation 之 一对多单向外键关联

假设，一个农场产出多种植物，具体的某一植物产于某一农场。

1 package net.yeah.fancydeepin.po;

2

3 import java.io.Serializable;

4 import java.util.Set;

5 import javax.persistence.Column;

6 import javax.persistence.Entity;

7 import javax.persistence.GeneratedValue;

8 import javax.persistence.Id;

9 import javax.persistence.OneToMany;

10 /**

11 * -----------------------------------------

12 * @描述 农场

13 * @作者 fancy

14 * @邮箱 fancydeepin@yeah.net

15 * @日期 2012-10-21 <p>

16 * -----------------------------------------

17 */

18 @Entity

19 public class Farm implements Serializable{

20

21 private static final long serialVersionUID = 1L;

22

23 private Integer id;

24 private String name;

25 private Set<Plant> plantSet;

26

27 @Id

28 @GeneratedValue

29 public Integer getId() {

30 return id;

31 }

32 @Column(length = 18, nullable = false)

33 public String getName() {

34 return name;

35 }

36 @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)

37 public Set<Plant> getPlantSet() {

38 return plantSet;

39 }

40 public void setId(Integer id) {

41 this.id = id;

42 }

43 public void setName(String name) {

44 this.name = name;

45 }

46 public void setPlantSet(Set<Plant> plantSet) {

47 this.plantSet = plantSet;

48 }

49 }

1 package net.yeah.fancydeepin.po;

2

3 import java.io.Serializable;

4 import javax.persistence.Column;

5 import javax.persistence.Entity;

6 import javax.persistence.GeneratedValue;

7 import javax.persistence.Id;

8 /**

9 * -----------------------------------------

10 * @描述 植物

11 * @作者 fancy

12 * @邮箱 fancydeepin@yeah.net

13 * @日期 2012-10-21 <p>

14 * -----------------------------------------

15 */

16 @Entity

17 public class Plant implements Serializable{

18

19 private static final long serialVersionUID = 1L;

20

21 private Integer id;

22 private String name;

23

24 @Id

25 @GeneratedValue

26 public Integer getId() {

27 return id;

28 }

29 @Column(length = 18, nullable = false)

30 public String getName() {

31 return name;

32 }

33 public void setId(Integer id) {

34 this.id = id;

35 }

36 public void setName(String name) {

37 this.name = name;

38 }

39

40 }

Junit 测试 :

1 @Test

2 public void createTable(){

3

4 new SchemaExport(new AnnotationConfiguration().configure()).create(true, true);

5 }

执行上面的单元测试，数据库中生成的表结构图 :



从上面的图可以看出，多出了一张中间表，这张中间表是冗余的，它没有必要存在，它之所以存在，这是因为 hibernate 在默认的情况下，

会将 OneToMany 当成是 ManyToMany 的一种特殊情况，故而生成了一张中间表。想要去掉这张中间表只需要加上 @JoinColumn 注解即可 :

1 @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)

2 @JoinColumn(name = "farm_id")

3 public Set<Plant> getPlantSet() {

4 return plantSet;

5 }

再次执行单元测试 ( 注意要手动删除之前生成的表，因为有中间表的存在，中间表参考了与它关联的主表，这种情况下 hibernate 无法帮我们删除表 )



级联保存 :

1 private static Session session;

2

3 @BeforeClass

4 public static void beforeClass(){

5

6 session = new AnnotationConfiguration().configure().buildSessionFactory().getCurrentSession();

7 }

8

9 @Test

10 public void insert(){

11 try {

12 Plant tomato = new Plant();

13 tomato.setName("番茄");

14 Plant cabbage = new Plant();

15 cabbage.setName("卷心菜");

16 Set<Plant> plantSet = new HashSet<Plant>();

17 plantSet.add(tomato);

18 plantSet.add(cabbage);

19 Farm farm = new Farm();

20 farm.setName("fancy-farm");

21 farm.setPlantSet(plantSet);

22 session.beginTransaction();

23 session.persist(farm);

24 session.getTransaction().commit();

25 } catch (Exception e) {

26 e.printStackTrace();

27 }

28 }