Why Objectify instead of JDO?

I am approaching to Gwt + Gae world.

My essential need is to send over the Gwt-Rpc wire my Entity classes, without duplicating them into DTOs.

Objectify promise to do that pretty well. It claims it will hide all the "Jdo complexity".

I ne开发者_运维技巧ver worked with Jpa or Jdo technologies. Where's all the complexity?

I mean, can you provide me some simple examples about complex tasks in JDO, made trivial by Objectify?

Maybe relationships?

I think JDO/JPA are easy to play with on "Hello World" level. But it changes as soon as you need something more real such as composite keys, multiply relationships between entities and etc. JDO GAE implementation is quite complex and hard to grasp for beginners, partly due to unsupported features, workarounds and extensions. JDO is designed to work "everywhere", which means it is highly abstracted and very general in its nature. Great for portability, but it also means that it might not be a perfect match for a specific engine like GAE with its quite unique datastore. The Datanucleus/JDO/JPA jars are quite big (~2.3 mb in total), while Objectify's jar is pretty small. JDO/JPA might perform class path scanning at startup to find and register your entities, which could add to the load time. The time spent would be proportional to the number of classes in your project.

As per example I think in terms of amount of code JDO/JPA sample will appear simpler than lots of DAO classes for Objectify, but in general, maintenance of Objectify code will be easier for an engineer because you don't need to walk through minefield thinking what you can break in JDO :)

One example of JDO's complexity is seeing how many different states an entity can be in. As an example of how this can be overwhelming at first, scroll to the bottom of this page and look at that state diagram. Objectify does not need such a state diagram.

Another tricky part of JDO is all the 'magic' that happens behind the scenes, which sometimes made debugging difficult. Of course this is not actually magic, just bytecode rewriting, but that along is tricky enough.

Finally, JDO is a generic API. It is designed to work with object stores, SQL databases, and who knows what else. The connection between a certain JDO concept and what will actually be happening in the datastore is sometimes difficult to see. Objectify's API is closely aligned with the datastore, making it easier to understand what is going on.