Memory Management in Kotlin: How the Garbage Collector Works

Memory and Garbage Collector in Kotlin: How Not to Leak RAM and Lose Performance

Have you ever wondered where your Kotlin application's RAM disappears? The code seems perfectly written, coroutines are flying, but after an hour of work, the program starts to "lag" or crashes with OutOfMemoryError. Sound familiar? Welcome to the club of those who want to understand how Kotlin manages memory and why the garbage collector is not evil, but your best friend (if you know how to be friends with it).

In this article, we will dive into the topic of memory management in Kotlin and break down how the Kotlin garbage collector works on different platforms. You will learn what the stack and heap are, how to avoid leaks, and why WeakReference can save your project. Let's go!

What is Memory in Kotlin? Stack, Heap, and Their Friends

Kotlin is a multiplatform language. It compiles to JVM bytecode, native code (via Kotlin/Native), or JavaScript (Kotlin/JS). And on each platform, memory is structured a bit differently, but the basics are the same everywhere.

Let's recall the fundamentals. When your program starts, the operating system allocates a memory region for it. Inside this region, there are two main "areas":

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• Stack — local variables, primitives, and references to objects are stored here. It works like a stack of plates: fast, predictable, but with a limited size. Going beyond the stack limit results in StackOverflowError.
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• Heap — a dump for all objects you create via new or a constructor (val list = mutableListOf()). The heap is large but slow, and this is where the Kotlin garbage collector "lives".
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On the JVM, the stack and heap work classically. In Kotlin/Native, the stack is the native thread stack, and the heap is managed by its own garbage collector. In Kotlin/JS, everything falls on the shoulders of the JavaScript engine (V8, SpiderMonkey).

The key point: references to objects are stored on the stack, while the objects themselves are on the heap. As long as an object has at least one "live" reference (from the stack or another object), the garbage collector will not touch it.

Garbage Collector in Kotlin: How It Works on the JVM

When you write Kotlin for the JVM (Android, server), you get the power of the Java Virtual Machine Garbage Collector (GC). This is not specific to Kotlin, but understanding its mechanics is essential.

Modern JVM uses G1GC (Garbage First) or ZGC (in newer versions). Here's how it works in simple terms:

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1. Mark — GC "walks" through all live objects, starting from root points (stack, static fields). Everything it reaches is alive.
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2. Sweep — everything not marked is considered garbage and is removed.
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3. Compact — live objects are moved to prevent fragmentation.
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In G1GC, the heap is divided into regions. GC collects the regions with the most garbage (hence the name). This reduces pauses but does not eliminate them completely.

In Kotlin, you can influence the garbage collector through JVM flags (for servers) or via GarbageCollectorMXBean (for monitoring). But most often, you just need to write code that doesn't interfere with the GC's work.

Example: How GC Sees Your Code

fun processData() {\n    val list = mutableListOf<String>()\n    for (i in 1..100_000) {\n        list.add("Element $i")\n    }\n    // Here list is alive until the method finishes\n    println(list.size)\n    // After exiting the method, list becomes garbage\n}\n

After processData() completes, the reference to list disappears from the stack, and the object becomes a candidate for collection. Simple, right?

Kotlin/Native Features: Garbage Collector Without JVM

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