Automatically generate constructors for services with Groovy

When using dependency injection frameworks like Spring or Micronaut, you will often have services, that depend on services, …. While back in the days, annotating fields with @Inject or @Autowired was acceptable, it felt out of favour, because you would never know what a service really depends on. So nowadays injection is usually done via the constructor.

But this also means, that you will find yourself often fiddling around with constructors and orders of arguments. You also will find yourself falling back to the help of your IDE. But if you have the slightest sense of order, you will also find, that the generated code does not keep your order of things: e.g. IntelliJ always adds new fields to the end of the c'tor - and also the assignment to the field inside the c'tor. So you either delete the whole c'tor and recreate again or you will end up using more IDE features to re-arrange your code.

Given that usually those c'tors are only used by CI automatically and maybe in your tests, can't we do better and just let the compiler generate the c'tor for us -- instead of moving words around with your editors?

Groovy comes with TupleConstructor, which can create a c'tor with explicit arguments (in contrast to MapConstructor, which is used to construct via passed in maps to make it look like Groovy would support named arguments). The defaults of TupleConstructor are very un-strict, though. By default, it creates a c'tor for each additional property where the default is null. Usually not what we want with DI, because it does not know which c'tor to use now or might use the wrong one and we end up in the place, that we wanted to avoid when going with c'tor based injection.

But luckily TupleConstructor can be configured to high degree. These settings will give you a pretty good default:

@groovy.transform.TupleConstructor(
        includeFields = true,  // add fields to the c'tor
        defaults = false // do not generate versions of the c'tor with null assignments
)

So now instead of writing this every time, you can make this a new annotation to easily use (and still be able to override the nuances, if you have to), with AnnotationCollector. E.g.

@groovy.transform.TupleConstructor(
        includeFields = true,  // add fields to the c'tor
        defaults = false // do not generate versions of the c'tor with null assignments
)
@groovy.transform.AnnotationCollector
@interface ServiceConstructor {}

Now you can use this annotation instead:

@Service
@ServiceConstructor(
        excludes = ['message'] // if you have transients, add them here
)
class MyService {

    protected final HelloService helloService

    transient protected String message

    @PostConstruct
    void init() {
        message = helloService.sayHello("World")
    }
}

This will generate the c'tor you expect: MyService(HelloService helloService). And if you need your FormatService too, you just add it as a field -- at the place you like. And the c'tor will honour your order in the code.

One downside though is the loss of making your IDE show the usages of the c'tor -- but then, this is often only useful for tests. In that case you can still use your IDE to generate the c'tor, maybe adjust, find the usages and fix them and then delete the c'tor again. There sadly seems not to be a flag to make TupleConstructor throw, if there are existing c'tors. It will by default just silently not generate a c'tor, which is not the worst.