stop -p停并生产保存点，报错kyro错误。

[nobleyd]

如题，报错如下。
2021-02-02 20:44:19
com.esotericsoftware.kryo.KryoException: Encountered unregistered class ID:
95
    at com.esotericsoftware.kryo.util.DefaultClassResolver.readClass(
DefaultClassResolver.java:119)
    at com.esotericsoftware.kryo.Kryo.readClass(Kryo.java:641)
    at com.esotericsoftware.kryo.Kryo.readClassAndObject(Kryo.java:752)
    at com.esotericsoftware.kryo.serializers.MapSerializer.read(
MapSerializer.java:135)
    at com.esotericsoftware.kryo.serializers.MapSerializer.read(
MapSerializer.java:21)
    at com.esotericsoftware.kryo.Kryo.readClassAndObject(Kryo.java:761)
    at org.apache.flink.api.java.typeutils.runtime.kryo.KryoSerializer
.deserialize(KryoSerializer.java:346)
    at org.apache.flink.api.java.typeutils.runtime.PojoSerializer
.deserialize(PojoSerializer.java:411)
    at org.apache.flink.streaming.runtime.streamrecord.
StreamElementSerializer.deserialize(StreamElementSerializer.java:202)
    at org.apache.flink.streaming.runtime.streamrecord.
StreamElementSerializer.deserialize(StreamElementSerializer.java:46)
    at org.apache.flink.runtime.plugable.NonReusingDeserializationDelegate
.read(NonReusingDeserializationDelegate.java:55)
    at org.apache.flink.runtime.io.network.api.serialization.
SpillingAdaptiveSpanningRecordDeserializer.getNextRecord(
SpillingAdaptiveSpanningRecordDeserializer.java:92)
    at org.apache.flink.streaming.runtime.io.StreamTaskNetworkInput
.emitNext(StreamTaskNetworkInput.java:145)
    at org.apache.flink.streaming.runtime.io.StreamOneInputProcessor
.processInput(StreamOneInputProcessor.java:67)
    at org.apache.flink.streaming.runtime.io.StreamTwoInputProcessor
.processInput(StreamTwoInputProcessor.java:92)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.processInput(
StreamTask.java:372)
    at org.apache.flink.streaming.runtime.tasks.mailbox.MailboxProcessor
.runMailboxLoop(MailboxProcessor.java:186)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.runMailboxLoop(
StreamTask.java:575)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask
.java:539)
    at org.apache.flink.runtime.taskmanager.Task.doRun(Task.java:722)
    at org.apache.flink.runtime.taskmanager.Task.run(Task.java:547)
    at java.lang.Thread.run(Thread.java:748)
近期遇到过不少跟序列化有关的错误。

想问下，（1）如上错误原因。（2）关于自定义序列化器，有什么类似于POJO序列化器一样的不需要用户指定schema的序列化器吗？
比如只需要注册一个实现类，不需要定义任何schema的。