特别是在ObjectInputStream的readUnshared()方法的上下文中,“反向引用”是什么意思?我在这个词中遇到了这个词IfreadUnsharediscalledtodeserializeaback-reference(thestreamrepresentationofanobjectwhichhasbeenwrittenpreviouslytothestream),anObjectStreamExceptionwillbethrown.引用:https://docs.oracle.com/javase/7/docs/api/java/io/ObjectInputSt