Hibernate一对一数据关联讲解

对于ORM而言，一个重要的特性就是对实体之间关联关系的管理。数据关联是ORM一个重要的特征，但往往也是导致性能低下的原因，在开发中要特别注意这一点。

一对一关联有两种类型：主键关联和唯一外键关联

一.主键关联：
一对一主键关联形式，即两张关联表通过主键形成一对一映射关系。
例如，一个中国公民只能有一份护照。

1.数据模型如下



2.表定义sql

DROP TABLE T_User;
DROP TABLE T_Passport;

CREATE TABLE T_Passport(
id INT NOT NULL
,serial VARCHAR ( 30 )
,expiry INT
, PRIMARY KEY (id)
);

CREATE TABLE T_User(
id INT NOT NULL AUTO_INCREMENT
,name VARCHAR ( 50 )
,age INT
, PRIMARY KEY (id)
, INDEX (id)
, CONSTRAINT FK_T_User_1 FOREIGN KEY (id)
REFERENCES T_Passport(id)
);

3.POJO类
TUser.java

packagecn.blogjava.start;

publicclassTUserimplementsjava.io.Serializable{
//Fields
privateIntegerid;
privateIntegerage;
privateStringname;
privateTPassportpassport;


//Constructors

publicIntegergetAge(){
returnage;
}

publicvoidsetAge(Integerage){
this.age=age;
}

publicTPassportgetPassport(){
returnpassport;
}

publicvoidsetPassport(TPassportpassport){
this.passport=passport;
}

/**defaultconstructor*/
publicTUser(){
}

/**constructorwithid*/
publicTUser(Integerid){
this.id=id;
}
//Propertyaccessors

publicIntegergetId(){
returnthis.id;
}

publicvoidsetId(Integerid){
this.id=id;
}

publicStringgetName(){
returnthis.name;
}

publicvoidsetName(Stringname){
this.name=name;
}

}

TPassport.java

packagecn.blogjava.start;

importjava.io.Serializable;

publicclassTPassportimplementsSerializable{
privateIntegerid;
privateStringserial;
privateIntegerexpiry;
privateTUseruser;

publicIntegergetExpiry(){
returnexpiry;
}
publicvoidsetExpiry(Integerexpiry){
this.expiry=expiry;
}
publicIntegergetId(){
returnid;
}
publicvoidsetId(Integerid){
this.id=id;
}
publicStringgetSerial(){
returnserial;
}
publicvoidsetSerial(Stringserial){
this.serial=serial;
}
publicTUsergetUser(){
returnuser;
}
publicvoidsetUser(TUseruser){
this.user=user;
}

}

3.配置文件
TUser.hbm.xml

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<classname="cn.blogjava.start.TUser"table="T_User"catalog="sample">
<idname="id"type="integer">
<columnname="id"/>
<generatorclass="native"/>
</id>
<propertyname="name"type="string">
<columnname="name"length="100"not-null="true"/>
</property>
<propertyname="age"type="java.lang.Integer"column="age"/>
<one-to-onename="passport"
class="cn.blogjava.start.TPassport"
cascade="all"
outer-join="true"
/>
</class>
</hibernate-mapping>

注意：outer-join="true"的设置，最后面的这样测试打印的结果:
Hibernate: select tuser3x0_.id as id18_1_, tuser3x0_.name as name18_1_, tuser3x0_.age as age18_1_, tpassport1_.id as id19_0_, tpassport1_.serial as serial19_0_, tpassport1_.expiry as expiry19_0_ from sample.t_user3 tuser3x0_ left outer join sample.t_passport tpassport1_ on tuser3x0_.id=tpassport1_.id where tuser3x0_.id=?
user name is byf
passport serial NO. is PCN123456
可以看到，当我们加载TUser实例时，Hibernate通过left outer join将t_user表及其关联的t_passpord完成读入。
如果我们将这里的节点的outer-join="false",那么Hibernate将执行下面的SQL语句执行：
Hibernate: select tuser3x0_.id as id10_, tuser3x0_.name as name10_, tuser3x0_.age as age10_ from sample.t_user3 tuser3x0_ where tuser3x0_.id=2
Hibernate: select tpassport0_.id as id11_0_, tpassport0_.serial as serial11_0_, tpassport0_.expiry as expiry11_0_ from sample.t_passport tpassport0_ where tpassport0_.id=?
user name is byf
passport serial NO. is PCN123456

设为outer-join="true"，这本也是Hibernate提升性能减少数据库访问的一个优化措施。
但是当outer-join="true"时,你的lazy属性无论设置为任何值,Collection都会初始加载(
one-to-one没有lazy 因此不必考虑)请大家谨慎设置outer-join="true"

TPassport.hbm.xml

<?xmlversion="1.0"?>
<!DOCTYPEhibernate-mappingPUBLIC"-//Hibernate/HibernateMappingDTD3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<classname="cn.blogjava.start.TPassport"table="T_Passport"catalog="sample">
<idname="id"type="integer"column="id">
<generatorclass="foreign">
<paramname="property">user</param>
</generator>
</id>
<propertyname="serial"type="string"column="serial"/>
<propertyname="expiry"type="java.lang.Integer"column="expiry"/>

<one-to-onename="user"
class="cn.blogjava.start.TUser"
constrained="true"
/>
</class>
</hibernate-mapping>

4.测试代码

packagecn.blogjava.start;

importjava.util.List;

importjunit.framework.Assert;
importjunit.framework.TestCase;

importorg.hibernate.HibernateException;
importorg.hibernate.Session;
importorg.hibernate.SessionFactory;
importorg.hibernate.Transaction;
importorg.hibernate.cfg.Configuration;


publicclassHibernateTestextendsTestCase{

Sessionsession=null;
protectedvoidsetUp(){
try{

Configurationconfig=newConfiguration().configure();
SessionFactorysessionFactory=config.buildSessionFactory();
session=sessionFactory.openSession();

}catch(HibernateExceptione){
e.printStackTrace();
}
}

protectedvoidtearDown(){
try{
session.close();
}catch(HibernateExceptione){
e.printStackTrace();
}
}

publicvoidtestInsert(){
Transactiontran=null;
try{

TUseruser=newTUser();
user.setName("byf");
user.setAge(newInteger(26));

TPassportpassport=newTPassport();
passport.setSerial("PCN123456");
passport.setExpiry(newInteger(20081010));

//设置相互关联
user.setPassport(passport);
passport.setUser(user);

tran=session.beginTransaction();
session.save(user);
session.flush();
tran.commit();
Assert.assertEquals(user.getId().intValue()>0,true);
}catch(HibernateExceptione){
e.printStackTrace();
Assert.fail(e.getMessage());
if(tran!=null){
try{
tran.rollback();
}catch(Exceptione1){
e1.printStackTrace();
}
}
}
}

/**
* 对象读取测试(Select方法)
*/
public void testSelect2(){
try {
TUser user = (TUser)session.load(TUser.class, new Integer(3));
System.out.println("user name is " + user.getName());
System.out.println("passport serial NO. is " + user.getPassport().getSerial());
Assert.assertEquals(user.getName(), "byf");
} catch (Exception e) {
e.printStackTrace();
Assert.fail(e.getMessage());
}
}
}