這里接上次的一些沒(méi)有說(shuō)完的，openJPA的2兩個(gè)常用屬性
<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema" /> <!-- 這個(gè)是自動(dòng)創(chuàng)建表的 -->
<property name="openjpa.Log" value="DefaultLevel=WARN,SQL=TRACE" /> <!-- 這是打印SQL語(yǔ)句 -->
這兩個(gè)是比較常用的屬性
今天說(shuō)的是一對(duì)多的關(guān)聯(lián)，這里有三個(gè)類(lèi)，非別是User，Post，Comment他們之間的關(guān)系是，User中有多個(gè)Post和
Comment，Post有多個(gè)Comment，而Comment有只有一個(gè)User和Post，這里我就不列出來(lái)表結(jié)構(gòu)了，因?yàn)榭梢杂肑PA的自動(dòng)生成
表
下面是User的關(guān)鍵代碼:
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 }
沒(méi)有什么可說(shuō)的，只說(shuō)兩點(diǎn)
1.@GeneratedValue(strategy =
GenerationType.SEQUENCE)這是說(shuō)為id自動(dòng)生成一個(gè)序列，這個(gè)選項(xiàng)湖自動(dòng)在數(shù)據(jù)庫(kù)中創(chuàng)建一個(gè)用來(lái)生成序列的表，一共有四種
-
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
這事Apache官方文檔說(shuō)的， 英文不好我就不翻譯了，應(yīng)該說(shuō)的很明白，也很容易懂
GenerationValue還有另外一個(gè)屬性generator這個(gè)我只在AUTO下實(shí)驗(yàn)了，他有下面幾種：
-
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),級(jí)聯(lián)，我這里是所有，級(jí)聯(lián)也是有4中（還是考的官方文檔）：
-
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.
具體的還是要看官方文檔，說(shuō)的很詳細(xì)
這里也可以使用其中的幾種，1種或著多重，例如：@ManyToOne(cascade=
{CascadeType.PERSIST,CascadeType.REMOVE,CascadeType.REFRESH,CascadeType.MERGE})
下來(lái)是Post和Comment的關(guān)鍵代碼：
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 }
下來(lái)的應(yīng)該沒(méi)有什么可說(shuō)的，下面是我的測(cè)試代碼：
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("男");
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();
源代碼（包含1，2的）:
JTATest.7z
posted on 2010-04-06 22:24
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