Hibernate 級聯操作，對映檔案中的預設的Lazy屬性導致的異常
hibernate中的延遲載入策略一定程度上降低了記憶體開銷，但是有時候使用會出現不想要的異常。
首先，hibernate hbm 中lazy屬性（true|false)，在hibernate中預設lazy是true。
Hibernate中允許使用延遲載入的地方主要有以下幾個地方：
<hibernate-mapping default-lazy=(true|false)”true”>：設定全域性的延遲載入策略。
<class lazy=(true|false)>：DTD沒設定預設值，推理預設值為true （在不同的hibernate中，lazy的預設值不同）
<property lazy=(true|false)>：設定欄位延遲載入，預設為false
<component lazy=(true|false)：預設為false
<subclass lazy=(true|false)>：預設設定為true
<join-subclass lazy=(true|false)>：預設設定為true
<union-subclass lazy=(true|false)>：預設設定為true
<many-to-one lazy=(proxy|no-proxy|false)>：預設為proxy
<one-to-one lazy=(proxy|no-proxy|false)>：預設為proxy
<map lazy=(true|extra|false)>：預設為true
<set lazy=(true|extra|false)>：預設為true
<bag lazy=(true|extra|false)>：預設為true
<ibag lazy=(true|extra|false)>：預設為true
<list lazy=(true|extra|false)>：預設為true
當我們在做級聯操作時，如果異常資訊如下。四月 25, 2017 9:23:19 上午 org.hibernate.proxy.pojo.javassist.JavassistProxyFactory getProxy
ERROR: HHH000142: Javassist Enhancement failed: cn.itsure.entity.User
java.lang.NoClassDefFoundError: javassist/util/proxy/Proxy
at org.hibernate.proxy.pojo.javassist.JavassistProxyFactory.getProxy(JavassistProxyFactory.java:123)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:671)
at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:4536)
at org.hibernate.event.internal.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:359)
at org.hibernate.event.internal.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:274)
at org.hibernate.event.internal.DefaultLoadEventListener.doOnLoad(DefaultLoadEventListener.java:121)
at org.hibernate.event.internal.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:89)
at org.hibernate.internal.SessionImpl.fireLoad(SessionImpl.java:1129)
at org.hibernate.internal.SessionImpl.internalLoad(SessionImpl.java:1022)
at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:639)
at org.hibernate.type.EntityType.resolve(EntityType.java:431)
at org.hibernate.engine.internal.TwoPhaseLoad.doInitializeEntity(TwoPhaseLoad.java:154)
at org.hibernate.engine.internal.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:128)
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.performTwoPhaseLoad(AbstractRowReader.java:238)
at org.hibernate.loader.plan.exec.process.internal.AbstractRowReader.finishUp(AbstractRowReader.java:209)
at org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl.extractResults(ResultSetProcessorImpl.java:133)
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:122)
at org.hibernate.loader.plan.exec.internal.AbstractLoadPlanBasedLoader.executeLoad(AbstractLoadPlanBasedLoader.java:86)
at org.hibernate.loader.entity.plan.AbstractLoadPlanBasedEntityLoader.load(AbstractLoadPlanBasedEntityLoader.java:167)
at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:4019)
at org.hibernate.event.internal.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:508)
at org.hibernate.event.internal.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:478)
at org.hibernate.event.internal.DefaultLoadEventListener.load(DefaultLoadEventListener.java:219)
at org.hibernate.event.internal.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:278)
at org.hibernate.event.internal.DefaultLoadEventListener.doOnLoad(DefaultLoadEventListener.java:121)
at org.hibernate.event.internal.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:89)
at org.hibernate.internal.SessionImpl.fireLoad(SessionImpl.java:1129)
at org.hibernate.internal.SessionImpl.access$2600(SessionImpl.java:164)
at org.hibernate.internal.SessionImpl$IdentifierLoadAccessImpl.load(SessionImpl.java:2696)
at org.hibernate.internal.SessionImpl.get(SessionImpl.java:975)
at cn.itsure.dao.demo3.updateUser(demo3.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
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.runNotIgnored(BlockJUnit4ClassRunner.java:79)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
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.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)
Caused by: java.lang.ClassNotFoundException: javassist.util.proxy.Proxy
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 54 more
四月 25, 2017 9:23:19 上午 org.hibernate.event.internal.DefaultLoadEventListener doOnLoad
INFO: HHH000327: Error performing load command : org.hibernate.HibernateException: HHH000142: Javassist Enhancement failed: cn.itsure.entity.User
四月 25, 2017 9:23:19 上午 org.hibernate.event.internal.DefaultLoadEventListener doOnLoad
INFO: HHH000327: Error performing load command : org.hibernate.HibernateException: HHH000142: Javassist Enhancement failed: cn.itsure.entity.User
異常分析
Hibernate中<class lazy=””>預設為true。如果，在load的時候只會返回一個代理類，並不會正在從資料庫中讀取資料。第一次用到時，會將所有普通屬性（set這種就不是）全部載入進來。如果第一次使用到時，session已經關閉將發生錯誤。
如果顯式是設定lazy=false，load的時候即會把所有普通屬性全部讀取進來。而且，返回的將是一個真正的該型別的物件(如Person)，而不是代理類。
而不同版本的hibernate在load的時候，lazy的預設值是不同的。這個可能是官方自己也在考慮這個lazy屬性值怎麼設定預設值。
異常處理辦法
<class 標籤中設定 lazy="false"