Okio – A modern I/O library for Android, Java, and Kotlin Multiplatform

Okio is a library that complements java.io and java.nio to make it much easier to access, store, and process your data. It started as a component of OkHttp, the capable HTTP client included in Android. It’s well-exercised and ready to solve new problems.

ByteStrings and Buffers

Okio is built around two types that pack a lot of capability into a straightforward API:

• ByteString is an immutable sequence of bytes. For character data, String is fundamental. ByteString is String’s long-lost brother, making it easy to treat binary data as a value. This class is ergonomic: it knows how to encode and decode itself as hex, base64, and UTF-8.
• Buffer is a mutable sequence of bytes. Like ArrayList, you don’t need to size your buffer in advance. You read and write buffers as a queue: write data to the end and read it from the front. There’s no obligation to manage positions, limits, or capacities.

Internally, ByteString and Buffer do some clever things to save CPU and memory. If you encode a UTF-8 string as a ByteString, it caches a reference to that string so that if you decode it later, there’s no work to do.

Buffer is implemented as a linked list of segments. When you move data from one buffer to another, it reassigns ownership of the segments rather than copying the data across. This approach is particularly helpful for multithreaded programs: a thread that talks to the network can exchange data with a worker thread without any copying or ceremony.

Sources and Sinks

An elegant part of the java.io design is how streams can be layered for transformations like encryption and compression. Okio includes its own stream types called Source and Sink that work like InputStream and OutputStream, but with some key differences:

• Timeouts. The streams provide access to the timeouts of the underlying I/O mechanism. Unlike the java.io socket streams, both read() and write() calls honor timeouts.
• Easy to implement. Source declares three methods: read(), close(), and timeout(). There are no hazards like available() or single-byte reads that cause correctness and performance surprises.
• Easy to use. Although implementations of Source and Sink have only three methods to write, callers are given a rich API with the BufferedSource and BufferedSink interfaces. These interfaces give you everything you need in one place.
• No artificial distinction between byte streams and char streams. It’s all data. Read and write it as bytes, UTF-8 strings, big-endian 32-bit integers, little-endian shorts; whatever you want. No more InputStreamReader!
• Easy to test. The Buffer class implements both BufferedSource and BufferedSink so your test code is simple and clear.

Sources and sinks interoperate with InputStream and OutputStream. You can view any Source as an InputStream, and you can view any InputStream as a Source. Similarly for Sink and OutputStream.

Presentations¶

A Few “Ok” Libraries (slides): An introduction to Okio and three libraries written with it.

Decoding the Secrets of Binary Data (slides): How data encoding works and how Okio does it.

Ok Multiplatform! (slides): How we changed Okio’s implementation language from Java to Kotlin.

Requirements¶

Okio 2.x supports Android 4.0.3+ (API level 15+) and Java 7+.

Okio 3.x supports Android 4.0.3+ (API level 15+) and Java 8+.

Okio depends on the Kotlin standard library. It is a small library with strong backward-compatibility.

Okio on GitHub: https://github.com/square/okio
Web-site: https://square.github.io/okio/
Platform: Android
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