hibernate many-to-one

看下面的例子

user和班级，多对一得关系。
目标：希望查询学生的时候，知道学生所在班级信息

public class User {

	private Long userId;
	private String userName;
	private GradeClass gradeClass;
           ...//省略get和set方法

User.hbm.xml

<hibernate-mapping>
		<class name="com.supben.pojo.User" table="User">
				<id name="userId" type="java.lang.Long">
						<column length="20" name="userId" />
						<generator class="identity"></generator>
				</id>

				<property name="userName" type="java.lang.String">
						<column name="userName" />
				</property>
<!--column="gradeClassId" 表示的是user表中和班级表关联字段gradeClassId -->
			 <many-to-one name="gradeClass" column="gradeClassId" class="com.supben.pojo.GradeClass" />   
		</class>
</hibernate-mapping>

GradeClass.hbm.xml，和平时的配置没有任何的不同！

<hibernate-mapping>
		<class name="com.supben.pojo.GradeClass" table="gradeclass">
				<id name="gradeClassId" type="java.lang.Long">
						<column length="20" name="gradeClassId" />
						<generator class="identity"></generator>
				</id>

				<property name="gradeClassName" type="java.lang.String">
						<column name="gradeClassName" />
				</property> 
		</class>

测试代码

public void testFind() {
		List<User> userList = userService.findAll();
		assertEquals(2, userList.size());
		System.out.println("*************************************");
		System.out.println(userList.get(0).getGradeClass().getGradeClassName());
                  ...//(省略dao和service中的查询方法)
	}

Hibernate: select this_.userId as userId1_0_, this_.userName as userName1_0_, this_.gradeClassId as gradeCla3_1_0_ from User this_
*************************************
1084 [main] ERROR org.hibernate.LazyInitializationException - could not initialize proxy - no Session
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:86)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:140)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
at com.supben.pojo.GradeClass_$$_javassist_1.getGradeClassName(GradeClass_$$_javassist_1.java)
at com.supben.service.UserServiceTest.testFind(UserServiceTest.java:21)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)


可以看到hibernate 3 many-to-one 默认是延迟加载的，出现了异常：不能初始化代理对象，session超时。

把配置文件改成lazy="false"呢，即这样配置

<many-to-one name="gradeClass" column="gradeClassId" class="com.supben.pojo.GradeClass" lazy="false" />  

运行结果一切正常，在拿到user对象之后，立即发出2条sql去拿到班级对象。

Hibernate: select this_.userId as userId1_0_, this_.userName as userName1_0_, this_.gradeClassId as gradeCla3_1_0_ from User this_
Hibernate: select gradeclass0_.gradeClassId as gradeCla1_0_0_, gradeclass0_.gradeClassName as gradeCla2_0_0_ from gradeclass gradeclass0_ where gradeclass0_.gradeClassId=?
Hibernate: select gradeclass0_.gradeClassId as gradeCla1_0_0_, gradeclass0_.gradeClassName as gradeCla2_0_0_ from gradeclass gradeclass0_ where gradeclass0_.gradeClassId=?
*************************************
101


优化：在配置文件里加上fetch="join"在运行，可以看到控制台的输出为：
Hibernate: select this_.userId as userId1_1_, this_.userName as userName1_1_, this_.gradeClassId as gradeCla3_1_1_, gradeclass2_.gradeClassId as gradeCla1_0_0_, gradeclass2_.gradeClassName as gradeCla2_0_0_ from User this_ left outer join gradeclass gradeclass2_ on this_.gradeClassId=gradeclass2_.gradeClassId
*************************************
101

只发出了一条sql就获得了所有的结果。这是因为hibernate默认是用fetch="select"的方式，即根据gradeClassId去select。改成了join，就会一次性查出

评论

2 楼 supben 2012-05-15  

Killer+bj 写道

你好：我想问一下，如果这样配置的话，那么Java dao 中该如何写呢？
直接：from 学生表 ，
还是怎么写法？谢谢了！

直接from学生表，就把班级也查出来了，因为many-to-one 默认不是lazy的！

如果是lazy的话，用hql的话，就应该强行JOIN 出来。

如FROM User u JOIN u.gradeClass g WHERE ....

1 楼 Killer+bj 2012-05-14  

你好：我想问一下，如果这样配置的话，那么Java dao 中该如何写呢？
直接：from 学生表 ，
还是怎么写法？谢谢了！