JarCastingOkio
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,
Stringis fundamental.ByteStringis 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:
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Timeouts. The streams provide access to the timeouts of the underlying I/O mechanism. Unlike the
java.iosocket streams, bothread()andwrite()calls honor timeouts. -
Easy to implement.
Sourcedeclares three methods:read(),close(), andtimeout(). There are no hazards likeavailable()or single-byte reads that cause correctness and performance surprises. -
Easy to use. Although implementations of
SourceandSinkhave only three methods to write, callers are given a rich API with theBufferedSourceandBufferedSinkinterfaces. 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
Bufferclass implements bothBufferedSourceandBufferedSinkso 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.
Dependable
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 {
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;
}
pngSource.close();
}
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
Download the latest JAR or grab via Maven:
<dependency>
<groupId>com.squareup.okio</groupId>
<artifactId>okio</artifactId>
<version>1.6.0</version>
</dependency>
or Gradle:
compile 'com.squareup.okio:okio:1.6.0'
Snapshots of the development version are available in Sonatype's snapshots repository.
