如果是一對多，多對一，甚至是多對多那效率就差別大了！！如果一對多lazy=”false”或者fetch=”join”會同時查詢出所有關聯的物件，對資料庫和對網路影響將是很大地。（lazy=”false” 採用多條查詢語句，比如，一對100:會有可能101條select查詢語句；fetch=”join” 會同時訪問兩個表的）

但是代理物件呼叫getClass()和getId()方法的時候不會立刻載入，.class和id的資訊是隨物件的！


domain物件

public class User {

	private Integer id;
	private String name;
	private Float wage;
	private Set<Book> bookSet = new HashSet<Book>();

public class Book {

	private Integer id;
	private String bookname;
	private User user;

對映檔案

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.itcast.hibernate.lazyOneToOne">

	<class name="User" table="user" lazy="false">
		<id name="id" unsaved-value="-1">
			<generator class="native" />
		</id>
		<property name="name" />
		<property name="wage"/>
		<set name="bookSet"  lazy="false" fetch="select">
		     <key column="u_id"/>
		     <one-to-many class="Book"/>
		</set>

	</class>
</hibernate-mapping>
 

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.itcast.hibernate.lazyOneToOne">

	<class name="Book" table="book">
		<id name="id" unsaved-value="-1">
			<generator class="native" />
		</id>
		<property name="bookname" />

		<many-to-one name="user" column="u_id"/>

	</class>
</hibernate-mapping>
 

測試程式碼：

User user = (User)session.get(User.class, 1);

此時的sql語句為：

Hibernate: 
    select
        user0_.id as id0_0_,
        user0_.name as name0_0_,
        user0_.wage as wage0_0_ 
    from
        user user0_ 
    where
        user0_.id=?
Hibernate: 
    select
        bookset0_.u_id as u3_1_,
        bookset0_.id as id1_,
        bookset0_.id as id1_0_,
        bookset0_.bookname as bookname1_0_,
        bookset0_.u_id as u3_1_0_ 
    from
        book bookset0_ 
    where
        bookset0_.u_id=?
 

如果對映檔案改為：

<set name="bookSet"  lazy="false" fetch="join">
		     <key column="u_id"/>
		     <one-to-many class="Book"/>
		</set>

sql語句為：

Hibernate: 
    select
        user0_.id as id0_1_,
        user0_.name as name0_1_,
        user0_.wage as wage0_1_,
        bookset1_.u_id as u3_3_,
        bookset1_.id as id3_,
        bookset1_.id as id1_0_,
        bookset1_.bookname as bookname1_0_,
        bookset1_.u_id as u3_1_0_ 
    from
        user user0_ 
    left outer join
        book bookset1_ 
            on user0_.id=bookset1_.u_id 
    where
        user0_.id=?

如果對映檔案為：

		<set name="bookSet"  lazy="true">
		     <key column="u_id"/>
		     <one-to-many class="Book"/>
		</set>

只是訪問一個表了：

Hibernate: 
    select
        user0_.id as id0_0_,
        user0_.name as name0_0_,
        user0_.wage as wage0_0_ 
    from
        user user0_ 
    where
        user0_.id=?