JPA学习笔记-EJB-03JPA主键生成策略总结--3

 

package eo;   import javax.persistence.AttributeOverride; import javax.persistence.AttributeOverrides; import javax.persistence.Column; import javax.persistence.EmbeddedId; import javax.persistence.Entity; import javax.persistence.Table;   /**  * TbStudents entity. @author MyEclipse Persistence Tools  */ @Entity @Table(name = "tb_students", catalog = "ejbjpa") public class TbStudents implements java.io.Serializable {       // Fields       private TbStudentsId id;     private String info;       // Constructors       /** default constructor */     public TbStudents() {     }       /** minimal constructor */     public TbStudents(TbStudentsId id) {         this.id = id;     }       /** full constructor */     public TbStudents(TbStudentsId id, String info) {        this.id = id;        this.info = info;     }       // Property accessors     @EmbeddedId     @AttributeOverrides( {            @AttributeOverride(name = "name", column = @Column(name = "name", nullable = false)),            @AttributeOverride(name = "no", column = @Column(name = "no", nullable = false)) })     public TbStudentsId getId() {        return this.id;     }       public void setId(TbStudentsId id) {        this.id = id;     }       @Column(name = "info")     public String getInfo() {        return this.info;     }       public void setInfo(String info) {        this.info = info;     } }

TbStudentsId类是主键类，TbStudents是实体类。TbStudentsId是作为TbStudents的一个特殊属性存在的。代码还是以嵌入式的（@Embeddable）方式表现主键的。

下面写个DAO接口

package dao;   import javax.ejb.Remote;   import eo.TbStudents;   @Remote public interface ITbStudentsService {         public void save (TbStudents tbStudents);     }

实现类如下：

package dao.impl;   import javax.ejb.Stateless; import javax.persistence.EntityManager; import javax.persistence.PersistenceContext;   import dao.ITbStudentsService; import eo.TbStudents;   @Stateless public class TbStudentsServiceImpl implements ITbStudentsService {       @PersistenceContext(unitName = "JPAEJBPU")     private EntityManager entityManager;       @Override     public void save(TbStudents tbStudents) {        entityManager.persist(tbStudents);     } }

按照前文章节部署到JBOSS上面后，写一个测试代码，代码如下：

public void test04() {        try {              Context ctx = getInitialContext();              Object object = ctx.lookup("TbStudentsServiceImpl/remote");              ITbStudentsService tbStudentsService = (ITbStudentsService) PortableRemoteObject                   .narrow(object, ITbStudentsService.class);                       //对主键对象赋值            TbStudentsId id = new TbStudentsId();            id.setName("Klose");            id.setNo("11");            //对实体赋值            TbStudents tbStudents = new TbStudents();            tbStudents.setId(id);            tbStudents.setInfo("拜仁");              tbStudentsService.save(tbStudents);          } catch (NamingException e) {            e.printStackTrace();        }     }

第一次执行此方法后：结果数据库如下：

之后我们不改变任何代码在执行一遍此测试方法，就会发生以下主键重复异常：

Caused by: java.sql.BatchUpdateException: Duplicate entry 'Klose-11' for key 'PRIMARY'