Polymorphism and inheritance in Avro schemas

Question

Is it possible to write an Avro schema/IDL that will generate a Java class that either extends a base class or implements an interface? It seems like the generated Java class extends the org.apache.avro.specific.SpecificRecordBase. So, the implements might be the way to go. But, I don't know if this is possible.

I have seen examples with suggestions to define an explicit "type" field in each specific schema, with more of an association than inheritance semantics.

I use my base class heavily in my factory classes and other parts of the code with generics like <T extends BaseObject>. Currently, I had it code generated from the JSON Schema, which supports inheritance.

Another side question: can you use IDL to define just records without the protocol definition? I think the answer is no because the compiler complains about the missing protocol keyword.

Help appreciated! Thanks.

Answer 1

I found a better way to solve this problem. Looking at the Schema generation source in Avro, I figured out that internally the class generation logic uses Velocity schemas to generate the classes.

I modified the record.vm template to also implement my specific interface. There is a way to specify the location of velocity directory using the templateDirectory configuration in the maven build plugin.

I also switched to using SpecificDatumWriter instead of reflectDatumWriter.

<plugin>
  <groupId>org.apache.avro</groupId>
  <artifactId>avro-maven-plugin</artifactId>
   <version>${avro.version}</version>
   <executions>
    <execution>
      <phase>generate-sources</phase>
      <goals>
        <goal>schema</goal>
      </goals>
      <configuration>
         <sourceDirectory>${basedir}/src/main/resources/avro/schema</sourceDirectory>
         <outputDirectory>${basedir}/target/java-gen</outputDirectory>
         <fieldVisibility>private</fieldVisibility>
         <stringType>String</stringType>
         <templateDirectory>${basedir}/src/main/resources/avro/velocity-templates/</templateDirectory>
       </configuration>
    </execution>
  </executions>
</plugin>