hibernate中get 与 load 区别

session.get 与 session.load区别
1. 对于get方法，hibernate会确认一下该id对应的数据是否存在，首先在session缓存中查找，然后在二级缓存中查找，还没有就查询数据库，数据库中没有就返回null。

2. load方法加载实体对象的时候，根据映射文件上类级别的lazy属性的配置(默认为true)，分情况讨论：

(1)若为true,则首先在Session缓存中查找，看看该id对应的对象是否 存在，不存在则使用延迟加载，返回实体的代理类对象(该代理类为实体类的子类，由CGLIB动态生成)。等到具体使用该对象(除获取OID以外)的时候， 再查询二级缓存和数据库，若仍没发现符合条件的记录，则会抛出一个ObjectNotFoundException。

(2)若为false,就跟get方法查找顺序一样，只是最终若没发现符合条件的记录，则会抛出一个ObjectNotFoundException。

这里get和load有两个重要区别:

如果未能发现符合条件的记录，get方法返回null，而load方法会抛出一个ObjectNotFoundException。
load方法可返回没有加载实体数据的代理类实例，而get方法永远返回有实体数据的对象。(对于load和get方法返回类型：好多书中都说：“get方法永远只返回实体类”，实际上并不正确，get方法如果在session缓存中找到了该id对应的对象，如果刚好该对象前面是被代理过的，如被load方法使用过，或者被其他关联对象延迟加载过，那么返回的还是原先的代理对象，而不是实体类对象，如果该代理对象还没有加载实体数据（就是id以外的其他属性数据），那么它会查询二级缓存或者数据库来加载数据，但是返回的还是代理对象，只不过已经加载了实体数据。)

以下是代码实例:

Category.hbm.xml:

<hibernate-mapping package="cn.itcast.bbs.entities">
<class name="Category" table="itcast_category" lazy="false" ><<<<<<<<<<<<<<1.

<id name="id" >
<column name="ID_" unique="true"></column>
<generator class="native"></generator>
</id>

<property name="name" column="NAME_"></property>

<property name="desc" column="DESC_"></property>
<property name="order" column="ORDER_"></property>

<set name="forums" order-by="ORDER_ ASC" lazy="true" ><<<<<<<<<<<<<<<<2.
<key column="CATEGORY_"></key>
<one-to-many class="Forum" />
</set>
</class>
</hibernate-mapping>

DaoImpl:

public Category get(int id) {
return (Category) getSession().get(Category.clazz, id);
}

public T load(int id) {
return Category) getSession().load(Category.clazz, id);
}

1.情况如下时

junit:

private CategoryDao categoryDao = new CategoryDaoImpl();

public void testGet(){
Session session = HibernateSessionFactory.getSession();
session.beginTransaction();
Category c = categoryDao.load(2);

//System.out.println(c.getId() + ", " + c.getName());<<<<<<<3.

//System.out.println( c.getForums());<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<4.
session.getTransaction().commit();
session.close();
}

a. 1.lazy="false" 2.lazy="true" 时:

Console:

Hibernate: select category0_.ID_ as ID1_0_0_, category0_.NAME_ as NAME2_0_0_, category0_.DESC_ as DESC3_0_0_, category0_.ORDER_ as ORDER4_0_0_ from itcast_category category0_ where category0_.ID_=?

打开注释3时

Console:

Hibernate: select category0_.ID_ as ID1_0_0_, category0_.NAME_ as NAME2_0_0_, category0_.DESC_ as DESC3_0_0_, category0_.ORDER_ as ORDER4_0_0_ from itcast_category category0_ where category0_.ID_=?
2, b

打开注释4时

Console:

Hibernate: select category0_.ID_ as ID1_0_0_, category0_.NAME_ as NAME2_0_0_, category0_.DESC_ as DESC3_0_0_, category0_.ORDER_ as ORDER4_0_0_ from itcast_category category0_ where category0_.ID_=?
2, b
Hibernate: select forums0_.CATEGORY_ as CATEGORY5_1_, forums0_.ID_ as ID1_1_, forums0_.ID_ as ID1_1_0_, forums0_.NAME_ as NAME2_1_0_, forums0_.DESC_ as DESC3_1_0_, forums0_.ORDER_ as ORDER4_1_0_, forums0_.CATEGORY_ as CATEGORY5_1_0_ from itcast_forum forums0_ where forums0_.CATEGORY_=? order by forums0_.ORDER_ ASC
[id=2,name=b1,order=2, id=3,name=b2,order=3]

b. 1.lazy="true" 2.lazy="true" 时:
Console:

打开注释3时

Console:

Hibernate: select category0_.ID_ as ID1_0_0_, category0_.NAME_ as NAME2_0_0_, category0_.DESC_ as DESC3_0_0_, category0_.ORDER_ as ORDER4_0_0_ from itcast_category category0_ where category0_.ID_=?
2, b

打开注释4时

Console:

Hibernate: select category0_.ID_ as ID1_0_0_, category0_.NAME_ as NAME2_0_0_, category0_.DESC_ as DESC3_0_0_, category0_.ORDER_ as ORDER4_0_0_ from itcast_category category0_ where category0_.ID_=?
2, b
Hibernate: select forums0_.CATEGORY_ as CATEGORY5_1_, forums0_.ID_ as ID1_1_, forums0_.ID_ as ID1_1_0_, forums0_.NAME_ as NAME2_1_0_, forums0_.DESC_ as DESC3_1_0_, forums0_.ORDER_ as ORDER4_1_0_, forums0_.CATEGORY_ as CATEGORY5_1_0_ from itcast_forum forums0_ where forums0_.CATEGORY_=? order by forums0_.ORDER_ ASC
[id=2,name=b1,order=2, id=3,name=b2,order=3]

更多示例,请大家自行测试,这里就不赘述了