Kotlin / Java Memory Leaks
Problem: Memory leaks when using Kotlin and Java
When using Kotlin and Java, memory leaks can occur in combination with Java Lambdas. Example: a listener is created in Kotlin, which is then registered with a Java class and allegedly unregistered again:
public class Calculator {
interface Listener {
void onResult(int result);
}
Set<Listener> listeners = new HashSet<>();
void addListener(Listener listener) {
listeners.add(listener);
}
void removeListener(Listener listener) {
listeners.remove(listener);
}
...
}fun main() {
val c = Calculator()
val listener = { sum: Int ->
println("sum: $sum")
}
c.addListener(listener)
c.removeListener(listener)
c.sum(10, 20)
}When running the Kotlin main-function you would not expect any output, but calling c.removeListener does not work as expected. To see the cause in the generated Java code, you can use Android Studio – click on "Tools → Kotlin → Show Kotlin Bytecode" followed by a click on "Decompile":
Object var10001 = listener;
if (listener != null) {
var10001 = new MainKt$sam$test_Calculator_Listener$0(listener);
}
c.addListener((Listener)var10001);
var10001 = listener;
if (listener != null) {
var10001 = new MainKt$sam$test_Calculator_Listener$0(listener);
}
c.removeListener((Listener)var10001);listener is wrapped in MainKt$sam$test_Calculator_Listener$0(listener) each time before it is passed to addListener or removeListener, which then are two different Java objects.
Solution: Modifying the declaration of listener
The declaration of listener can slightly be modified by declaring the type to avoid the need for a generated wrapper:
val listener = Calculator.Listener {
sum: Int -> println("sum: $sum")
}
c.addListener(listener)
c.removeListener(listener)Listener listener = (Listener)null.INSTANCE;
c.addListener(listener);
c.removeListener(listener);
Further Aspects
- Kotlin Under the Hood: https://www.youtube.com/watch?v=Ta5wBJsC39s
- Early detection of memory leaks in Android apps with the LeakCanary Library
---
Author: Marco Pfattner / Senior Software Architect/ Business Division Automotive Bavaria
Download Toilet Paper #119: Kotlin/Java Memory Leaks (pdf)
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