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