hibernate 懒加载异常分析： failed to lazily initialize a collection of role

初始环境：hibernate 每个用户对应一个小组 。user--group（many to one）。group--user（one to many）  

Group代码片段

@OneToMany(mappedBy="group" )
	@JoinColumn(name="groupid")
	public Set getUsers() {
		return users;
	}

User代码片段

@ManyToOne
	@JoinColumn(name="groupid")
	public Group getGroup() {
		return group;
	}

在执行这段代码时

	@Test
	public void testGetGroup(){
		//testSaveGroup();
		
		Session se=sf.getCurrentSession();
		se.beginTransaction();
		Group g=(Group)se.get(Group.class, 1);
		
		se.getTransaction().commit();
		for(User u : g.getUsers()){
			//g.getUsers() 
			System.out.println("------------------");
			System.out.println(u.getName());
		}
	}

会出现异常：

18:26:28,624 ERROR org.hibernate.LazyInitializationException:42 - failed to lazily initialize a collection of role: com.hibernate.crud.Group.users, no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.hibernate.crud.Group.users, no session or session was closed
	at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:380)
	at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:372)
	at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:365)
	at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:108)
	at org.hibernate.collection.PersistentSet.iterator(PersistentSet.java:186)
	at com.hibernate.crud.test.testGetGroup(test.java:69)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:31)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

也就是懒加载异常

failed to lazily initialize a collection of role: com.hibernate.crud.Group.users, no session or session was closed

经过马士兵老师的讲解，基本了解常形成的原因，现在大致谈一谈，当做个人笔记。

这是因为Hibernate Annotation的默认的FetchType在ManyToOne是EAGER的,在OneToMany上默认的是LAZY.

就是说这段代码中，User是FetchType.EAGER。 而Group是FetchType.LAZY。

具体是因为在这段代码中

@Test
	public void testGetGroup(){
		//testSaveGroup();
		
		Session se=sf.getCurrentSession();
		se.beginTransaction();
		Group g=(Group)se.get(Group.class, 1);
		
		se.getTransaction().commit();
		for(User u : g.getUsers()){
			//g.getUsers() 
			System.out.println("------------------");
			System.out.println(u.getName());
		}
	}

执行 

Group g=(Group)se.get(Group.class, 1);

查询到的结果是

 select group0_.id as id1_0_, group0_.name as name1_0_ from t_group group0_ where group0_.id=?

只有group表的数据。

而要执行

for(User u : g.getUsers()){    

还需要连接数据库，但是session已经提交关闭了。所以会报 failed to lazily initialize a collection of role 异常。

要解决这个问题
1.可以在Group中设置FetchType.EAGER,得到对应的User表，缓存。如下：

@OneToMany(mappedBy="group",fetch=FetchType.EAGER )
	@JoinColumn(name="groupid")
	public Set getUsers() {
		return users;
	}

2.将提交放在最后。如下：

@Test
	public void testGetGroup(){
		//testSaveGroup();
		
		Session se=sf.getCurrentSession();
		se.beginTransaction();
		Group g=(Group)se.get(Group.class, 1);
		
		for(User u : g.getUsers()){
			//g.getUsers() 
			System.out.println("------------------");
			System.out.println(u.getName());
		}
		se.getTransaction().commit();
	}
	

现在正在学习hibernate，以后有其他方法或者错误，会有所更新。