Add lazy loading support to fields referencing other resources about neba

[olaf-otto]

The introduction of lazy-loading forces adressing a couple of general-nature issues. Most prominently, lazy-loading does not work in conjunction with null values. Imagine a resource model "A" referencing a resource model "B":

a : A {
  b : B
}

To signal whether "b" exists or not, "a.b" can be null. However, to determine whether that is the case, one must actually attempt to load and adapt "B" from the persistence layer - which is exactly what we do not want for lazy loading.

Usually, one injects a proxy object for lazily loaded references and loads the actually referenced object when the proxy is accessed. However, this would mean that any lazy reference is never null, and that the (poor) client accessing the proxied reference will find out that the hidden target of the reference is null by means of an NPE when accessing a proxied method. He would have no useful means to find out if the reference is there - or not.

This issue can only be prevented by following the value-holder pattern, like so:

a : A {
    b : Optional<B>
}

Here, b is of type Optional, which offers ways to determine if the reference exists. This pattern is employed by languages that strive to abolish null, e.g. in Scala's Option.

The only kind of references that can be made lazy transparantly are collections, since they have a natural empty state - and NEBA guarantees that non - @unmapped collections are not null.

I thus suggest to implement lazy loading like so:

• By default, make collection-valued references, such as @children or @reference with collection-types, lazy
• In addition, provide a ValueHolder interface that can explicitly be used in @ResourceModels to signal that references are lazy-loaded. This also allows declaring:

a : A {
  b : Optional<Collection<B>>
}

Thus, users can choose the style of collection lazy-loading: Explicit or implicit.

In addition, I do suggest that we make the value holder interface API compatible to the Java 8 Optional type, to ensure smooth transitioning from older java versions in the future.

[olaf-otto]

Feature branch: https://github.com/unic/neba/tree/feature/neba-15

[olaf-otto]

Implemented and shipped with 3.4.0