Write custom AssertJ assertions

#java #testing monday, january 01, 2018

AssertJ is an alternative matching library to the widely used Hamcrest matchers. For my own projects I in fact have changed to solely use AssertJ — I just find the fluid interfaces and extensibility quite appealing.

You can write custom assertions as follows:

Imagine a coffee with a strength and a drink type, such as Espresso or Latte. A custom CoffeeAssert validates coffee instances based on their custom business logic — in this case their properties.

public class CoffeeAssert extends AbstractAssert<CoffeeAssert, Coffee> {

    public CoffeeAssert(Coffee actual) {
        super(actual, CoffeeAssert.class);
    }

    public static CoffeeAssert assertThat(Coffee actual) {
        return new CoffeeAssert(actual);
    }

    public CoffeeAssert hasType(Coffee.Type type) {
        isNotNull();

        if (actual.getType() != type) {
            failWithMessage("Expected the coffee type to be <%s> but was <%s>", type, actual.getType());
        }

        return this;
    }

    // hasStrength(Strength) omitted ...

    public CoffeeAssert isNotDecaf() {
        isNotNull();

        if (actual.getStrength() == Coffee.Strength.DECAF) {
            failWithMessage("Expected a coffee but got decaf!");
        }

        return this;
    }
}

Coffee instances can then simply be validated using the custom assertion. The static import of the assertThat has to refer to CoffeeAssert.

import static com.example.coffee.CoffeeAssert.assertThat;
...

Coffee coffee = new Coffee();
coffee.setStrength(Strength.STRONG);
coffee.setType(Type.ESPRESSO);

assertThat(coffee)
    .hasType(Type.ESPRESSO)
    .isNotDecaf();

The use of custom assertions can vastly improve the quality of your test code.

This post was reposted from my newsletter issue 012

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