CODEWITHSUNDEEP
javadesign-patternscallback
Walfrat
Walfrat

Reputation: 5353

A very specific usage of callbacks in Java

This question is about a specific usage of a callback pattern. By callback i mean an interface from which i can define method(s) that is (are) optionnaly (= with a default set to 'do nothing', thanks Java 8) called from a lower layer in my application. My "application" is in fact a product which may have a lot of changes between client projects, so i need to separates somethings in order to reuse what won't change (technical code, integration of technologies) from the rest (model, rules).

Let's take an example :

I developped a Search Service which is based upon Apache CXF JAX-RS Search. This service parses a FIQL query which can only handle AND/OR condition with =/</&gt/LIKE/... condition to create a JPA criteria query. I can't use a a condition like 'isNull'.

Using a specific interface i can define a callback that will be called when i got the criteria query from apache CXF layer in my search service and add my condition to the existing ones before the query is executed. This condition are defined on the upper layer of my searchService (RestController). This is in order to reduce code duplicate, like retuning a criteria query and finalize it in every methods where i need it. And because using @Transactional in CXF JAX-RS controller does not work well Spring proxy and CXF work (some JAX-RS annotation are ignored);

First question : does this example seems to be a good idea in terms of design ?

Now another example : i have an object which have some basic fields created from a service layer. But i want to be able to set others non-nullable fields not related to the service's process before the entity is persisted. These fields may move from a projects to another so i'd like to not have to change the signature of my service's method every time we add / remove columns. So again i'm considering using a callback pattern to be able to set within the same transaction and before object is persisted by the Service layer.

Second question : What about this example ?

Global question : Except the classic usage of callback for events : is this a pratice to use this pattern for some specific usage or is there any better way to handle it ?

If you need some code sample ask me, i'll make some (can't post my current code).

Upvotes: 1

Views: 196

Answers (1)

Michał Komorowski
Michał Komorowski

Reputation: 6238

I wouldn't say that what you've described is a very specific usage of "an interface from which i can define method(s) that is (are) optionally called from a lower layer". I think that it is reasonable and also quite common solution.

Your doubts may be due to the naming. I'd rather use the term command pattern here. It seems to me that it is less confusing. Your approach also resembles the strategy pattern i.e. you provide (inject) an object which performs some calculations. Depending, on the context you inject objects that behave in a different way (for example add different conditions to a query).

To sum up callbacks/commands are not only used for events. I'd even say that events are specific usage of them. Command/callback pattern is used whenever we need to encapsulate an operation within an object and transfer/pass it somehow (by the way, in Java there is no other way to do so but for example in C++ there are pointers to methods, in C# there are delegates...).

As to your second example. I'm not sure if I understand it correctly. Why can't you simply populate all required fields of an object before calling the service?

Upvotes: 1

Related Questions