one-to-many的懒加载及原理分析

如果是一对多，多对一，甚至是多对多那效率就差别大了！！如果一对多lazy=”false”或者fetch=”join”会同时查询出所有关联的对象，对数据库和对网络影响将是很大地。（lazy=”false” 采用多条查询语句，比如，一对100:会有可能101条select查询语句；fetch=”join” 会同时访问两个表的）

但是代理对象调用getClass()和getId()方法的时候不会立刻加载，.class和id的信息是随对象的！


domain对象

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;

映射文件

<?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>

测试代码：

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

此时的sql语句为：

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=?

如果映射文件改为：

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

sql语句为：

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=?

如果映射文件为：

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

只是访问一个表了：

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