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这里接上次的一些没有说完的,openJPA的2两个常用属性
<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema" /> <!-- 这个是自动创建表的 -->
<property name="openjpa.Log" value="DefaultLevel=WARN,SQL=TRACE" />  <!-- 这是打印SQL语句 -->
这两个是比较常用的属性
今天说的是一对多的关联,这里有三个类,非别是User,Post,Comment他们之间的关系是,User中有多个Post和 Comment,Post有多个Comment,而Comment有只有一个User和Post,这里我就不列出来表结构了,因为可以用JPA的自动生成 表
下面是User的关键代码:
 1 @Entity
 2 public class User {
 3   @Id
 4   @GeneratedValue(strategy = GenerationType.SEQUENCE)
 5   private int id;
 6   private String name;
 7   private String password;
 8   private String gender;
 9   @OneToMany(cascade = CascadeType.ALL)
10   private List<Comment> comments;
11   @OneToMany(cascade = CascadeType.ALL)
12   private List<Post> posts;
13   // ………………
14 }
没有什么可说的,只说两点
1.@GeneratedValue(strategy = GenerationType.SEQUENCE)这是说为id自动生成一个序列,这个选项湖自动在数据库中创建一个用来生成序列的表,一共有四种
  • GeneratorType.AUTO: The default. Assign the field a generated value, leaving the details to the JPA vendor.

  • GenerationType.IDENTITY: The database will assign an identity value on insert.

  • GenerationType.SEQUENCE: Use a datastore sequence to generate a field value.

  • GenerationType.TABLE: Use a sequence table to generate a field value

这事Apache官方文档说的, 英文不好我就不翻译了,应该说的很明白,也很容易懂
GenerationValue还有另外一个属性generator这个我只在AUTO下实验了,他有下面几种:
  • uuid-string: OpenJPA will generate a 128-bit type 1 UUID unique within the network, represented as a 16-character string. For more information on UUIDs, see the IETF UUID draft specification at: http://www1.ics.uci.edu/~ejw/authoring/uuid-guid/

  • uuid-hex: Same as uuid-string, but represents the type 1 UUID as a 32-character hexadecimal string.

  • uuid-type4-string: OpenJPA will generate a 128-bit type 4 pseudo-random UUID, represented as a 16-character string. For more information on UUIDs, see the IETF UUID draft specification at: http://www1.ics.uci.edu/~ejw/authoring/uuid-guid/

  • uuid-type4-hex: Same as uuid-type4-string , but represents the type 4 UUID as a 32-character hexadecimal string.


2.@OneToMany(cascade = CascadeType.ALL),级联,我这里是所有,级联也是有4中(还是考的官方文档):
  • CascadeType.PERSIST: When persisting an entity, also persist the entities held in this field. We suggest liberal application of this cascade rule, because if the EntityManager finds a field that references a new entity during flush, and the field does not use CascadeType.PERSIST, it is an error.

  • CascadeType.REMOVE: When deleting an entity, also delete the entities held in this field.

  • CascadeType.REFRESH: When refreshing an entity, also refresh the entities held in this field.

  • CascadeType.MERGE: When merging entity state, also merge the entities held in this field.

具体的还是要看官方文档,说的很详细
这里也可以使用其中的几种,1种或着多重,例如:@ManyToOne(cascade= {CascadeType.PERSIST,CascadeType.REMOVE,CascadeType.REFRESH,CascadeType.MERGE})
下来是Post和Comment的关键代码:
 1 @Entity
 2 public class Post {
 3   @Id
 4   @GeneratedValue(strategy = GenerationType.SEQUENCE)
 5   private int id;
 6   private Date createDate;
 7   private String title;
 8   private String content;
 9   @ManyToOne(cascade = CascadeType.ALL)
10   private User author;
11   @OneToMany(cascade = CascadeType.ALL)
12   private List<Comment> comments;
13   // ………………
14 }
15 
16 @Entity
17 public class Comment {
18   @Id
19   @GeneratedValue(strategy = GenerationType.SEQUENCE)
20   private int id;
21   private Date createDate;
22   private String title;
23   private String content;
24   @ManyToOne(cascade = CascadeType.ALL)
25   private User author;
26   @ManyToOne(cascade = CascadeType.ALL)
27   private Post post;
28   // ………………
29 }
下来的应该没有什么可说的,下面是我的测试代码:
 1  EntityManagerFactory factory = Persistence.createEntityManagerFactory("test");
 2 EntityManager em = factory.createEntityManager();
 3 em.getTransaction().begin();
 4 User user = new User();
 5 user.setName("Innate");
 6 user.setPassword("Innate");
 7 user.setGender("");
 8 List<Post> posts = new ArrayList<Post>();
 9 
10 for (int i = 0; i < 5; i++) {
11   Post post = new Post();
12   post.setAuthor(user);
13   post.setContent("test");
14   post.setCreateDate(new Date());
15   post.setTitle("Innate");
16   posts.add(post);
17   List<Comment> comments = new ArrayList<Comment>();
18   for (int j = 0; j < 5; j++) {
19     Comment comment = new Comment();
20     comment.setAuthor(user);
21     comment.setContent("testtest");
22     comment.setCreateDate(new Date());
23     comment.setPost(post);
24     comment.setTitle("InnateInnate");
25     comments.add(comment);
26   }
27   post.setComments(comments);
28   user.setComments(comments);
29 }
30 user.setPosts(posts);
31 em.persist(user);
32 em.getTransaction().commit();
源代码(包含1,2的):JTATest.7z
posted on 2010-04-06 22:24 天独 阅读(581) 评论(1)  编辑  收藏 所属分类: Java

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