A modern I/O API for Java
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Latest commit 5049e1c Feb 15, 2017 @JakeWharton JakeWharton committed on GitHub Merge pull request #287 from square/jw/2017-02-13/error-prone
Add error-prone compiler.



Okio is a new library that complements java.io and java.nio to make it much easier to access, store, and process your data.

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.


Okio started as a component of OkHttp, the capable HTTP+SPDY client included in Android. It's well-exercised and ready to solve new problems.

Example: a PNG decoder

Decoding the chunks of a PNG file demonstrates Okio in practice.

private static final ByteString PNG_HEADER = ByteString.decodeHex("89504e470d0a1a0a");

public void decodePng(InputStream in) throws IOException {
  try (BufferedSource pngSource = Okio.buffer(Okio.source(in))) {
    ByteString header = pngSource.readByteString(PNG_HEADER.size());
    if (!header.equals(PNG_HEADER)) {
      throw new IOException("Not a PNG.");

    while (true) {
      Buffer chunk = new Buffer();

      // Each chunk is a length, type, data, and CRC offset.
      int length = pngSource.readInt();
      String type = pngSource.readUtf8(4);
      pngSource.readFully(chunk, length);
      int crc = pngSource.readInt();

      decodeChunk(type, chunk);
      if (type.equals("IEND")) break;

private void decodeChunk(String type, Buffer chunk) {
  if (type.equals("IHDR")) {
    int width = chunk.readInt();
    int height = chunk.readInt();
    System.out.printf("%08x: %s %d x %d%n", chunk.size(), type, width, height);
  } else {
    System.out.printf("%08x: %s%n", chunk.size(), type);


Download the latest JAR or grab via Maven:


or Gradle:

compile 'com.squareup.okio:okio:1.11.0'

Snapshots of the development version are available in Sonatype's snapshots repository.


If you are using ProGuard you might need to add the following option:

-dontwarn okio.**


Copyright 2013 Square, Inc.

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
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