Hibernate里让程序产生主键就出错了请帮我看看

但我想用外部程序控制 <generator class="Assigned"/>
程序产生主键：Cat cat=new Cat();
//cat.setName("tom");
cat.setSex("z");
TestCat.insertCat(cat);
运行就出错了
net.sf.hibernate.MappingException: could not instantiate id generator
at net.sf.hibernate.id.IdentifierGeneratorFactory.create(IdentifierGeneratorFactory.java:82)
at net.sf.hibernate.mapping.Simplevalue.createIdentifierGenerator(Simplevalue.java:80)
at net.sf.hibernate.persister.AbstractEntityPersister.<init>(AbstractEntityPersister.java:631)
at net.sf.hibernate.persister.EntityPersister.<init>(EntityPersister.java:715)
at net.sf.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:41)
at net.sf.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:137)
at net.sf.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:750)
at Test.TestCat.insertCat(TestCat.java:33)
at Test.TestCat.main(TestCat.java:157)
Caused by: java.lang.ClassNotFoundException: Assigned
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:164)
at net.sf.hibernate.util.ReflectHelper.classForName(ReflectHelper.java:86)
at net.sf.hibernate.id.IdentifierGeneratorFactory.create(IdentifierGeneratorFactory.java:76)
... 8 more
那这是什么原因呢？