10.2.2 实现Domain Object
各Domain Object之间存在的关联关系,由图10.8可以清楚地看到,其中AuctionUser和Item两个持久化类之间的关系尤为复杂,它们之间存在两种1对N的关系,分别是所属和赢取两种关系。
下面是AuctionUser类的代码:
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| package org.crazyit.auction.domain;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
@Entity
@Table(name="auction_user")
public class AuctionUser
{
@Id
@Column(name="user_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Integer id;
private String username;
private String userpass;
private String email;
@OneToMany(targetEntity=Item.class ,
mappedBy="owner")
private Set<Item> itemsByOwner = new HashSet<>();
@OneToMany(targetEntity=Item.class ,
mappedBy="winer")
private Set<Item> itemsByWiner = new HashSet<>();
@OneToMany(targetEntity=Bid.class ,
mappedBy="bidUser")
private Set<Bid> bids = new HashSet<>();
public AuctionUser()
{
}
public AuctionUser(Integer id , String username
, String userpass , String email)
{
this.id = id;
this.username = username;
this.userpass = userpass;
this.email = email;
}
public void setId(Integer id)
{
this.id = id;
}
public Integer getId()
{
return this.id;
}
public void setUsername(String username)
{
this.username = username;
}
public String getUsername()
{
return this.username;
}
public void setUserpass(String userpass)
{
this.userpass = userpass;
}
public String getUserpass()
{
return this.userpass;
}
public void setEmail(String email)
{
this.email = email;
}
public String getEmail()
{
return this.email;
}
public void setItemsByOwner(Set<Item> itemsByOwner)
{
this.itemsByOwner = itemsByOwner;
}
public Set<Item> getItemsByOwner()
{
return this.itemsByOwner;
}
public void setItemsByWiner(Set<Item> itemsByWiner)
{
this.itemsByWiner = itemsByWiner;
}
public Set<Item> getItemsByWiner()
{
return this.itemsByWiner;
}
public void setBids(Set<Bid> bids)
{
this.bids = bids;
}
public Set<Bid> getBids()
{
return this.bids;
}
}
|
在AuctionUser 类里保留了两个Set<Item>属性:itemsByOwner和itemsByWiner,其中itemsByOwner用于访问属于该用户的全部拍卖物品,而itemsByWiner用于访问该用户赢取的全部拍卖物品,这两个属性的关联实体都是Item。此外,AuctionUser类里还有一个bids属性,该属性用于访问该用户参与的全部竞价记录。
本示例没有使用映射文件来管理Hibernate实体,而是采用Annotation来管理持久化实体。在该程序中,在定义AuctionUser类时使用了@Entity修饰,该Annotation可将该类映射成持久化实体。除此之外,AuctionUser实体和3个关联实体存在1对N的关系,因此需要在AuctionUser类的itemsByOwner、itemsByWiner和bids属性上使用@OneToMany修饰。
本系统将把所有1对N关联都映射成双向关联,因此我们为3个@OneToMany元素都指定了mappedBy属性,这表明AuctionUser实体(1的一端)不控制关联关系。根据Hibernate的建议:对于双向1对N关联,不要让1的一端控制关系,而应该让N的一端控制关联关系,这样可以保证更好的性能。
Item类除了和AuctionUser之间存在两种N对1的关联关系外,还和Bid之间存在1对N的关联关系,和Kind之间存在N对1的关联关系,和State之间存在N对1的关联关系。下面是Item类的代码:
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| package org.crazyit.auction.domain;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;
@Entity
@Table(name="item")
public class Item
{
@Id
@Column(name="item_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Integer id;
@Column(name="item_remark")
private String itemRemark;
@Column(name="item_name")
private String itemName;
@Column(name="item_desc")
private String itemDesc;
private Date addtime;
private Date endtime;
@Column(name="init_price")
private double initPrice;
@Column(name="max_price")
private double maxPrice;
@ManyToOne(targetEntity=AuctionUser.class)
@JoinColumn(name="owner_id", nullable=false)
private AuctionUser owner;
@ManyToOne(targetEntity=AuctionUser.class)
@JoinColumn(name="winer_id", nullable=true)
private AuctionUser winer;
@ManyToOne(targetEntity=Kind.class)
@JoinColumn(name="kind_id", nullable=false)
private Kind kind;
@ManyToOne(targetEntity=State.class)
@JoinColumn(name="state_id", nullable=false)
private State itemState;
@OneToMany(targetEntity=Bid.class ,
mappedBy="bidItem")
private Set<Bid> bids = new HashSet<Bid>();
public Item()
{
}
public Item(Integer id , String itemRemark , String itemName ,
String itemDesc , Date addtime , Date endtime ,
double initPrice , double maxPrice , AuctionUser owner)
{
this.id = id;
this.itemRemark = itemRemark;
this.itemName = itemName;
this.itemDesc = itemDesc;
this.addtime = addtime;
this.endtime = endtime;
this.initPrice = initPrice;
this.maxPrice = maxPrice;
this.owner = owner;
}
public void setId(Integer id)
{
this.id = id;
}
public Integer getId()
{
return this.id;
}
public void setItemRemark(String itemRemark)
{
this.itemRemark = itemRemark;
}
public String getItemRemark()
{
return this.itemRemark;
}
public void setItemName(String itemName)
{
this.itemName = itemName;
}
public String getItemName()
{
return this.itemName;
}
public void setItemDesc(String itemDesc)
{
this.itemDesc = itemDesc;
}
public String getItemDesc()
{
return this.itemDesc;
}
public void setAddtime(Date addtime)
{
this.addtime = addtime;
}
public Date getAddtime()
{
return this.addtime;
}
public void setEndtime(Date endtime)
{
this.endtime = endtime;
}
public Date getEndtime()
{
return this.endtime;
}
public void setInitPrice(double initPrice)
{
this.initPrice = initPrice;
}
public double getInitPrice()
{
return this.initPrice;
}
public void setMaxPrice(double maxPrice)
{
this.maxPrice = maxPrice;
}
public double getMaxPrice()
{
return this.maxPrice;
}
public void setOwner(AuctionUser owner)
{
this.owner = owner;
}
public AuctionUser getOwner()
{
return this.owner;
}
public void setKind(Kind kind)
{
this.kind = kind;
}
public Kind getKind()
{
return this.kind;
}
public void setWiner(AuctionUser winer)
{
this.winer = winer;
}
public AuctionUser getWiner()
{
return this.winer;
}
public void setItemState(State itemState)
{
this.itemState = itemState;
}
public State getItemState()
{
return this.itemState;
}
public void setBids(Set<Bid> bids)
{
this.bids = bids;
}
public Set<Bid> getBids()
{
return this.bids;
}
}
|
这里的5行粗体字代码为Item类映射了它关联的4个关联实体,其中
owner引用该物品的所属者,winer 引用该物品的赢取者,这两个属性都引用到AuctionUser 实体。
Item 类和AuctionUser、Kind、State之间存在N对1的关联,因此需要使用@ManyToOne来修饰这些属性;
Item类和Bid之间存在1对N的关联,因此需要使用@OneToMany来修饰该属性。
@ManyToOne的fetch属性默认为FetchType.EAGER,该属性将会取消延迟加载。对于N对1的关联映射,N的一端的实体只有一个关联实体,如果取消了延迟加载,那么系统加载N的一端的实体时,1的一端的实体同时也会被加载—这不会有太大的问题,因为只是额外多加载了一条记录。
在@ManyToOne中取消延迟加载不会有太大的性能下降。但对于@OneToMany等修饰集合的Annotation则尽量不要取消延迟加载。一旦为修饰集合属性的Annotation取消延迟加载,则意味着加载主表记录时,引用该记录的所有从表记录也会同时被加载,这会产生一个问题:我们无法预料有多少条从表记录引用该主表记录,同时加载这些从表记录可能引起巨大的性能下降。
本系统中还有Bid、State和Kind三个实体,但这三个实体都比Item和AuctionUser实体更简单。理解了Item和AuctionUser的映射关系,自然也就掌握了Bid、State和Kind实体的实现方法,故此处不再赘述。
原文链接: 10.2.2 实现Domain Object