[cdi-dev] [JBoss JIRA] (CDI-565) Unsatisfied dependency should be null?

Martin Andersson (JIRA) issues at jboss.org
Wed Oct 7 17:10:00 EDT 2015

    [ https://issues.jboss.org/browse/CDI-565?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13116194#comment-13116194 ] 

Martin Andersson commented on CDI-565:

I'm not sure I fully understood you on that "dropping behavior"-thing. NPE:s is kind of what I try to advertise here =)

{{null}} is and will always be a valid candidate to represent the absence, or lack of, a value. *YES*, we all agree that it is easy to misuse {{null}} and {{null}} should be avoided if we so can. My point is definitely not that {{null}} is a god sent gift. Please reread my comments and see if you can answer the questions I asked, for example, what is the difference between {{Map}} and the CDI container (when it comes to dependency lookup)? Also compare {{EntityManager.find()}} from the JPA specification which I believe is another comparable interface.

Everything is an API. I don't understand this notion really or how it relates to the topic of discussion. I googled the CDI specification, {{null}} return values are used everywhere. Would you like to advocate that we setup tens of new exception types into our type system instead?

Speaking of API:s, I would appreciate an {{Optional}} return type from a service only if it is conceivable and often enough the case that no return value is present. For example: {{tryCatchAFish()}}, {{getLotteryWinner()}}. Or, if it is conceivably so that the client doesn't bother about the presence of the value, he's sole intent is to pass it around somewhere. For example: {{myDataSource.getSomeData().ifPresent(someValueContainer::setThatField)}}. I myself overused {{Optional}} once it was out in JDK 8, singing praise to Jesus every minute of the day. Only to discover that my overuse of it introduced complex boilerplate code. Eventually I got so lazy that I dereferenced those optionals anyway ({{Optional.get().doSomething()}} and so it was only a matter of time before I found myself back in square one. But this time, not only did I have NPE:s, all my clients had to make an extra method call all over the place for no apparent gain. More than anything else, I learnt by my own mistake that if a parameter is optional for the client, then accept {{null}}. Don't force the clients to type {{Optional.empty()}} instead of the oh so simple blueish keyword {{null}}. Null as parameters make the life easier for the implementations too.

Also, if you know a "modern language" that had any success avoiding {{null}}, please let me know. That would be interesting. For example, how would I create a new 10 element array? Am I required to have all ten elements right of the bat?

Guys, what is the most intuitive thing to do here? Why do you believe that the dependency lookup should have different semantics than {{Map.get()}}?

> Unsatisfied dependency should be null?
> --------------------------------------
>                 Key: CDI-565
>                 URL: https://issues.jboss.org/browse/CDI-565
>             Project: CDI Specification Issues
>          Issue Type: Feature Request
>          Components: Beans
>            Reporter: Martin Andersson
> What if I am writing a component that call a collaborator only if that class exist during runtime? It is amazingly intuitive to write code like this:
> {code:java}
> class MyComponent
> {
>     @Inject
>     SomeCollaborator collaborator;
>     public void someMethod() {
>         // .. do something
>         if (collaborator != null) {
>             collaborator.callback();
>         }
>     }
> }
> {code}
> But the specification currently says in section "5.2.2. Unsatisfied and ambiguous dependencies" that this code should cause a "deployment problem". IIRC, GlassFish 4.1 and WildFly 9 doesn't actually crash during deployment. They will inject {{null}}. WebLogic 12, I just learned, do crash during deployment.
> I believe it is unfortunate to have this null value logic for no apparent reason. It is inevitably so that the application code would crash anyways as soon as it tries to dereference a null reference. And you probably agree with me that there's a general guideline established in the developer community which say you shouldn't do null pointer checks all over the code because null pointers will crash exactly when and where the absence of a value really is a problem.
> The work around is to inject an {{Instance}} of my type and iterate through all of them, or do any other form of programmatic lookup. However, me personally, I've had this requirement far too many times now. It is often the case that a component I write has a "subframework" in place such that when I want to affect how the application performs, I can just add in new classes of a particular type and it is scoped up. Please don't even make the notion of a design smell out of your own lack of creativity, if you want to see a concrete example then of course I am more than happy to provide you with that. Just saying =)
> Something so intuitive and present in our every day coding life as a "null return value" should be present in the CDI specification too. I mean that is what the specification in essence is; one huge {{lookupInstanceOf(class)}}-method. Reading this method name, would you really expect it to crash, or return {{null}}?
> How about adding in a new annotation such that the injection point accept null values for unsatisfied dependencies, but if the injection point has an annotation {{@Required}}, then an unsatisfied dependency do crash for this injection point?
> You will probably say "let's do it the other way around so that we don't brake backward compatibility" by creating an {{@Optional}} annotation. Hey, if I am abusive to my wife and she divorce me. Should I continue to be abusive to my next wife so that I don't break backward compatibility? Our number one goal should be to define the most awesome specification and API possible - nothing else. If we keep leaving small piles of poo everywhere, then we will inevitably end up deep in shit.
> Thank you all for your hard work and time devoted to making all of our lives so much greater.

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