Lazy Loading

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Lazy Loading
This is the manual for older MicroStream versions (Version < 5.0).
The new documentation (Version >= 5.0) is located at:
​https://docs.microstream.one/​
In this chapter it is explained how Lazy Loading is done with MicroStream.
Of course, it's not really about the technical implementation of procrastination, but about efficiency: why bloat the limited RAM with stuff before you even need it?
Classic example:
The application has self-contained data areas that contain a large amount of data. The data for an area is not loaded if the area is not worked at all. Instead, you only load a tiny amount of "head data" for each area (name or other for displaying purposes) and the actual data only when the application really needs it. E.g. fiscal years with hundreds of thousands or millions of sales. One million revenue records for 2010, one million for 2011, for 2012, etc. In 2019, most of the time only 2019 and 2018 will be needed. The previous few, and the year 2000 sales are not of great interest anymore. Therefore: load data only when needed. Super efficient.
For example let's say the app "MyBusinessApp" has a root instance class, looking like this:
public class MyBusinessApp
{
    // ...
​
    private HashMap<Integer, BusinessYear> businessYears = new HashMap<>(); 
​
    // ...
}
The business year hold the turnovers:
public class BusinessYear
{
    // ...
​
    private ArrayList<Turnover> turnovers = new ArrayList<>();
​
    // ...
}
This approach would be a problem: During initialization, the root instance would be loaded, from there its HashMap with all BusinessYear instances, each with its ArrayList and thus all conversions. For all years. 20 years of approximately 1 million sales makes 20 million entities, which are pumped directly into the RAM at the start. It does not matter if someone needs it or not. We don't want it that way.
It would be nice if you could simply add a "lazy" to the turnover list. And that's exactly how it works:
public class BusinessYear
{
    // ...
​
    private Lazy<ArrayList<Turnover>> turnovers  = ...; // we will get to that
​
    // ...
}
And bingo, the turnovers are now loaded lazily.
Of course, this is no longer an ArrayList<Turnover> field, which is now magically loaded lazy, but this is now a Lazy field and the instances of this type are typed generically to ArrayList<Turnover> . Lazy is just a simple class whose instances internally hold an ID and a reference to the actual thing (here the ArrayList instance). If the internal reference is zero, the reserved ID is used to reload it. If it is not null, it is simply returned. So just a reference intermediate instance. Similar to the JDK's WeakReference, just not JVM-weak, but storage-lazy.
What do you have to do now to get the actual ArrayList<Turnover> instance?
ArrayList<Turnover> turnovers = this.turnovers.get();
Just as with WeakReference, or simply as one would expect from a reference intermediate type in general: a simple get method.
The .get() call reloads the data as needed. But you do not want to mess around with that yourself. No "SELECT bla FROM turnovers WHERE ID =" + this.turnovers.getId(). Since you want to program your application you don' t have to mess around with low-level database ID-loading stuff. That's what the MicroStream Code does internally. You do not even need to access the ID, you just have to say "get!".
That's it.
What about the "..." ?
There are different strategies, what you write here. Analogous to the code example before it would be simply:
private Lazy<ArrayList<Turnover>> turnovers = Lazy.Reference(new ArrayList<>());
So always a new ArrayList instance, wrapped in a Lazy instance. If the actual ArrayList reference should be null at first, it works the same way:
private Lazy<ArrayList<Turnover>> turnovers = Lazy.Reference(null);
The this.turnovers.get() also just always returns null. Completely transparent.
But you could also do this:
private Lazy<ArrayList<Turnover>> turnovers = null;
If there is no list, then you do not make any intermediate reference instance for any list. A separate instance for null is indeed a bit ... meh.
But that has a nasty problem elsewhere: this.turnovers.get() does not work then. Because NullPointerException.
Anytime you need to write this here, the readability of code is not exactly conducive:
return this.turnovers == null ? null : this.turnovers.get();
But there is a simple solution: Just move this check into a static util method. Just like that:
return Lazy.get(this.turnovers);
This is the same .get(), just with a static null-check around it. This always puts you on the safe side.
In Short
For Lazy Loading, simply wrap Lazy<> around the actual field and then call .get() or maybe better Lazy.get(...).
It's as simple as that.
The full example can be found on GitHub.
Side Note
Why do you have to replace your actual instance with a lazy loading intermediate instance and fiddle around with generics? Why is not something like this:
@Lazy
private ArrayList<Turnover> turnovers = new ArrayList<>();
Put simply:
If it were just that it would be bare Java bytecode for accessing an ArrayList. There would be no way for a middleware library to get access and look it up and perhaps reload it. What's written there is an ArrayList reference. There is no lazy anymore. Either, the instance is null, or it is not null. If you wanted to reach in there, you would have to start with bytecode manipulation. Technically possible, but something you really don't want in your application.
So there must always be some form of intermediary.
Hibernate solves this through its own collection implementations that do lazy loading internally. Although the lazy loading is nicely hidden in some way (or not, if you need an annotation for that), it also comes with all sorts of limitations. You can only use interfaces instead of concrete classes for collections. At first, the instance is not the one you dictate, the code becomes intransparent and difficult to debug, you have to use a collection, even if it's just a single instance, and so on. You want to be able to write anything you want and you want full insight and control (debugability, etc.) over the code.
All this can be done with the tiny Lazy Interim Reference class. No restrictions, no incomprehensible "magic" under the hood (proxy instances and stuff) and also usable for individual instances.
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Outline
What about the "..." ?
In Short
Side Note