What are the main problems that can arise when using by lazy {} for initialization, and what does it mean?

by lazy in Kotlin means that the property value will be computed only upon first access, not when the object is created. This is useful for optimization when initialization is resource-intensive or not always needed.

Main issues:

• Thread safety: By default, by lazy is synchronized (LazyThreadSafetyMode.SYNCHRONIZED), which can cause overhead in multithreaded environments. Other modes include:

  • LazyThreadSafetyMode.PUBLICATION: Does not block the first access but may compute the value multiple times in multithreading, though the result will be the same.
  • LazyThreadSafetyMode.NONE: Does not guarantee thread safety. Dangerous to use in multithreaded scenarios without external synchronization.

• Initialization exceptions: If the initializing lambda throws an exception, it will occur on the first property access, which may be non-obvious and hard to catch. On subsequent accesses, the same exception will be thrown again.

• Testing complexity: Lazy initialization can complicate testing, as it must be ensured that the property is initialized at the right moment.

• Unclear initialization timing: For external code, it is not obvious when exactly initialization will occur. This can be problematic if initialization has side effects.

• Serialization: If a lazy property contains a non-serializable object, problems may arise when attempting to serialize an object containing this property.

• Limited access to context: Inside the by lazy lambda, only this object and external variables captured at creation are accessible.

Example of usage and choosing thread safety mode:

// Thread-safe by default (for most cases)
val heavyResource: HeavyResource by lazy {
    // Long initialization
    HeavyResource()
}

// For single-threaded scenarios or when external synchronization is assured
val uiComponent: UIComponent by lazy(LazyThreadSafetyMode.NONE) {
    UIComponent()
}

// When access from multiple threads is possible but exact initialization timing is not critical
val sharedData: List<String> by lazy(LazyThreadSafetyMode.PUBLICATION) {
    fetchDataFromNetwork()
}

class HeavyResource
class UIComponent
fun fetchDataFromNetwork(): List<String> = listOf("data")