CODEWITHSUNDEEP
jsongwt
paul
paul

Reputation: 494

converting gwt shared object to json

Could someone explain to me why in GWT you cannot convert a client/shared pojo (that implements Serializable) into a JSON object without jumping through a load of hoops like using the AutoBeanFactory (e.g GWT (Client) = How to convert Object to JSON and send to Server? ) or creating javascript overlay objects (and so extends JavaScriptObject)

GWT compiles your client objects into a javascript object, so why can't it then simply convert your javascript to JSON if you ask it to?

The GWT JSON library supplied only allows you to JSONify java objects that extend JavaScriptObject

I am obviously misunderstanding something about GWT since a GWT compiles a simple java POJO into a javascript object and in javascript you can JSON.stringify it into JSON so why not in GWT?

Upvotes: 2

Views: 1469

Answers (1)

Colin Alworth
Colin Alworth

Reputation: 18331

GWT compiles your app, it doesn't just convert it. It does take advantage of the prototype object in JavaScript to build classes as it needs, usually following your class hierarchy (and any GWT classes you use), but it makes many other changes:

Optimizations:

  • Tightens up types - if you refer to something as List, but it can only be an ArrayList, it rewrites the type declarations. This by itself doesnt give much, but it lets other steps do better work, such as
  • Making methods static - if nothing ever overrides ArrayList.add, for example, this will turn any calls it can prove are to ArrayList.add into a static call, preventing the need for dynamic dispatch, and allowing the 'this' string in the final JS to be replaces with a shorter arg name. This will prevent a JS object from having a method you expect it to have.
  • Inline Methods - if a method is simple enough, and is called in few enough places, the compiler might remove the method entirely, since it knows all places where it is called. This will directly affect your use case.
  • Removes/Inlines unreferenced fields - if you read to a field but only write it once, it will assume that the original value is a constant. If you don't read it, there is no reason to assign it. Values that the compiler can't tell will ever be used don't need to be using up space in the js and time in the browser. This also will directly affect treating gwt'd Java as JS.

After these, among others, the compiler will rename fields, arguments, and types to be as small as possible - rarely will a field or argument be longer than 1 character when this is complete, since those are most frequently used and have the smallest scope, so can be reused the most often by the compiler. This too will affect trying to treat objects as JSON.

The libraries that allow you to export GWT objects as JSON do so by making some other assumption.

  • JavaScriptObject (JSO) isn't a real Java object, but actually represents a JavaScript instance, so you can cast back and forth at will - the JSNI you write will emerge relatively unoptimized, as the compiler can't tell if you are trying to talk to an external library.
  • AutoBeans are generated to assume that they should have the ability to write out JSON, so specific methods to encode objects are written in. They will be subject to the same rules as the other Java that is compiled - code that isn't used may be removed, code that is only called one way might be tightened up or inlined.
  • Libraries that can export JS compile in Java details into the final executable, making it bigger, but giving you the ability to treat these Java objects like JS in some limited way.

One last point, since you are talking both about JSON and Javascript - Some normal JS isn't suitable for writing out as JSON. Date objects don't have a consistent way to serialize that is recognized by JSON. Non-tree object graphs can't be serialized:

var obj = {};
obj.prop = {};
obj.prop.obj = obj;

Autobeans come with a built in checker for these circular references, and I would hope the JSO serialization does as well.

Upvotes: 2

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