cannot simultaneously fetch multiple bags 问题的解决办法

我出现问题如下：

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

at org.hibernate.loader.BasicLoader.postInstantiate(BasicLoader.java:94)

at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:119)

at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:71)

at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:54)

at org.hibernate.loader.entity.BatchingEntityLoader.createBatchingEntityLoader(BatchingEntityLoader.java:133)

at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1914)

at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1937)

at org.hibernate.persister.entity.AbstractEntityPersister.createLoaders(AbstractEntityPersister.java:3205)

at org.hibernate.persister.entity.AbstractEntityPersister.postInstantiate(AbstractEntityPersister.java:3191)

at org.hibernate.persister.entity.SingleTableEntityPersister.postInstantiate(SingleTableEntityPersister.java:728)

at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:348)

at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1872)

at com.fzb.flashShopping.entity.EntityTest.testSchemaExport(EntityTest.java:24)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

at java.lang.reflect.Method.invoke(Method.java:597)

at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:59)

at org.junit.internal.runners.MethodRoadie.runTestMethod(MethodRoadie.java:98)

at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:79)

at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:87)

at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:77)

at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:42)

at org.junit.internal.runners.JUnit4ClassRunner.invokeTestMethod(JUnit4ClassRunner.java:88)

at org.junit.internal.runners.JUnit4ClassRunner.runMethods(JUnit4ClassRunner.java:51)

at org.junit.internal.runners.JUnit4ClassRunner$1.run(JUnit4ClassRunner.java:44)

at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:27)

at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:37)

at org.junit.internal.runners.JUnit4ClassRunner.run(JUnit4ClassRunner.java:42)

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)

 

问题是什么时候出现的呢？

当一个实体对象中包含多于一个non-lazy获取策略时，比如@OneToMany，@ManyToMany或者@ElementCollection时，获取策略为(fetch = FetchType.EAGER)

 

出现问题的原因：

当(fetch = FetchType.EAGER)多余一个时，持久框架抓取一方的对象时，同时又将多方的对象加载进容器中，多方又可能关联其它对象，Hibernate实现的JPA，默认最高抓取深度含本身级为四级(它有个属性配置是0-3),若多方(第二级)存在重复值，则第三级中抓取的值就无法映射，就会出现 multiple bags。

解决方法：

1、将(fetch = FetchType.EAGER)改为(fetch = FetchType.LAZY)

2、将List修改成Set集合，即推荐@ManyToMany或@OneToMany的Many方此时用Set容器来存放，而不用List集合。

3、改变FetchMode为@Fetch(FetchMode.SUBSELECT)，即发送另外一条select语句抓取前面查询到的所有实体对象的关联实体。

4、在对应的属性上添加@IndexColumn，该注解允许你指明存放索引值的字段，目的跟Set容器不允许重复元素的道理一样。

推荐的处理办法：

方法2;

方法3和方法4是Hibernate特有的，非JPA标准;

如果可以用方法1，那就不会出现这个问题。

参考：

1、http://blog.eyallupu.com/2010/06/hibernate-exception-simultaneously.html
2、http://dyldragon.iteye.com/blog/788385
3、http://howsun.blog.sohu.com/119020926.html

转自 : http://www.quanlei.com/2011/10/2817.html