How to solve java exceptions: NoClassDefFoundError

Serialization and deserialization in java Development for Web projects developed based on the GAE platform include Json and Gson, I encountered an exception in the deserialization process when using Gson, as shown in:

There is no problem in the program, but an error occurs during running.

Then, the run as -- configuration is complete.

Before modification:

Select Bootstrap Entries and click Add JARs to Add several Gson packages introduced in the project:

OK, there is no serialized NoClassDefFoundError error. In fact, the NoClassDefFoundError error is a series and may be caused by other reasons, however, "com/google/gson/reflect/TypeToken" here is a type.