MapStruct

In the Java sphere we often develop the same data class several times. Each layer has its own variant and they are named almost the same, with some prefix or suffix or just the package name to distinguish. The set of attributes is the same (or almost the same), they have setters and getters. Or maybe only getters.

Nobody wants to write business logic two or three or four times, no matter how much support we have for copying code between the layers. And there OO is gone. We have to use anemic data objects, which was clearly introduced as an antipattern by Martin Fowler some years ago.

Since we are now using new paradigms and every couple of years, we no longer care and no longer know. OO was 25 years ago. Now we do FP and Microservices. And new frameworks. And many layers.

So, where does this come from?

First of all, the database access layer is Hibernate. I do not know why, because I think that plain JDBC would be easier, but Hibernate is already there and cannot be removed by arguments. Now Hibernate came with the promise that we can just use plain objects (POJOs) for our data and mirror database tables to classes and columns to attributes. Some XML-stuff had to be written and everything worked. Only writing the XML was such a pain that people immediately jumped to the annotations alternative once it was there. It was better and still is. But now the POJOs are obviously cluttered with Hibernate or JPA annotations. So they have to stay in the database layer. Actually there is a much stronger argument for this. Objects contain other objects and collections of objects. And possibly everything, if we go deeper recursively. So accessing the database should be a reasonably fast operations, so some attributes are loaded lazily. That means, they are only really loaded, when we need them. Which can go terribly wrong, because the transaction is no longer around and it is too late.

Also we have our idea what data classes have to look like, so there are some layers where we want no-argument-constructors and setters and getters, some layers where we want final attributes and constructors with all attributes and only getters and again others where we prefer to use a builder. And yes, each layer has its rules that need to be followed.

So, we keep them in the DB layer and map them to almost identical service layer objects without annotations. Then we work with these, write our business logic. Procedural programming mostly. Because the objects cannot have business logic. So we have classes with methods that are kind of behaving like static methods, but are non-static, because the framework wants it like that.

And then again, we build more and more layers, because each concern needs to be dealt in its own layer. And requires its own set of data objects, possibly with its own annotations for REST or SOAP or JSON or XML or whatever.

So, how do we move data between layers? At each layer boundary the data needs to be copied to the sister objects in the new layer. Now this is kind of stupid, programming something like

class HouseL1 {
private final int a;
private final String b;

public void HouseL1(HouseL2 l2) {
this.a = l2.getA();
this.b = l2.getB();
....
}
}

or with builder or with setters and getters, it is a lot of ugly work. And even worse, all sub-objects and collections of sub-objects have to be mapped. And their sub objects. And we possibly have to stop somewhere.

So we would like to avoid doing all this tedious stuff.

What can we do?

Is Java really the right language? Of course, we are writing enterprise software and we need type safety. Not real type safety like Scala, but a little bit of it feels good.

Reconsider our whole architecture and simplify it. Maybe it is possible to get rid of some layers and write much simpler software that does the same thing, only much faster and with less bugs. Ok, I’m only kidding here. We are talking enterprise software here. And yes, sometimes the layers do have real purposes and make sense.

Try to use the same data objects in all layers anyway? It has been tried. It works, but only in very simple settings.

Create the source code for the mapping. You can write a script for this or find one or find a tool or whatever. Parse the data classes and create the source code for the transformation methods. Or just write hibernate classes and create all other layers with their preferred setup in terms of mutability and construction and annotations from that.

But we are in Java, so why not use reflection and figure out at runtime how to map it. Find a library to do it for you or write your own. Performance? No problem. We use enterprise servers, of course.

So, in the end it is a good possibility to have a Java-tool that creates the source code for the transformation as part of the compile process.

MapStruct does exactly that. We write in interface for our transformations. With some annotations non obvious mapping behavior can be specified. Keep that list small and try to make it possible to automatically recognize the mappings, where possible. Then an extension to the maven compiler-plugin is added, that involves MapStruct to create an implementation for this interface at compile time and of course compile the implementation. And voila, it works. Even for classes with builders, if only the builder uses method names that are identical to the corresponding attribute name without „with“-prefix. So deal with it, name the methods of the builder like that.

And yes, we should get rid of setters, where we do not really need them. And we should not write constructors with 20 parameters, because the parameters will get messed up. In a language like Java, that does not yet have named parameters. And we do not want to couple layers, so constructors that use the sister class from another layer are not a good idea either, if we have more than two layers or so. So there we go with a builder..

In the end of the day, we can write good software with any reasonably good language and framework. But it is worth investigating how to do certain things. And it is really worth asking the question, why we are doing this at times when it is possible to make such choices.