http://blog.csdn.net/princetoad/archive/2007/10/10/1817693.aspx
一對(duì)一單向,顧名思義,就是只要求從A方到達(dá)B方,而不需要從B方到達(dá)A方,典型的例子就是,一個(gè)人對(duì)應(yīng)一個(gè)地址,因?yàn)楝F(xiàn)實(shí)生活中,一個(gè)地址可能住很多 人,所以一般我們只需要根據(jù)人查到它的地址,而不太會(huì)需要從一個(gè)地址去查誰(shuí)住在那里,不過(guò),真的有這種需求的話,我們就要以用另外一種關(guān)系來(lái)實(shí)現(xiàn)了,這個(gè) 以后再講
首先我們聲明一個(gè)人的實(shí)體類,Person
/**//*
* Person.java
*
* Created on 2007-9-15, 0:11:58
*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package lbf.entitybean.test1;
import java.io.Serializable;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
/** *//**
*
* @author Admin
*/
@Entity
public class Person implements Serializable 
{
private static final long serialVersionUID = 1L;
private Long id;
private String name;
private String sex;
private int age;
private Address address;
@OneToOne(cascade=CascadeType.ALL,optional=true)
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public void setId(Long id) {
this.id = id;
}
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Long getId() {
return id;
}
}
再來(lái)看看Address的實(shí)體BEAN定義
/**//*
* Address.java
*
* Created on 2007-9-15, 0:13:50
*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package lbf.entitybean.test1;
import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
/** *//**
*
* @author Admin
*/
@Entity
public class Address implements Serializable {
private static final long serialVersionUID = 1L;
private Long id;
private String country,province,city,street;
private int postcode;
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public int getPostcode() {
return postcode;
}
public void setPostcode(int postcode) {
this.postcode = postcode;
}
public String getStreet() {
return street;
}
public void setStreet(String street) {
this.street = street;
}
public void setId(Long id) {
this.id = id;
}
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Long getId() {
return id;
}
}
以上便是兩個(gè)類的定義,從Person里面我們可以看到如下的代碼
@OneToOne(cascade=CascadeType.ALL,optional=true)
public Address getAddress() {
return address;
}
在這里的@OneToOne就是注釋是一對(duì)一的關(guān)系,其中cascade是表示級(jí)聯(lián)的關(guān)系,是級(jí)聯(lián)刪除還是級(jí)聯(lián)更新,還是所有,我們這里選擇的是所有,這 樣的好處就是,我們?cè)诓迦隤erson的時(shí)候,就順帶的把Person里面包含的Address一起插入數(shù)據(jù)庫(kù)里面,當(dāng)我們刪除Person的時(shí)候,也是 把這個(gè)Person對(duì)應(yīng)的Address從數(shù)據(jù)庫(kù)里面刪除,否則就需要我們?nèi)斯さ膭h除兩遍或者插入兩遍等等,optional表示這個(gè)成員是不是可選的, 我們這里是可選的,也就是說(shuō)一個(gè)人可以沒(méi)有地址(比如流浪漢:)).
然后我們發(fā)現(xiàn),在Address里面只有一些簡(jiǎn)單的EntityBean的注釋,并沒(méi)有表示關(guān)系的注釋,這是因?yàn)楸纠且粚?duì)一單向的實(shí)現(xiàn),Person里 面有 Address,而Address卻什么都不知道,它對(duì)應(yīng)到數(shù)據(jù)庫(kù)里面只是一張簡(jiǎn)單的表而已,Person對(duì)應(yīng)到數(shù)據(jù)庫(kù)里面就有一個(gè)指向 Address的外鍵.我們也可以增加注釋指定外鍵的列的名字,如下:
@OneToOne(cascade=CascadeType.ALL,optional=true)
@JoinColumn(name="addressID")
public Address getAddress() {
return address;
}
如果我們不加的話,也是可以通過(guò)的,在JBOSS里面,它會(huì)自動(dòng)幫你生成你指向這個(gè)類的類名加上下劃線再加上id的列,也就是默認(rèn)列名是:address_id.
如果是主鍵相關(guān)聯(lián)的話,那么可以運(yùn)用如下注釋
@OneToOne(cascade={CascadeType.ALL})
@PrimaryKeyJoinColumn
public Address getAddress( ) {
return homeAddress;
}
它表示兩張表的關(guān)聯(lián)是根據(jù)兩張表的主鍵的
以下是一些注釋的定義,我們看一下可以了解一下這些注釋有哪些方法,
public @interface JoinColumn
{
String name( ) default "";
String referencedColumnName( ) default "";
boolean unique( ) default false;
boolean nullable( ) default true;
boolean insertable( ) default true;
boolean updatable( ) default true;
String columnDefinition( ) default "";
String table( ) default "";
}
public @interface OneToOne
{
Class targetEntity( ) default void.class;
CascadeType[] cascade( ) default {};
FetchType
fetch( ) default EAGER;
boolean optional( ) default true;
String mappedBy( ) default "";
}
public @interface PrimaryKeyJoinColumn
{
String name( ) default "";
String referencedColumnName( ) default "";
String columnDefinition( ) default "";
}
好了,一對(duì)一單向差不多就這么些吧,明天再看一下一對(duì)一雙向的情況.