hibernate加载策略之lazy

一、hibernate的加载策略有两种：

1）即时加载--get加载数据

使用get获取数据，会立即查询，先到缓存中查找，找不到再到数据库中查找。

2）延迟加载（懒加载lazy）--load加载

不会立即查找，当需要的时候才会查找。

容易造成LazyInitializationException异常。如果出现这个异常，大部分是因为session被关闭了，还要从session中获取数据造成的。

但是懒加载也是有优势的，那就是效率高。因为我们查数据时不一定要把所有数据都查询出来。

注：load支持延迟加载，get不支持延迟加载。如果没有设置延迟加载，那么load也会立即加载对象。

二、class的lazy

class默认情况下是支持懒加载的（在*.hbm.xml中的class中有一个属性lazy="true"），所以在load方法时，是懒加载，但是当我们给成lazy="false"时，就不支持懒加载了，这里即使使用load方法，也会是即时加载。



我们来看项目案例：



Book实体类

public class Book {
private int id;
private String name;
private double price;
private String author;
private Date pubDate;
public Book() {
}
public Book(String name, double price) {
super();
this.name = name;
this.price = price;
}
@Override
public String toString() {
return "Book [id=" + id + ", name=" + name + ", price=" + price
+ ", author=" + author + ", pubDate=" + pubDate + "]";
}
//省略get/set
}

Book.hbm.xml配置文件
<hibernate-mapping package="com.test.pojo">
<class name="Book" table="book" lazy="false">
<id name="id">
<generator class="identity" />
</id>
<property name="author" />
<property name="name" column="book_name" />
<property name="price" />
<property name="pubDate" />
</class>
</hibernate-mapping> HibernateUtil类
public class HibernateUtil {
private static Configuration cfg=null;
private static SessionFactory factory=null;
private static Session session=null;
static{
cfg=new Configuration().configure();
factory=cfg.buildSessionFactory(new StandardServiceRegistryBuilder()
.applySettings(cfg.getProperties()).build());
}
public static Session getSession(){
if(factory!=null)
return session=factory.openSession();
factory=cfg.buildSessionFactory(new StandardServiceRegistryBuilder()
.applySettings(cfg.getProperties()).build());
return session=factory.openSession();
}
public static void closeSession(){
if(session!=null && session.isOpen())
session.close();
}
}hibernate.cfg.xml配置文件
<hibernate-configuration>
<session-factory>
<property name="connection.driver_class">
com.mysql.jdbc.Driver
</property>
<property name="connection.url">
jdbc:mysql:///hibernatetest
</property>
<property name="connection.username">root</property>
<property name="connection.password">root</property>
<property name="dialect">
org.hibernate.dialect.MySQLDialect
</property>
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<property name="hbm2ddl.auto">update</property>
<mapping resource="com/test/pojo/Book.hbm.xml" />
</session-factory>
</hibernate-configuration>HibernateTest测试类
public class HibernateTest {
@Test
public void testCreateDB(){
Configuration cfg=new Configuration().configure();
SchemaExport se=new SchemaExport(cfg);
se.create(true, true);
}
@Test
public void testSave(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
Book book=new Book();
book.setName("读者");
book.setPrice(21.5);
book.setAuthor("读者出版传媒有限公司");
book.setPubDate(new Date());
Book book1=new Book();
book1.setName("蓝血人");
book1.setPrice(50);
book1.setAuthor("卫斯理");
book1.setPubDate(new Date());
Book book2=new Book();
book2.setName("傲慢与偏见");
book2.setPrice(80);
book2.setAuthor("简.奥斯丁");
book2.setPubDate(new Date());
Book book3=new Book();
book3.setName("中国历史");
book3.setPrice(21.5);
book3.setAuthor("人民");
book3.setPubDate(new Date());
Book book4=new Book();
book4.setName("七龙珠");
book4.setPrice(21.5);
book4.setAuthor("不知道");
book4.setPubDate(new Date());
Book book5=new Book();
book5.setName("名侦探柯南");
book5.setPrice(50);
book5.setAuthor("宫崎骏");
book5.setPubDate(new Date());

session.save(book);
session.save(book1);
session.save(book2);
session.save(book3);
session.save(book4);
session.save(book5);
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testGet(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
Book book=(Book) session.get(Book.class, 1);
System.out.println("书名："+book.getName());
tx.commit();
HibernateUtil.closeSession();
System.out.println("书名："+book.getName());
}
@Test
public void testLoad(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
Book book=(Book) session.load(Book.class, 1);
// System.out.println("书名："+book.getName());
tx.commit();
HibernateUtil.closeSession();
//当Book.hbm.xml配置class中的lazy="true"时，以下操作会报错
//open session in view
System.out.println("书名："+book.getName());
}
}

===========================================================
三、现在我们再看另一种懒加载情况

项目结构如下：



Book实体类

public class Book {
private int id;
private String name;
private double price;
private String author;
private Date pubDate;
private Category category;
public Book() {
}
public Book(String name, double price) {
super();
this.name = name;
this.price = price;
}
@Override
public String toString() {
return "Book [id=" + id + ", name=" + name + ", price=" + price
+ ", author=" + author + ", pubDate=" + pubDate + "]";
}
//省略get/set
}Book.hbm.xml配置文件
<hibernate-mapping package="com.test.pojo">
<class name="Book" table="book">
<id name="id">
<generator class="identity" />
</id>
<many-to-one name="category" class="Category" column="category_id"
cascade="save-update"/>
<property name="author" />
<property name="name" column="book_name" />
<property name="price" />
<property name="pubDate" />
</class>
</hibernate-mapping>

Category实体类
public class Category {
private int id;
private String name;
private Set<Book> books=new HashSet<Book>();
//省略get/set
}Category.hbm.xml配置文件
<hibernate-mapping package="com.test.pojo">
<class name="Category" >
<id name="id" >
<generator class="identity" />
</id>
<property name="name" />
<set name="books" inverse="true" lazy="true">
<key>
<column name="category_id" />
</key>
<one-to-many class="Book" />
</set>
</class>

</hibernate-mapping>

HibernateTest
public class HibernateTest {
@Test
public void testCreateDB(){
Configuration cfg=new Configuration().configure();
SchemaExport se=new SchemaExport(cfg);
se.create(true, true);
}
@Test
public void testSave(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
Category category=new Category();
category.setName("文学");
Category category1=new Category();
category1.setName("科幻");
Category category2=new Category();
category2.setName("历史");
Category category3=new Category();
category3.setName("言情");
Category category4=new Category();
category4.setName("漫画");
Category category5=new Category();
category5.setName("悬疑");
Book book=new Book();
book.setName("读者");
book.setPrice(21.5);
book.setAuthor("读者出版传媒有限公司");
book.setPubDate(new Date());
book.setCategory(category);
Book book1=new Book();
book1.setName("蓝血人");
book1.setPrice(50);
book1.setAuthor("卫斯理");
book1.setPubDate(new Date());
book1.setCategory(category1);
Book book2=new Book();
book2.setName("傲慢与偏见");
book2.setPrice(80);
book2.setAuthor("简.奥斯丁");
book2.setPubDate(new Date());
book2.setCategory(category3);
Book book3=new Book();
book3.setName("中国历史");
book3.setPrice(21.5);
book3.setAuthor("人民");
book3.setPubDate(new Date());
book3.setCategory(category2);
Book book4=new Book();
book4.setName("七龙珠");
book4.setPrice(21.5);
book4.setAuthor("不知道");
book4.setPubDate(new Date());
book4.setCategory(category4);
Book book5=new Book();
book5.setName("名侦探柯南");
book5.setPrice(50);
book5.setAuthor("宫崎骏");
book5.setPubDate(new Date());
book5.setCategory(category5);

session.save(book);
session.save(book1);
session.save(book2);
session.save(book3);
session.save(book4);
session.save(book5);
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testGet(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
/**
* 当Category.hbm.xml中的set中的lazy属性是true（默认为true）时，
* 先查询一端的Category，只会先查询Category数据；
*
* 当执行下面for中的category.getBooks时才会去查询Book数据，这时就是懒加载
*
* 但是当我们把Category.hbm.xml中的set中的lazy属性设为false时，
* 会在查询Category的时候，把Book也查询出来，这时就是即时加载
*/

Category category=(Category) session.get(Category.class, 1);
System.out.println("分类名:"+category.getName());
for(Iterator<Book> iter=category.getBooks().iterator();iter.hasNext();){
System.out.println(iter.next().getName());
}
tx.commit();
HibernateUtil.closeSession();
}
@Test
public void testLoad(){
Session session=HibernateUtil.getSession();
Transaction tx=session.beginTransaction();
Category category=(Category) session.get(Category.class, 1);
System.out.println("分类名:"+category.getName());
System.out.println("对应的书大小："+category.getBooks().size());
tx.commit();
HibernateUtil.closeSession();
}
}断点运行testLoad，当运行到下面这句时，并没有查询数据的sql语句打印

Category category=(Category) session.get(Category.class, 1);

当执行到下面这句时，才会打印出sql语句
System.out.println("分类名:"+category.getName());控制台打印sql语句如下：
Hibernate:
select
category0_.id as id1_0_0_,
category0_.name as name2_0_0_
from
Category category0_
where
category0_.id=?

继续运行
System.out.println("对应的书大小："+category.getBooks().size());执行这句后，打印sql语句如下：
Hibernate:
select
books0_.category_id as category2_0_0_,
books0_.id as id1_1_0_,
books0_.id as id1_1_1_,
books0_.category_id as category2_1_1_,
books0_.author as author3_1_1_,
books0_.book_name as book_nam4_1_1_,
books0_.price as price5_1_1_,
books0_.pubDate as pubDate6_1_1_
from
book books0_
where
books0_.category_id=?当把Category.hbm.xml中set的lazy改为extra时，再执行
System.out.println("对应的书大小："+category.getBooks().size());打印sql语句如下：
Hibernate:
select
count(id)
from
book
where
category_id =?

总结：
1、set/list默认下是lazy=true的。支持懒加载，但是当使用size()的时候依然要去查询整个set集合的内容。

2、lazy=false时，立即查询所有集合的内容。

3、lazy=extra时，比较智能，支持懒加载，当使用size()的时候不会查询整个集合，仅仅查询集合元素的个数。当需要使用集合元素的内容时，再去查询集合内容。

四、单端关联上的lazy：（many-to-one,one-to-one），默认是支持懒加载的lazy=proxy。



五、property上也有懒加载，默认lazy=false。如果property要去支持懒加载，需要用到第三方增强，且大部分情况下用不到property懒加载，只有当是大对象Blob、Clob时才会用到。