Kotlin Code Smell 23 - Singletons

Problem

• Coupling

• Testability

• Accidental implementation problems

• Multi-threading issues

• Static methods polluting

• Object creation contract violation

• Bijection mismatch

• Memory issues

• Premature Optimization

Solution

1. Avoid singletons altogether.

2. Use contextual unique objects instead of relying on a global instance.

3. Benchmark object creation to ensure performance is not adversely affected.

Examples

• Database Access

• Globals

• Loggers

• Helper classes

Sample Code

Wrong

// God is the archetypal singleton example
class God {
    // In Kotlin, the companion object is always a singleton
    companion object {
        private var instance: God? = null

        fun getInstance(): God {
            if (instance == null) {
                instance = God()
            }

            return requireNotNull(instance)
        }
    }
}

// Why should we be aware of getInstance when creating an object?
val jewishGod = God.getInstance()

Right

interface Religion {
    // Define behavior
}

class God {
    // There can be as many as you wish
}

class PolytheisticReligion(private val gods: Collection<God>) : Religion
class MonotheisticReligion(private val godAllMighty: God) : Religion

// According to Judaism, there's only one God,
// but this does not hold in other religions.
// Under this context, God is unique. We cannot create or change
// a new one.
val jewishGod = God()
val judaism = MonotheisticReligion(jewishGod)

val jupiter = God()
val saturn = God()
val mythologicalReligion = PolytheisticReligion(listOf(jupiter, saturn))

// Gods are unique (or not) according to context.
// We can create test religions with or without unicity.
// This is less coupled since we break the direct reference to the
// God class.
// The God class has the single responsibility to create gods, not
// to manage them.

Conclusion

The use of singletons is a historical mistake that has already been acknowledged by the community. Nevertheless, lazy developers bring it up again and again. We need to reach a consensus on its drawbacks and strive for better design patterns.

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Credits

• Code Smell 32 - Singletons by Maxi Contieri