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Declarative and Immutable Pipeline of Transformations

A couple of months in the past I made a small Java library, which is price explaining for the reason that design of its courses and interfaces is fairly uncommon. It’s very a lot object-oriented for a reasonably crucial process: constructing a pipeline of doc transformations. The aim was to do that in a declarative and immutable manner, and in Java. Properly, as a lot because it’s attainable.

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Let’s say you may have a doc, and you’ve got a group of transformations, every of which is able to do one thing with the doc. Every transformation, for instance, is a small piece of Java code. You wish to construct an inventory of transformations after which cross a doc by this record.

First, I made an interface Shift (as a substitute of the ceaselessly used and boring “transformation”):

interface Shift {

  Doc apply(Doc doc);


Then I made an interface Prepare (that is the identify I made up for the gathering of transformations) and its default implementation:

interface Prepare {

  Prepare with(Shift shift);

  Iterator<Shift> iterator();


class TrDefault implements Prepare {

  personal remaining Iterable<Shift> record;


  Prepare with(Shift shift) {

    remaining Assortment<Shift> gadgets = new LinkedList<>();

    for (remaining Shift merchandise : this.record) {




    return new TrDefault(gadgets);



  public Iterator<Shift> iterator() {

      return this.record.iterator();



Ah, I forgot to let you know. I’m an enormous fan of immutable objects. That’s why the Prepare doesn’t have a technique add, however as a substitute has with. The distinction is that add modifies the article, whereas with makes a brand new one.

Now, I can construct a prepare of shifts with TrDefault, a easy default implementation of Prepare, assuming ShiftA and ShiftB are already carried out:

Prepare prepare = new TrDefault()

  .with(new ShiftA())

  .with(new ShiftB());

Then I created an Xsline class (it’s “XSL” + “pipeline”, since in my case I’m managing XML paperwork and remodel them utilizing XSL stylesheets). An occasion of this class encapsulates an occasion of Prepare after which passes a doc by all its transformations:

Doc enter = …;

Doc output = new Xsline(prepare).cross(enter);

Up to now so good.

Now, I would like all my transformations to log themselves. I created StLogged, a decorator of Shift, which encapsulates the unique Shift, decorates its methodology apply, and prints a message to the console when the transformation is accomplished:

class StLogged implements Shift {

  personal remaining Shift origin;


  Doc apply(Doc earlier than) {

    Doc after = origin.apply(earlier than);

    System.out.println(“Transformation accomplished!”);

    return after;



Now, I’ve to do that:

Prepare prepare = new TrDefault()

  .with(new StLogged(new ShiftA()))

  .with(new StLogged(new ShiftB()));

Seems like a duplication of recent StLogged(, particularly with a group of some dozen shifts. To do away with this duplication I created a decorator for Prepare, which on the fly decorates shifts that it encapsulates, utilizing StLogged:

Prepare prepare = new TrLogged(new TrDefault())

  .with(new ShiftA()))

  .with(new ShiftB());

In my case, all shifts are doing XSL transformations, taking XSL stylesheets from information obtainable in classpath. That’s why the code seems to be like this:

Prepare prepare = new TrLogged(new TrDefault())

  .with(new StXSL(“stylesheet-a.xsl”)))

  .with(new StXSL(“stylesheet-b.xsl”)));

There may be an apparent duplication of recent StXSL(…), however I can’t merely do away with it, for the reason that methodology with expects an occasion of Shift, not a String. To resolve this, I made the Prepare generic and created TrClasspath decorator:

Prepare<String> prepare = new TrClasspath<>(new TrDefault<>())



TrClasspath.with() accepts String, turns it into StXSL and passes to TrDefault.with().

Take note of the snippet above: the prepare is now of sort Prepare<String>, not Prepare<Shift>, as can be required by Xsline. The query now’s: how will we get again to Prepare<Shift>?

Ah, I forgot to say. I wished to design this library with one necessary precept in thoughts, steered in 2014: all objects could solely implement strategies from their interfaces. That’s why, I couldn’t simply add a technique getEncapsulatedTrain() to TrClasspath.

I launched a brand new interface Prepare.Short-term<T> with a single methodology again() returning Prepare<T>. The category TrClasspath implements it and I can do that:

Prepare<Shift> prepare = new TrClasspath<>(new TrDefault<>())




Subsequent I made a decision to do away with the duplication of .with() calls. Clearly, it might be simpler to have the power to supply an inventory of file names as an array of String and construct the prepare from it. I created a brand new class TrBulk, which does precisely that:

Iterable<String> names = Arrays.asList(




Prepare<Shift> prepare = new TrBulk<>(

  new TrClasspath<>(

    new TrDefault<>()



With this design I can assemble the prepare in virtually any attainable manner.

See, for instance, how we use it right here and right here.




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