Gotcha: PicoContainer requires Zero-Argument Constructors

At the time of writing, Cucumber (7.1.0) still recommends using PicoContainer as the default dependency injection framework.

Although this works for a lot of cases that folks are building for, there is unfortunately a big gotcha that I've hit a few times, and have since avoided using PicoContainer due to this problem.

Let us say that we're following a common practice of using constructor injection for objects, such as the below code snippet:

package io.cucumber.skeleton;

import io.cucumber.java.en.Given;

public class StepDefinitions {

    private final Belly belly;

    public StepDefinitions(Belly belly) {
        this.belly = belly;
    }

    @Given("I have {int} cukes in my belly")
    public void I_have_cukes_in_my_belly(int cukes) {
        belly.eat(cukes);
    }
}

And the following Belly class definition:

package io.cucumber.skeleton;

public class Belly {
    public void eat(int cukes) {

    }
}

(Example code can be found on a branch on GitLab)

This works absolutely fine, as our Belly class can be instantiated with the implicit zero-args constructor that Java provides.

However, if we make our Belly class require some configuration via the constructor, such as:

 package io.cucumber.skeleton;

 public class Belly {
+
+    public Belly(int initial) {
+        // TODO: something with initial
+    }
+
     public void eat(int cukes) {

     }
 }

We notice that running this leads to the following error when executing the tests:

RunCucumberTest > Cucumber > Belly > io.cucumber.skeleton.RunCucumberTest.Belly - a few cukes STANDARD_OUT

    Scenario: a few cukes               # io/cucumber/skeleton/belly.feature:3
      Given I have 42 cukes in my belly # io.cucumber.skeleton.StepDefinitions.I_have_cukes_in_my_belly(int)
          org.picocontainer.injectors.AbstractInjector$UnsatisfiableDependenciesException: io.cucumber.skeleton.Belly has
            unsatisfied dependency 'class java.lang.Integer' for constructor
            'public io.cucumber.skeleton.Belly(int)' from org.picocontainer.DefaultPicoContainer@2f05be7f:2<|

Unfortunately, PicoContainer's reliance on a zero-args constructor has been broken, which means we can no longer use this class.

It may not be so much of a problem if we've written all the code we're injecting, so can add convenience zero-args constructors with default implementations, but it's very unlikely to be the case that we're not using any dependencies.

Yes, we could create i.e. a Config object that provides a container for the dependencies, but this then results in not-great dependency injection, and means we're sidestepping good dependency injection for the case of our library.

A common solution to this is to create default wrapper objects for each of our classes, i.e. a BellyWrapper which produces a zero-args contructor.

If you hit this, I recommend looking at the other options, such as those called out in the Cucumber docs for Sharing state between steps, or my article on using Using Dagger for Dependency Injection with Cucumber Tests.

See also discussion on the Cucumber issue tracker.