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From David Leangen <o...@leangen.net>
Subject Re: [Converter] Serializer
Date Mon, 22 Aug 2016 23:50:00 GMT

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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