能帮我解释一下为什么抛出下面的
线程"main“javax.xml.bind.JAXBException中的异常: com.jaxb.model.copy.copy.Snapshot类及其任何超类都不为该上下文所知。在com.sun.xml.bind.v2.runtime.JAXBContextImpl.getBeanInfo(JAXBContextImpl.java:588) at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:482) at com.sun.xml.bind.v2.r