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From David Bosschaert <david.bosscha...@gmail.com>
Subject Re: [Converter] Serializer
Date Tue, 23 Aug 2016 15:35:48 GMT
Hi David,

In that case you'd need to store the type information somewhere which,
depending on the target serialization format may or may not be possible.
For example in plain JSON you don't really have a place to store such
metadata, although you could add it in a 'special' "__metadata" key or
something.

I think this may be a use-case that could for some people be handy but for
others it might pollute the generated serialization with stuff that they
don't really want to see.

I guess it could be a special feature of a certain Codec. Codecs could be
configured via some mechanism (e.g. ConfigAdmin) to support this feature.
Then you should be able to get the behaviour you are looking for by calling:

MyDTO dto = (MyDTO) myCodec.decode(Object.class).from(json)

So my feeling is that something like this might already be possible given
the current API, although not every codec might support it. Maybe we can
try to expand one of the codecs that is in the Felix codebase to support
this enabled via a configuration setting?

Thoughts?

David

On 23 August 2016 at 01:50, David Leangen <osgi@leangen.net> wrote:

>
> Thanks, David B.,
>
> The use cases are indeed very similar, but for one difference, I think.
> This line:
>
>   MyDTO dto2 = jsonCodec.decode(MyDTO.class).from(json);
>
> You see, when you decode from JSON in this line, you already know in
> advance that you are deserialising a MyDTO object, so you feed it into the
> decoder. Easy schmeasy. But what do you do if you don’t know in advance
> what type the object is? That is the problem with deserialisation.
>
> A generic serialiser has this very problem. Its job is to serialise (very
> easy) and deserialise (difficult because it somehow needs to know what the
> type is). If this were XML, you would somehow need to first determine the
> schema before you could deserialise into a statically-typed object. If you
> know the schema, then you can convert. JSON is no different. What is really
> cool about the DTO, though, is that the DTO class becomes the schema. So,
> all we would need to do so solve this problem is serialise the DTO type
> along with the data. Upon deserialisation, we could instantiate a class
> object (assuming with Class.forName(“dtoTypeName”)), then we’re good.
>
> So, my suggestion is to, as part of the API, serialise the type
> information along with the data. That way, when calling the API, however,
> all you would need to do to deserialise is this:
>
>   // Note the absence of type information, which is different from the
> line above.
>   // The line above requires knowledge of the type in advance.
>   Object dto2 = jsonCodec.deserializeFrom(json);
>
>
> Of course, this cruft could be done outside of the service. However, it
> seems like an important use case to consider, and would make the
> DTO/Converter/Codec more complete and more compelling, for such a small
> addition.
>
> wdyt?
>
> Cheers,
> =David
>
>
> > On Aug 23, 2016, at 6:23 AM, David Bosschaert <
> david.bosschaert@gmail.com> wrote:
> >
> > Hi David,
> >
> > Maybe I'm missing something, but isn't this the same use-case as what is
> > done in the unit test JsonCodecTest.testDTO()? [1]
> >
> > Basically there it has a DTO with an embedded DTO.
> > This is then converted into JSON with the JSON codec. The JSON contains
> the
> > embedded content as an embedded JSON object.
> > Then that JSON is converted back into the DTO and the embedded DTO is
> > automatically filled as part of that process.
> >
> > Or is your case different?
> >
> > Cheers,
> >
> > David
> >
> > [1]
> > https://svn.apache.org/viewvc/felix/trunk/converter/src/
> test/java/org/apache/felix/converter/impl/json/JsonCodecTest.java?view=
> markup#l102
> >
> > On 21 August 2016 at 06:46, David Leangen <osgi@leangen.net> wrote:
> >
> >>
> >> Hi Johan,
> >>
> >> Thanks for your thoughts. Are you able to elaborate a little more?
> >>
> >>> Whenever you serialize and say “Hey’ let’s use polymorphism", you
are
> >> pretty
> >>> much bound to fail.
> >>
> >> I’m not sure why you say this. I thought that by including the schema
> >> information, that is exactly what we were avoiding…
> >>
> >> I’m not sure I’m getting your idea about the command pattern. Would you
> >> mind showing me a quick example?
> >>
> >> Cheers,
> >> =David
> >>
> >>
> >>
> >>> On Aug 21, 2016, at 2:42 PM, Johan Edstrom <seijoed@gmail.com> wrote:
> >>>
> >>> Whenever you serialize and say “Hey’ let’s use polymorphism", you
are
> >> pretty
> >>> much bound to fail.
> >>>
> >>> Use something like a command pattern identifying what you are doing
> >>> so you can pick the de-serializer.
> >>>
> >>>
> >>>
> >>>> On Aug 20, 2016, at 11:32 PM, David Leangen <osgi@leangen.net>
wrote:
> >>>>
> >>>>
> >>>> I had a few thoughts about this topic.
> >>>>
> >>>> 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.
> >>>>
> >>>> 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:
> >>>>
> >>>> SomeDTO dto = ...
> >>>> codec.encode( dto ).serializeTo( out );
> >>>>
> >>>> Or maybe:
> >>>>
> >>>> codec.withSchema( SomeDTO.class).encode( dto ).to( out );
> >>>>
> >>>> 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:
> >>>>
> >>>> Object o = codec.deserializeFrom( in )
> >>>>
> >>>> Since the schema is included in the input data, the serializer will
> >> behind the scene do something like:
> >>>>
> >>>>
> >>>> Map<String, Object> map = codec.decode( Map.class ).from( in );
> >>>> // Get the schema type, which is SomeDTO.class
> >>>> ...
> >>>> Object o = converter.convert( map.get(“data”) ).to( SomeDTO.class
);
> >>>>
> >>>>
> >>>> wdyt?
> >>>>
> >>>> =David
> >>>>
> >>>>
> >>>>
> >>>>> On Aug 20, 2016, at 3:23 PM, David Leangen <osgi@leangen.net>
wrote:
> >>>>>
> >>>>>
> >>>>> Hi,
> >>>>>
> >>>>> I am trying to use the Convert/Codec as a serializer, but it has
been
> >> a bit of a struggle so far.
> >>>>>
> >>>>> I have a “deep” 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.
> >>>>>
> >>>>> I have tried adding a Rule using an adapter, and setting the
> >> codec.with(thatAdapter), but that didn’t work out so well, either.
> >>>>>
> >>>>> 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.
> >>>>>
> >>>>> I think that this code _should_ work, which means that there is
> likely
> >> a bug in the code. Even so…
> >>>>>
> >>>>> This is a very convoluted way of working, which doesn’t seem right
to
> >> me. Am I missing some concept?
> >>>>>
> >>>>>
> >>>>> Example below.
> >>>>>
> >>>>> Cheers,
> >>>>> =David
> >>>>>
> >>>>>
> >>>>> 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;
> >>>>>
> >>>>> public DTOSerializer( Converter aConverter, Codec aCodec, Class<E>
> >> aType )
> >>>>> {
> >>>>>    // Very convoluted attempt at adding a rule to try to make the
> >> conversion of my “PutCommand” work during deserialization
> >>>>>     converter = aConverter.getAdapter()
> >>>>>             .rule(
> >>>>>                     PutCommand.class,
> >>>>>                     Map.class,
> >>>>>                     dto -> { Map map = new HashMap<>();
map.put(
> >> "key", dto.key ); map.put( "entity", dto.entity ); return map; },
> >>>>>                     map -> { PutCommand c = new PutCommand();
c.key =
> >> (String)map.get( "key" ); c.entity = aConverter.convert( map.get(
> "entity"
> >> ) ).to( aType ); return c; } );
> >>>>>     codec = aCodec.with( converter );
> >>>>> }
> >>>>>
> >>>>> @Override
> >>>>> public Object readObject( InputStream in )
> >>>>>         throws Exception
> >>>>> {
> >>>>>    // This is the raw data
> >>>>>     final Map<String, Object> map = codec.decode( Map.class
).from(
> >> in );
> >>>>>    // The name of the object type to cast to
> >>>>>     final String commandTypeName = (String)map.get( "command" );
> >>>>>     final Class<?> commandType = 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 = converter.convert( map.get( "payload"
)
> >> ).to( commandType );
> >>>>>     return command;
> >>>>> }
> >>>>>
> >>>>> @Override
> >>>>> public void writeObject( OutputStream out, Object object )
> >>>>>         throws Exception
> >>>>> {
> >>>>>     final Map<String, Object> map = 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 );
> >>>>> }
> >>>>> }
> >>>>>
> >>>>
> >>>
> >>
> >>
>
>

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