Tuesday, May 13, 2008

Flexjson limitations

The old saying goes "If it sounds too good to be true, it is too good to be true". Well this applied to Flexjson. While this tool is very capable of externalizing an object to the JSON format, there are several shortcomings which make it difficult to use at the moment. The contributors have mentioned they are working to address these issues. Currently Flexjson only can process primitives, wrappers, Strings and Dates. Objects (that are not collections) are they themselves externalized into JSON. In complex data models, you may not wish to serialize the entire downstream object. You may wish to only serialize it's ID. This (and my gripping about the implementation) is shown in one of my previous posts on JAXB and XmlID. Currently Flexjson has no clean way of supporting this. The only was to attempt this would be through the usage of include() and exclude() on the JSONSerializer. The downside of this, is that you essentially need a specific handler for each object you want to serialize since the attributes/conditions of inclusion or exclusion will change. Lets look at an example:
public class Stamp implements Serializable {
  private Long id;
  private String description;
  private Country country;
  // ... other attributes and methods

public class Country implements Serializable {
   private Long id;
   private String name;
   // ...
So in this example, if we wanted to serialize all of the Stamps to JSON, by default the country would be serialized for each stamp. In a typical system, we might have 200 countries and 50,000 stamps. This means that our countries are fully serilized in a redundant fashion many times over. In this situation what we really want is the country id field. We can get this in the following way:
  Writer out = // ... some writer like a PrintWriter
  Collection stamps = stampService.getAll( );
  JSONSerializer serializer = new JSONSerializer( );
  serializer = serializer.include("id","description","country.id").exclude("*");
  for( Stamp s: stamps ) { 
     out.write( serializer.serialize( s ) );

While this works, If your object has many properties and object relationships, this can get a little exhaustive setting up the include and exclude parameters. It also means you either (a) need a introspective tool to read this from your beans or (b) you need to provide some handler for each bean to setup the includes and excludes properly. I personally have great faith in the Open Source community, and will look forward to leveraging the next version of Flexjson to cleaner handle this situation with a Transformer (Transformers today only handle Strings, primitives and dates). Until then I suppose I'll have to come up with some solution that is tied to my object model.

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