Sample code used in https://dalelane.co.uk/blog/?p=3781

WordsThatStartWithD.java

import org.apache.avro.LogicalType;
import org.apache.avro.LogicalTypes.LogicalTypeFactory;
import org.apache.avro.Schema;

public class WordsThatStartWithD extends LogicalType {

    public static final String PROPERTY_NAME = "words-that-start-with-D";

    private static final String INVALID_TYPE = "Logical type 'words-that-start-with-D' must be strings";

    public WordsThatStartWithD() {
        super(PROPERTY_NAME);
    }

    @Override
    public void validate(Schema schema) {
        super.validate(schema);
        if (schema.getType() != Schema.Type.STRING) {
            throw new IllegalArgumentException(INVALID_TYPE);
        }
    }


    public static class TypeFactory implements LogicalTypeFactory {
        private WordsThatStartWithD type = new WordsThatStartWithD();
        @Override
        public LogicalType fromSchema(Schema schema) {
            return type;
        }
    }
}