Ok, I have tried to answer the question by myself:
1. ReflectData ignores built-in classes
========================================
I found the following code inside the Method "private Collection<Field>
getFields(Class recordClass)":
if (c.getPackage() != null
&& c.getPackage().getName().startsWith("java."))
break;
The reason why built-in classes are ignored might be documented
somewhere. I didn’t find anything about the reason.
[AVRO-797] might have something to do with this issue.
The fields within the Properties.class (inherited from Hashtable) are
declared as transient. Even if the code above is comment out, the class
is not serialized through Avro. But: Java is able to serialize this
class?!?
Why is Java able to serialize this class and Avro not?
======================================================
First: The serialization process of java does have the same problem as
Avro: For sure, the fields are transient for boths APIs. But here is the
extra edge: A closer look into the class hashtable shows that the
specific methods "readObject" and "writeObject" (used for serialization)
are overwritten. Avro does not consider these methods (whichs write
specific binary-content direct into the outputstream).
I think that's the reason why the class is serialized by the JAVA
Serialization and not by Avro.
How can this problem be solved?
===============================
Don't use built-in classes in your data model.
Regards
Stefan
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