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In-memory Cache for Kotlin Multiplatform.

Work in progress.

cache4k provides a simple in-memory key-value cache for Kotlin Multiplatform, with support for time-based (expiration) and size-based evictions.

Note that only the new Kotlin Native memory model is supported.

The following targets are currently supported:

  • jvm
  • js
  • wasmJs
  • iosX64
  • iosArm64
  • iosSimulatorArm64
  • macosX64
  • macosArm64
  • tvosX64
  • tvosArm64
  • tvosSimulatorArm64
  • watchosX64
  • watchosArm64
  • watchosSimulatorArm64
  • linuxX64
  • linuxArm64
  • mingwX64


Dependencies are hosted on Maven Central.


dependencies {


kotlin {
    sourceSets {
        commonMain {
            dependencies {


Writing and reading cache entries

To create a new Cache instance using Long for the key and String for the value:

val cache = Cache.Builder<Long, String>().build()

To start writing entries to the cache:

cache.put(1, "dog")
cache.put(2, "cat")

To read a cache entry by key:

cache.get(1) // returns "dog"
cache.get(2) // returns "cat"
cache.get(3) // returns null

To overwrite an existing cache entry:

cache.put(1, "dog")
cache.put(1, "bird")
cache.get(1) // returns "bird"

Cache loader

Cache provides an API for getting cached value by key and using the provided loader: suspend () -> Value lambda to compute and cache the value automatically if none exists.

runBlockingTest {
    val cache = Cache.Builder<Long, User>().build()

    val userId = 1L
    val user = cache.get(userId) {
        fetchUserById(userId) // potentially expensive call (might be a suspend function)

    // value successfully computed by the loader will be cached automatically

Note that loader is executed on the caller’s coroutine context. Concurrent calls from multiple threads using the same key will be blocked. Assuming the 1st call successfully computes a new value, none of the loader from the other calls will be executed and the cached value computed by the first loader will be returned for those calls.

Any exceptions thrown by the loader will be propagated to the caller of this function.

Expirations and evictions

By default, Cache has an unlimited number of entries which never expire. But a cache can be configured to support both time-based expirations and size-based evictions.

Time-based expiration

Expiration time can be specified for entries in the cache.

Expire after access

To set the maximum time an entry can live in the cache since the last access (also known as ** time-to-idle**), where “access” means reading the cache, adding a new cache entry, or ** replacing an existing entry with a new one**:

val cache = Cache.Builder<Long, String>()

An entry in this cache will be removed if it has not been read or replaced after 24 hours since it’s been written into the cache.

Expire after write

To set the maximum time an entry can live in the cache since the last write (also known as ** time-to-live**), where “write” means adding a new cache entry or replacing an existing entry with a new one:

val cache = Cache.Builder<Long, String>()

An entry in this cache will be removed if it has not been replaced after 30 minutes since it’s been written into the cache.

Note that cache entries are not removed immediately upon expiration at exact time. Expirations are checked in each interaction with the cache.

Size-based eviction

To set the maximum number of entries to be kept in the cache:

val cache = Cache.Builder<Long, String>()

Once there are more than 100 entries in this cache, the least recently used one will be removed, where “used” means reading the cache, adding a new cache entry, or replacing an existing entry with a new one.

Getting all cache entries as a Map

To get a copy of the current cache entries as a Map:

val cache = Cache.Builder<Long, String>()

cache.put(1, "dog")
cache.put(2, "cat")

    .isEqualTo(mapOf(1L to "dog", 2L to "cat"))

Note that calling asMap() has no effect on the access expiry of the cache.

Deleting cache entries

Cache entries can also be deleted explicitly.

To delete a cache entry for a given key:

val cache = Cache.Builder<Long, String>().build()
cache.put(1, "dog")



To delete all entries in the cache:


Event listener

You can set an event listener as a lambda:

val cache1 = Cache.Builder<Long, String>()
    .eventListener { event ->
        println("onEvent: $event")

Or declare it as a class and share logic across many stores:

class FileDeleteEventListener : CacheEventListener<Long, File> {
    override fun onEvent(event: CacheEvent<Long, File>) {
        when(event) {
            is CacheEvent.Created -> {}
            is CacheEvent.Updated -> event.oldValue.delete()
            is CacheEvent.Evicted -> event.value.delete()
            is CacheEvent.Expired -> event.value.delete()
            is CacheEvent.Removed -> event.value.delete()
val fileDeleteEventListener = FileDeleteEventListener()

val cache1 = Cache.Builder<Long, File>()

val cache2 = Cache.Builder<Long, File>()

Cache entry event firing behaviors for mutative methods:

Initial value Operation New value Event
{} put(K, V) {K: V} Created(K, V)
{K: V1} put(K, V2) {K: V2} Updated(K, V1, V2)
{K: V} invalidate(K) {} Removed(K, V)
{K1: V1, K2: V2} invalidateAll() {} Removed(K1, V1), Removed(K2, V2)
{K: V} any operation, K expired {} Expired(K, V)
{K1: V1} put(K2, V2), K1 evicted {K2: V2} Created(K2, V2), Evicted(K1, V1)

Unit testing cache expirations

To test logic that depends on cache expiration, pass in a FakeTimeSource when building a Cache so you can programmatically advance the reading of the time source:

fun cacheEntryEvictedAfterExpiration() {
    private val fakeTimeSource = FakeTimeSource()
    val cache = Cache.Builder<Long, String>()

    cache.put(1, "dog")

    // just before expiry
    fakeTimeSource += 1.minutes - 1.nanoseconds


    // now expires
    fakeTimeSource += 1.nanoseconds



The library was ported from a kotlin / JVM cache which I contributed to dropbox/Store to help unblock Store’s multiplatform support ( it was reverted before the 1.0 release as multiplatform wasn’t a priority). Many thanks to Store’s owners and contributors for reviewing and improving the original implementation.


Copyright 2021 Yang Chen

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.


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