Generic conversion from POJO to Avro Record

Question

I'm looking for a way to convert a POJO to an avro object in a generic way. The implementation should be robust to any changes of the POJO-class. I have achieved it but filling the avro record explicitly (see example below).

Is there a way to get rid of the hard-coded field names and just fill the avro record from the object? Is reflection the only way, or does avro provide this functionality out of the box?

import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import org.apache.avro.Schema;
import org.apache.avro.generic.GenericData.Record;
import org.apache.avro.reflect.ReflectData;

public class PojoToAvroExample {

    static class PojoParent {
        public final Map<String, String> aMap = new HashMap<String, String>();
        public final Map<String, Integer> anotherMap = new HashMap<String, Integer>();
    }

    static class Pojo extends PojoParent {
        public String uid;
        public Date eventTime;
    }

    static Pojo createPojo() {
        Pojo foo = new Pojo();
        foo.uid = "123";
        foo.eventTime = new Date();
        foo.aMap.put("key", "val");
        foo.anotherMap.put("key", 42);
        return foo;
    }

    public static void main(String[] args) {
        // extract the avro schema corresponding to Pojo class
        Schema schema = ReflectData.get().getSchema(Pojo.class);
        System.out.println("extracted avro schema: " + schema);
        // create avro record corresponding to schema
        Record avroRecord = new Record(schema);
        System.out.println("corresponding empty avro record: " + avroRecord);

        Pojo foo = createPojo();
        // TODO: to be replaced by generic variant:
        // something like avroRecord.importValuesFrom(foo);
        avroRecord.put("uid", foo.uid);
        avroRecord.put("eventTime", foo.eventTime);
        avroRecord.put("aMap", foo.aMap);
        avroRecord.put("anotherMap", foo.anotherMap);
        System.out.println("expected avro record: " + avroRecord);
    }
}

Answer 1

Are you using Spring?

I build a mapper for that using a Spring feature. But it is also possible to build such a mapper via raw reflection utils too:

import org.apache.avro.Schema;
import org.apache.avro.generic.GenericData;
import org.apache.avro.reflect.ReflectData;
import org.springframework.beans.PropertyAccessorFactory;
import org.springframework.util.Assert;

public class GenericRecordMapper {

    public static GenericData.Record mapObjectToRecord(Object object) {
        Assert.notNull(object, "object must not be null");
        final Schema schema = ReflectData.get().getSchema(object.getClass());
        final GenericData.Record record = new GenericData.Record(schema);
        schema.getFields().forEach(r -> record.put(r.name(), PropertyAccessorFactory.forDirectFieldAccess(object).getPropertyValue(r.name())));
        return record;
    }

    public static <T> T mapRecordToObject(GenericData.Record record, T object) {
        Assert.notNull(record, "record must not be null");
        Assert.notNull(object, "object must not be null");
        final Schema schema = ReflectData.get().getSchema(object.getClass());
        Assert.isTrue(schema.getFields().equals(record.getSchema().getFields()), "Schema fields didn't match");
        record.getSchema().getFields().forEach(d -> PropertyAccessorFactory.forDirectFieldAccess(object).setPropertyValue(d.name(), record.get(d.name()) == null ? record.get(d.name()) : record.get(d.name()).toString()));
        return object;
    }

}

With this mapper you can generate a GenericData.Record which can be easily serialized to avro. When you deserialize an Avro ByteArray you can use it to rebuild a POJO from deserialized record:

Serialize

byte[] serialized = avroSerializer.serialize("topic", GenericRecordMapper.mapObjectToRecord(yourPojo));

Deserialize

GenericData.Record deserialized = (GenericData.Record) avroDeserializer.deserialize("topic", serialized);

YourPojo yourPojo = GenericRecordMapper.mapRecordToObject(deserialized, new YourPojo());

Answer 2

Here is generic way to convert

public static <V> byte[] toBytesGeneric(final V v, final Class<V> cls) {
        final ByteArrayOutputStream bout = new ByteArrayOutputStream();
        final Schema schema = ReflectData.get().getSchema(cls);
        final DatumWriter<V> writer = new ReflectDatumWriter<V>(schema);
        final BinaryEncoder binEncoder = EncoderFactory.get().binaryEncoder(bout, null);
        try {
            writer.write(v, binEncoder);
            binEncoder.flush();
        } catch (final Exception e) {
            throw new RuntimeException(e);
        }


        return bout.toByteArray();
    }

public static void main(String[] args) {
    PojoClass pojoObject = new PojoClass();
    toBytesGeneric(pojoObject, PojoClass.class);
}

Answer 3

With jackson/avro, it's very easy to convert pojo to byte[], similar to jackson/json:

byte[] avroData = avroMapper.writer(schema).writeValueAsBytes(pojo);

p.s.
jackson handles not only JSON, but also XML/Avro/Protobuf/YAML etc, with very similar classes and APIs.

Answer 4

Two steps to convert any pojo class to avro genric record

1. Using jackson/avro, to convert the pojo into bytes with Avro Mapper.

2. Using Avro GenericDatumReader to read it as Generic Record.

public class AvroConverter{

 public static GenericRecord convertToGenericRecord(String schemaPath, SomeClass someObject){
  Schema schema = new Schema.Parser().setValidate(true).parse(new ClassPathResource(schemaPath).getFile());
  AvroSchema avSchema = new AvroSchema(schema);
  ObjectWritter writter = new AvroMapper().writer(avSchema);
  final byte[] bytes = writter.writeValueAsBytes(someObject);
  GenericDatumReader<Object> genericRecordReader = new GenericDatumReader<>(avSchema);
  return (GenericRecord) genericRecordReader.read(null, DecoderFactory.get().binaryDecoder(bytes, null));
 }

}

Gradle Depedency

 // https://mvnrepository.com/artifact/com.fasterxml.jackson.dataformat/jackson-dataformat-avro
    implementation 'com.fasterxml.jackson.dataformat:jackson-dataformat-avro'