EJB——JPA持久化数据

        继续前面的学习，我们现在要将实体持久化到数据库中，首先认识JPA实体，如下是Bit实体：

package ejb3inaction.example.persistence;

import java.io.Serializable;

import java.sql.Date;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Table;

@Entity//是POJO成为实体

@Table(name = "BIDS")//指定表映射

public class Bid implements Serializable {

private Date bidDate;

private Long bidId;

private Double bidPrice;

private Long itemId;

private String bidderId;

public Bid() {

}

public Bid(String bidderId, Long itemId, Double bidPrice) {

this.itemId = itemId;

this.bidderId = bidderId;

this.bidPrice = bidPrice;

}

@Id//包含实体id

@GeneratedValue(strategy = GenerationType.AUTO)//自动生成id

@Column(name = "BID_ID")//指定列映射

public Long getBidId() {

return bidId;

}

@Column(name = "BID_DATE")

public Date getBidDate() {

return bidDate;

}

public void setBidDate(Date bidDate) {

this.bidDate = bidDate;

}

public void setBidId(Long bidId) {

this.bidId = bidId;

}

@Column(name = "BID_PRICE")

public Double getBidPrice() {

return bidPrice;

}

public void setBidPrice(Double bidPrice) {

this.bidPrice = bidPrice;

}

@Column(name = "BID_ITEM_ID")

public Long getItemId() {

return itemId;

}

public void setItemId(Long itemId) {

this.itemId = itemId;

}

@Column(name = "BID_BIDDER")

public String getBidderId() {

return bidderId;

}

public void setBidderId(String bidderId) {

this.bidderId = bidderId;

}

}

         @Entity注解表示Bid类是JPA实体。Bid是不需要业务接口的POJO，与会话bean和消息驱动bean不同。@Table注解通知JPA Bid实体映射到BIDS表。@Column注解表示字段，@Id注解使bidId属性成为实体主键。@GeneratedValue注解被设置为GenerationType.AUTO，表示实体保存到数据库中时，持久化提供器自动生成主键。

        实体不能自己保存到数据库，JPA的EntityManager通过读取配置ORM配置执行一系列操作，并通过基于API的接口提供实体持久化服务。下面代码是PlaceBid EJB如何使用EntityManager API持久化Bid实体：

package ejb3inaction.example.buslogic;

import javax.ejb.Stateless;

import javax.persistence.EntityManager;

import javax.persistence.PersistenceContext;

import ejb3inaction.example.persistence.Bid;

@Stateless(name = "PlaceBid")

public class PlaceBidBean implements PlaceBid {

//注入EntityManager实例

@PersistenceContext(unitName = "test")

private EntityManager em;

public PlaceBidBean() {

}

public Bid addBid(Bid bid) {

System.out.println("Adding bid, bidder ID=" + bid.getBidderId()

+ ", item ID=" + bid.getItemId() + ", bid amount="

+ bid.getBidPrice() + ".");

return save(bid);

}

private Bid save(Bid bid) {

	  //持久化实体实例

em.persist(bid);

System.out.println("Your bid your item id:" + bid.getItemId()

+ "was successful");

System.out.println("Your bid id is: " + bid.getBidId());

return bid;

}

}

      管理器读取Bid实体上的ORM映射注解，判断如何把实体保存到数据库中。通过@PersistenceContext注解把EntityManager注入到实体中。@ PersistenceContext注解的unitName参数指向特有的持久化单元。