本文主要針對(duì)一對(duì)多情況下讀取父類(lèi)的子集合時(shí)，hibernate 的lazy屬性在其中的影響進(jìn)行總結(jié)。(以下代碼運(yùn)行在jdk1.5,jboss eclipse ide 1.5，hibernate 3.1環(huán)境下)
假設(shè)有：父類(lèi) Person? (含有Set類(lèi)型屬性Address)，
??????????????? 子類(lèi) Address(碰巧集合的名字和子類(lèi)的名字都是Address,不要混淆了)
?????????????? ? Person.hbm.xml? 主要片段：
??????? <id
??????????? name="idx"
??????????? column="idx"
??????????? type="long"
??????? >
??????????? <generator class="identity">
??????????? </generator>
??????? </id>
??????? <property
??????????? name="age"
??????????? type="int"
??????????? update="true"
??????????? insert="true"
??????????? column="age"
??????? />
??????? <property
??????????? name="name"
??????????? type="java.lang.String"
??????????? update="true"
??????????? insert="true"
??????????? column="name"
??????? />
??????? <set
??????????? name="address"
??????????? table="address"
??????????? lazy="true"
??????????? cascade="none"
??????????? sort="unsorted"
??????? >
??????????? <key
??????????? >
??????????????? <column
??????????????????? name="personidx"
??????????????? />
??????????? </key>
??????????? <one-to-many
????????????????? class="com.abc.common.pojo.Address"
??????????? />
??????? </set>
????(1)在session 的周期內(nèi)，無(wú)論lazy 設(shè)為true or false,?不會(huì)有任何限制。訪問(wèn)父子數(shù)據(jù)的代碼如下所示?：???????
??? ?//打開(kāi)session??
???? session = HibernateUtil.currentSession();
?????PersonDAO dao = new PersonDAO();
???? Person person = null;
???? person = (Person)dao.findByPrimaryKey(4);
?????Set addressSet = person.getAddress();
??? Address[] addressAry = new Address[addressSet.size()];
??? Address address = null ;
?? ?addressSet.toArray(addressAry);
????
??? for(int i=0 ;i<addressAry.length;i++){
?????????????????? ?................??????
???? }
?? //session關(guān)閉
??? session.close();
?????if (session.isOpen()){
??????? ? HibernateUtil.closeSession();
??? }
(2)在session的周期外，訪問(wèn)父子數(shù)據(jù)的代碼如下所示?：
???? //打開(kāi)session??
???? session = HibernateUtil.currentSession();
?????PersonDAO dao = new PersonDAO();
???? Person person = null;
???? person = (Person)dao.findByPrimaryKey(4);
?????/**********************
????? *留待后續(xù)處理
????? *********************/
???? session.close();
????
???? //session關(guān)閉之后才訪問(wèn)person的子集
?????Set addressSet = person.getAddress();
??? Address[] addressAry = new Address[addressSet.size()];
??? Address address = null ;
?? ?addressSet.toArray(addressAry);
????
??? for(int i=0 ;i<addressAry.length;i++){
?????????????????? ?................??????
???? }
?????if (session.isOpen()){
??????? ? HibernateUtil.closeSession();
??? }
?? 此時(shí)，上述代碼的運(yùn)行結(jié)果根據(jù)lazy的設(shè)置的不同而不同
lazy=false
???結(jié)果：可以訪問(wèn)得到Person和Address的數(shù)據(jù)
lazy= true
??? 根據(jù)代碼的寫(xiě)法有不同
??? (1)代碼其他處不做任何處理，則拋出異常
???????? org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role:?
???????? 。。。。。。
??? (2)如果做一些處理如下，將上述那段代碼中的"留待后續(xù)處理"換成以下代碼
?????????? Hibernate.initialize(person.getAddress());? 則可以訪問(wèn)得到Person和Address的數(shù)據(jù)
???????? 實(shí)際編寫(xiě)時(shí)，不會(huì)象上述這樣的寫(xiě)法，即將
???????? Hibernate.initialize(person.getAddress());和person.getAddress()在同一個(gè)方法里面調(diào)用。他們往往出現(xiàn)在應(yīng)用程序的不同層次中(前者出現(xiàn)在DAO層居多，而后者則出現(xiàn)在web層居多).