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Subject: Re: [Converter] Serializer
From: David Leangen <osgi@leangen.net>
In-Reply-To: <056390CA-64B1-4DBA-ADCB-4DE3DF9CA335@gmail.com>
Date: Sun, 21 Aug 2016 14:46:19 +0900
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Message-Id: <20402349-BEA0-49AF-AE22-28C7793876F8@leangen.net>
References: <940E5AF8-E462-4639-868A-ED7C04E2BF50@leangen.net> <4A2EB6ED-93EF-401A-8379-47C65FA0DD8A@leangen.net> <056390CA-64B1-4DBA-ADCB-4DE3DF9CA335@gmail.com>
To: dev@felix.apache.org
archived-at: Sun, 21 Aug 2016 05:46:39 -0000


Hi Johan,

Thanks for your thoughts. Are you able to elaborate a little more?

> Whenever you serialize and say =E2=80=9CHey=E2=80=99 let=E2=80=99s use =
polymorphism", you are pretty=20
> much bound to fail.

I=E2=80=99m not sure why you say this. I thought that by including the =
schema information, that is exactly what we were avoiding=E2=80=A6

I=E2=80=99m not sure I=E2=80=99m getting your idea about the command =
pattern. Would you mind showing me a quick example?

Cheers,
=3DDavid


> On Aug 21, 2016, at 2:42 PM, Johan Edstrom <seijoed@gmail.com> wrote:
>=20
> Whenever you serialize and say =E2=80=9CHey=E2=80=99 let=E2=80=99s use =
polymorphism", you are pretty=20
> much bound to fail.
>=20
> Use something like a command pattern identifying what you are doing=20
> so you can pick the de-serializer.=20
>=20
>=20
>=20
>> On Aug 20, 2016, at 11:32 PM, David Leangen <osgi@leangen.net> wrote:
>>=20
>>=20
>> I had a few thoughts about this topic.
>>=20
>> In order to properly deserialise, we would need the schema. Since the =
DTO type *is* the schema, then somehow the schema needs to be serialised =
as well, or at least known upon serialisation.
>>=20
>> If the API were to have a toSchema() method, that would simplify =
things a lot. A client should also be able to do something like this:
>>=20
>> SomeDTO dto =3D ...
>> codec.encode( dto ).serializeTo( out );
>>=20
>> Or maybe:
>>=20
>> codec.withSchema( SomeDTO.class).encode( dto ).to( out );
>>=20
>> The SomeDTO schema would also be serialised (perhaps simply using the =
fully-qualified class name as an alias?), so then the client could do =
this:
>>=20
>> Object o =3D codec.deserializeFrom( in )
>>=20
>> Since the schema is included in the input data, the serializer will =
behind the scene do something like:
>>=20
>>=20
>>  Map<String, Object> map =3D codec.decode( Map.class ).from( in );
>>  // Get the schema type, which is SomeDTO.class
>>  ...
>>  Object o =3D converter.convert( map.get(=E2=80=9Cdata=E2=80=9D) =
).to( SomeDTO.class );
>>=20
>>=20
>> wdyt?
>>=20
>> =3DDavid
>>=20
>>=20
>>=20
>>> On Aug 20, 2016, at 3:23 PM, David Leangen <osgi@leangen.net> wrote:
>>>=20
>>>=20
>>> Hi,
>>>=20
>>> I am trying to use the Convert/Codec as a serializer, but it has =
been a bit of a struggle so far.
>>>=20
>>> I have a =E2=80=9Cdeep=E2=80=9D object structure (i.e. at least one =
level of embedded objects). Writing the data as a JSON string works just =
fine. However, when deserialising, I am having trouble. The data object =
uses generics, so it is not possible to determine the type at runtime. =
My embedded objects end up being deserialised as Maps, which of course =
causes a ClassCastException later on in the code.
>>>=20
>>> I have tried adding a Rule using an adapter, and setting the =
codec.with(thatAdapter), but that didn=E2=80=99t work out so well, =
either.
>>>=20
>>> Here is an example of my attempt so far. I am working with =
Prevayler, so I need to serialize/deserialze a command object, which =
contains some data.
>>>=20
>>> I think that this code _should_ work, which means that there is =
likely a bug in the code. Even so=E2=80=A6
>>>=20
>>> This is a very convoluted way of working, which doesn=E2=80=99t seem =
right to me. Am I missing some concept?
>>>=20
>>>=20
>>> Example below.
>>>=20
>>> Cheers,
>>> =3DDavid
>>>=20
>>>=20
>>> public class DTOSerializer<E>
>>>      implements Serializer // This is the interface provided by =
Prevayler
>>> {
>>>  private final Converter converter;
>>>  private final Codec codec;
>>>  private final Class<E> type;
>>>=20
>>>  public DTOSerializer( Converter aConverter, Codec aCodec, Class<E> =
aType )
>>>  {
>>> 	// Very convoluted attempt at adding a rule to try to make the =
conversion of my =E2=80=9CPutCommand=E2=80=9D work during =
deserialization
>>>      converter =3D aConverter.getAdapter()
>>>              .rule(=20
>>>                      PutCommand.class,=20
>>>                      Map.class,=20
>>>                      dto -> { Map map =3D new HashMap<>(); map.put( =
"key", dto.key ); map.put( "entity", dto.entity ); return map; },=20
>>>                      map -> { PutCommand c =3D new PutCommand(); =
c.key =3D (String)map.get( "key" ); c.entity =3D aConverter.convert( =
map.get( "entity" ) ).to( aType ); return c; } );
>>>      codec =3D aCodec.with( converter );
>>>  }
>>>=20
>>>  @Override
>>>  public Object readObject( InputStream in )
>>>          throws Exception
>>>  {
>>> 	// This is the raw data
>>>      final Map<String, Object> map =3D codec.decode( Map.class =
).from( in );
>>> 	// The name of the object type to cast to
>>>      final String commandTypeName =3D (String)map.get( "command" );
>>>      final Class<?> commandType =3D Class.forName( commandTypeName =
);
>>> 	// This indeed returns an object of the correct type,
>>> 	// HOWEVER, the embedded objects are not of the correct type, =
they are of type Map
>>>      final Object command =3D converter.convert( map.get( "payload" =
) ).to( commandType );
>>>      return command;
>>>  }
>>>=20
>>>  @Override
>>>  public void writeObject( OutputStream out, Object object )
>>>          throws Exception
>>>  {
>>>      final Map<String, Object> map =3D new HashMap<>();
>>> 	// Serialize the name of the command so I know what to cast it =
do when deserializing
>>>      map.put( "command", object.getClass().getName() );
>>> 	// This is the actual payload, which is nothing more than the =
command object
>>>      map.put( "payload", object );
>>>      codec.encode( map ).to( out );
>>>  }
>>> }
>>>=20
>>=20
>=20