JPA'Many2Many
Many2Many关系：
Document(主动)<---->(被动)KMFile
class Document
主动关联设置：指定中间表，及字段。
@ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.PERSIST}, targetEntity=KMFile.class)
@JoinTable(inverseJoinColumns={@JoinColumn(name="file_id")}, name="LINK_KMFile", joinColumns={@JoinColumn(name="linker_id")})
public List<KMFile> getFiles() {
return fileList;
}
class KMFile
被动关联设置：只需设置mappedBy，即本体在主体中的属性(Document#getFiles)。
@ManyToMany(fetch=FetchType.LAZY, cascade={CascadeType.PERSIST}, mappedBy="files", targetEntity=Document.class)
public List<Document> getDocuments() {
return docList;
}
使用示例：
public class Test{ static EntityManagerFactory emf = Persistence .createEntityManagerFactory(GlobConfig.PERSISTENCE_UNIT_NAME); static EntityManager em = emf.createEntityManager(); public static void main(String[] a) throws Exception { em.getTransaction().begin(); // 持久化对象 long docId = UUIDUtil.uuid(); Document doc = new Document(); doc.setId(docId); doc.setTitle("AAA"); long fileId = UUIDUtil.uuid(); KMFile file = new KMFile(); file.setId(fileId); file.setTitle("BBB"); //这里也可以:file.getDocuments().add(doc);em.persist(file);即持久化无主动关联、被动关联之区分。 doc.getFiles().add(file); em.persist(doc); em.flush(); // 持久化关系(必要，否则关联表无记录)：注意需使用查询返回对象(带jpa session信息) file = em.find(KMFile.class, fileId); doc = em.find(Document.class, docId); file.getDocuments().add(doc); //下面这条不能执行，因为之前已持久化doc.getFiles().add(file); //如果持久化之前doc没有添加getFiles().add，即各自独立保存，那么维持关系要相互add。 // doc.getFiles().add(file); //查询验证 Query query = em.createQuery("SELECT d FROM Document d where d.id=:docId"); query.setParameter("docId", docId); Document d=(Document)query.getSingleResult(); System.out.println("文档："+d.getTitle()); d.getFiles().size(); System.out.println("文档-附件："+d.getFiles().get(0).getTitle()); query = em.createQuery("SELECT f FROM KMFile f where f.id=:fileId"); query.setParameter("fileId", fileId); KMFile f=(KMFile)query.getSingleResult(); System.out.println("附件："+f.getTitle()); f.getDocuments().size(); System.out.println("附件-文档："+f.getDocuments().get(0).getTitle()); em.getTransaction().commit(); em.close(); emf.close(); } }
Logger调为Fine，以输出SQL(本例使用derby)，执行结果输出：
--INSERT INTO Document (ID, CREATETIME, LASTMODIFIED, TITLE, DESCRIPTION, OWNER_ID) VALUES (?, ?, ?, ?, ?, ?)
bind => [4508551237913941891, null, null, AAA, null, null]
--INSERT INTO KMFile (ID, CREATETIME, NAME, TYPE, SIZE, LASTMODIFIED, TITLE) VALUES (?, ?, ?, ?, ?, ?, ?)
bind => [4508551237913941892, null, null, 0, 0, null, BBB]
--INSERT INTO LINK_KMFile (file_id, linker_id) VALUES (?, ?)
bind => [4508551237913941892, 4508551237913941891]
--SELECT ID, CREATETIME, LASTMODIFIED, TITLE, DESCRIPTION, OWNER_ID FROM Document WHERE (ID = CAST (? AS BIGINT ))
bind => [4508551237913941891]
文档：AAA
文档-附件：BBB
--SELECT ID, CREATETIME, NAME, TYPE, SIZE, LASTMODIFIED, TITLE FROM KMFile WHERE (ID = CAST (? AS BIGINT ))
bind => [4508551237913941892]
附件：BBB
附件-文档：AAA
Relationships: http://www.devx.com/Java/Article/33906/0/page/2