kotlinx-io

A multiplatform Kotlin library providing basic IO primitives. kotlinx-io is based on Okio but does not preserve backward compatibility with it.

Overview

kotlinx-io is built around Buffer - a mutable sequence of bytes.

Buffer works like a queue, allowing to read data from its head or to write data to its tail. Buffer provides functions to read and write data of different built-in types, and to copy data to or from other Buffers. Depending on the target platform, extension functions allowing data exchange with platform-specific types are also available.

A Buffer consists of segments organized as a linked list: segments allow reducing memory allocations during the buffer's expansion and copy, with the latter achieved by delegating or sharing the ownership over the underlying buffer's segments with other buffers.

kotlinx-io provides interfaces representing data sources and destinations - Source and Sink, and in addition to the mutable Buffer the library also provides an immutable sequence of bytes - ByteString.

An experimental filesystem support is shipped under the kotlinx.io.files package, which includes the FileSystem interface and its default implementation - SystemFileSystem.

FileSystem provides basic operations for working with files and directories, which are represented by yet another class under the same package - Path.

There are several kotlinx-io modules:

• kotlinx-io-bytestring - provides ByteString.
• kotlinx-io-core - provides IO primitives (Buffer, Source, Sink), filesystems support, depends on kotlinx-io-bytestring.
• kotlinx-io-okio - bridges kotlinx-io and Okio ByteString, kotlinx.io.RawSource and okio.Source, kotlinx.io.RawSink and okio.Sink.

Using in your projects

Note that the library is experimental, and the API is subject to change.

Gradle

Make sure that you have mavenCentral() in the list of repositories:

repositories {
    mavenCentral()
}

Add the library to dependencies:

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-io-core:0.8.2")
}

In multiplatform projects, add a dependency to the commonMain source set dependencies:

kotlin {
    sourceSets {
        commonMain {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-io-core:0.8.2")
            }
        }
    }
}

Maven

Add the library to dependencies:

<dependency>
    <groupId>org.jetbrains.kotlinx</groupId>
    <artifactId>kotlinx-io-core-jvm</artifactId>
    <version>0.8.2</version>
</dependency>

JPMS support

On JVM, kotlinx-io supports Java Modules:

• kotlinx-io-bytestring library provides kotlinx.io.bytestring module;
• kotlinx-io-core library provides kotlinx.io.core module.
• kotlinx-io-okio library provides kotlinx.io.okio module.

Read this article for details on how to configure a Gradle project to utilize JPMS.

Android

kotlinx-io is not tested on Android on a regular basis, but the library is compatible with Android 5.0+ (API level 21+).

Segment pooling

As an optimization, on some platforms Buffer's segments are pooled, meaning that everytime a buffer needs a new segment, an attempt will be made to take a pre-allocated segment from a pool, and if the pool is empty, only then a new segment will be created. Everytime a segment is no longer needed, it will be placed back into a pool (unless the pool is already full).

Currently, the pooling is only supported on JVM (and Android).

The segment pool has a two-level structure, with a smaller first-level pool aimed to serve requests as quickly as possible, and a larger second-level pool. While the first-level pool sizing could not be configured at the moment, the size of the second-level pool could be adjusted using the kotlinx.io.pool.size.bytes system property. The property accepts numeric values only. On Android, the second-level pool size is 0 by default, for all other JVM environments it is 4194304 (4 megabytes) by default.

While the size could be changed using the system property, it is necessary to note that its value will be read during the pool initialization, and later changes of the property will not affect the pool size. In other words, the property has to be updated before any calls to the kotlinx-io API.

On JVM, the easiest way to achieve that is to supply the property value via command line JVM flag (-Pkotlinx.io.pool.size.bytes=XXX).

On Android, it is a bit trickier. One of the ways to set up the property before running any kotlinx-io-related code is by overriding Application.onCreate and setting the property there:

package org.example

import android.app.Application

class MySegmentHungryApp : Application() {
    override fun onCreate() {
        System.setProperty("kotlinx.io.pool.size.bytes", "2097152" /* or whatever value suites your case */)
        super.onCreate()
    }
}

Then, the application class needs to be explicitly registered in the manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <application 
       android:name="org.example.MySegmentHungryApp"
       ...
    >

    </application>
</manifest>

Contributing

Read the Contributing Guidelines.

Code of Conduct

This project and the corresponding community are governed by the JetBrains Open Source and Community Code of Conduct. Please make sure you read it.

License

kotlinx-io is licensed under the Apache 2.0 License.

Credits

Thanks to everyone involved in the project.

An honorable mention goes to the developers of Okio that served as the foundation for kotlinx-io and to Jesse Wilson, for the help with Okio adaption, his suggestions, assistance and guidance with kotlinx-io development.