Delegating in custom Jackson deserializer

user_11779675 0
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The situation is the following: I have managed to get Jackson to deserialize the following generic ResponseWrapper<T>.

static final class ResponseWrapper<T> {
    
    private ResponseData <T> response;
    
    protected static final class ResponseData<T> {
    
        private int    status;
        private T      data;
    
    }
    
}

Using the following ParameterizedTypeReference

public static <T> ParameterizedTypeReference <ResponseWrapper<T>> typeReferenceOf ( Class<T> tClass ) {
    return ParameterizedTypeReference.forType( ParameterizedTypeImpl.make( ResponseWrapper.class, new Type[]{ tClass }, null ) );
}

The problem: I need to handle a situation where the deseralization of T will fail because the value of data will not be an object but a String instead. I need to capture the exception and assign this value to another property in ResponseData, for example String errorMessage. My take on it has been to annotate the response property with @JsonDeserialize( using = ResponseDeserializer.class ) but I do not know how to implement the JsonDeserializer properly so that it delegates the deserialization to Jackson's implementations and I simply capture the exception when it occurs.

For more context, I am using WebClient as the HTTP client and I deal with responses using an exchange Function<ClientResponse, Mono<ResponseWrapper<T>> where the deserialization takes place.

json-deserialization jackson generics java
1 Answers

In order to get the generic type, I had ResponseDeserializer implement ContextualDeserializer

@Override
public JsonDeserializer<?> createContextual ( DeserializationContext context, BeanProperty property ) throws JsonMappingException {

   JavaType wrapperType = property.getType();
   JavaType valueType = wrapperType.containedType(0);
   return new ResponseDeserializer<>( valueType );

}

That allowed me to create concrete instances of the deserializer with the specific JavaType. The following is the complete solution:

private static final class ResponseDeserializer<T> extends JsonObjectDeserializer<ResponseWrapper.ResponseData<T>> implements ContextualDeserializer  {
   
   private final JavaType javaType;

   public ResponseDeserializer ( ) {
       this.javaType = null;
   }
   
   public ResponseDeserializer ( JavaType javaType ) {
       this.javaType = javaType;
   }
   
   @Override
   public JsonDeserializer<?> createContextual ( DeserializationContext context, BeanProperty property ) throws JsonMappingException {

       JavaType wrapperType = property.getType();
       JavaType valueType = wrapperType.containedType(0);
       return new ResponseDeserializer<>( valueType );
       
   }

   @Override
   protected ResponseWrapper.ResponseData<T> deserializeObject ( JsonParser jsonParser, DeserializationContext context, ObjectCodec codec, JsonNode tree ) throws IOException {
       
       int status = tree.get( "status" ).asInt();
       JsonNode dataNode = tree.get( "data" );
       
       try {

           final T data; {
               
               if ( dataNode == null ) {
                   data = null;
               } else {
                   JsonParser dataParser = dataNode.traverse();
                   JsonToken token = dataParser.nextToken(); // handle?
                   data = context.readValue( dataParser, javaType );
               }
               
           };
           
           return new ResponseWrapper.ResponseData<T>( status, data );
           
       } catch ( InvalidDefinitionException e ) {
           
           String errorMessage = dataNode.asText();
           return new ResponseWrapper.ResponseData<T>( status, errorMessage );
           
       }

   }
   
}
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