okio

by square

square / okio

A modern I/O library for Android, Kotlin, and Java.

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Okio

See the project website for documentation and APIs.

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.

Recipes

We've written some recipes that demonstrate how to solve common problems with Okio. Read through them to learn about how everything works together. Cut-and-paste these examples freely; that's what they're for.

Read a text file line-by-line (Java/Kotlin)

Use

Okio.source(File)
to open a source stream to read a file. The returned
Source
interface is very small and has limited uses. Instead we wrap the source with a buffer. This has two benefits:
  • It makes the API more powerful. Instead of the basic methods offered by

    Source
    ,
    BufferedSource
    has dozens of methods to address most common problems concisely.
  • It makes your program run faster. Buffering allows Okio to get more done with fewer I/O operations.

Each

Source
that is opened needs to be closed. The code that opens the stream is responsible for making sure it is closed.

=== "Java"

Here we use Java's `try` blocks to close our sources automatically.

public void readLines(File file) throws IOException {
  try (Source fileSource = Okio.source(file);
       BufferedSource bufferedSource = Okio.buffer(fileSource)) {

    while (true) {
      String line = bufferedSource.readUtf8Line();
      if (line == null) break;

      if (line.contains("square")) {
        System.out.println(line);
      }
    }

  }
}

=== "Kotlin"

Note that static `Okio` methods become extension functions (`Okio.source(file)` => 
`file.source()`), and `use` is used to automatically close the streams:

@Throws(IOException::class)
fun readLines(file: File) {
  file.source().use { fileSource ->
    fileSource.buffer().use { bufferedFileSource ->
      while (true) {
        val line = bufferedFileSource.readUtf8Line() ?: break
        if ("square" in line) {
          println(line)
        }
      }
    }
  }
}

The

readUtf8Line()
API reads all of the data until the next line delimiter – either
\n
,
\r\n
, or the end of the file. It returns that data as a string, omitting the delimiter at the end. When it encounters empty lines the method will return an empty string. If there isn’t any more data to read it will return null.

The above program can be written more compactly by inlining the

fileSource
variable and by using a fancy
for
loop instead of a
while
:
public void readLines(File file) throws IOException {
  try (BufferedSource source = Okio.buffer(Okio.source(file))) {
    for (String line; (line = source.readUtf8Line()) != null; ) {
      if (line.contains("square")) {
        System.out.println(line);
      }
    }
  }
}

In Kotlin, we can wrap invocations of

source.readUtf8Line()
into the
generateSequence
builder to create a sequence of lines that will end once null is returned. Plus, transforming streams is easy thanks to the extension functions:
@Throws(IOException::class)
fun readLines(file: File) {
  file.source().buffer().use { source ->
    generateSequence { source.readUtf8Line() }
      .filter { line -> "square" in line }
      .forEach(::println)
  }
}

The

readUtf8Line()
method is suitable for parsing most files. For certain use-cases you may also consider
readUtf8LineStrict()
. It is similar but it requires that each line is terminated by
\n
or
\r\n
. If it encounters the end of the file before that it will throw an
EOFException
. The strict variant also permits a byte limit to defend against malformed input.
public void readLines(File file) throws IOException {
  try (BufferedSource source = Okio.buffer(Okio.source(file))) {
    while (!source.exhausted()) {
      String line = source.readUtf8LineStrict(1024L);
      if (line.contains("square")) {
        System.out.println(line);
      }
    }
  }
}

Here's a similar example written in Kotlin:

@Throws(IOException::class)
fun readLines(file: File) {
  file.source().buffer().use { source ->
    while (!source.exhausted()) {
      val line = source.readUtf8LineStrict(1024)
      if ("square" in line) {
        println(line)
      }
    }
  }
}

Write a text file (Java/Kotlin)

Above we used a

Source
and a
BufferedSource
to read a file. To write, we use a
Sink
and a
BufferedSink
. The advantages of buffering are the same: a more capable API and better performance.
public void writeEnv(File file) throws IOException {
  try (Sink fileSink = Okio.sink(file);
       BufferedSink bufferedSink = Okio.buffer(fileSink)) {

for (Map.Entry<string string> entry : System.getenv().entrySet()) {
  bufferedSink.writeUtf8(entry.getKey());
  bufferedSink.writeUtf8("=");
  bufferedSink.writeUtf8(entry.getValue());
  bufferedSink.writeUtf8("\n");
}

} }

There isn’t an API to write a line of input; instead we manually insert our own newline character. Most programs should hardcode

"\n"
as the newline character. In rare situations you may use
System.lineSeparator()
instead of
"\n"
: it returns
"\r\n"
on Windows and
"\n"
everywhere else.

We can write the above program more compactly by inlining the

fileSink
variable and by taking advantage of method chaining:

=== "Java"

```Java
public void writeEnv(File file) throws IOException {
  try (BufferedSink sink = Okio.buffer(Okio.sink(file))) {
    for (Map.Entry entry : System.getenv().entrySet()) {
      sink.writeUtf8(entry.getKey())
          .writeUtf8("=")
          .writeUtf8(entry.getValue())
          .writeUtf8("\n");
    }
  }
}
```

=== "Kotlin"

```Kotlin
@Throws(IOException::class)
fun writeEnv(file: File) {
  file.sink().buffer().use { sink ->
    for ((key, value) in System.getenv()) {
      sink.writeUtf8(key)
      sink.writeUtf8("=")
      sink.writeUtf8(value)
      sink.writeUtf8("\n")
    }
  }
}
```

In the above code we make four calls to

writeUtf8()
. Making four calls is more efficient than the code below because the VM doesn’t have to create and garbage collect a temporary string.
sink.writeUtf8(entry.getKey() + "=" + entry.getValue() + "\n"); // Slower!

UTF-8 (Java/Kotlin)

In the above APIs you can see that Okio really likes UTF-8. Early computer systems suffered many incompatible character encodings: ISO-8859-1, ShiftJIS, ASCII, EBCDIC, etc. Writing software to support multiple character sets was awful and we didn’t even have emoji! Today we're lucky that the world has standardized on UTF-8 everywhere, with some rare uses of other charsets in legacy systems.

If you need another character set,

readString()
and
writeString()
are there for you. These methods require that you specify a character set. Otherwise you may accidentally create data that is only readable by the local computer. Most programs should use the UTF-8 methods only.

When encoding strings you need to be mindful of the different ways that strings are represented and encoded. When a glyph has an accent or another adornment it may be represented as a single complex code point (

é
) or as a simple code point (
e
) followed by its modifiers (
´
). When the entire glyph is a single code point that’s called NFC; when it’s multiple it’s NFD.

Though we use UTF-8 whenever we read or write strings in I/O, when they are in memory Java Strings use an obsolete character encoding called UTF-16. It is a bad encoding because it uses a 16-bit

char
for most characters, but some don’t fit. In particular, most emoji use two Java chars. This is problematic because
String.length()
returns a surprising result: the number of UTF-16 chars and not the natural number of glyphs.

| | Café 🍩 | Café 🍩 | | --------------------: | :---------------------------| :------------------------------| | Form | NFC | NFD | | Code Points |

c  a  f  é    ␣   🍩     
|
c  a  f  e  ´    ␣   🍩     
| | UTF-8 bytes |
43 61 66 c3a9 20 f09f8da9
|
43 61 66 65 cc81 20 f09f8da9
| | String.codePointCount | 6 | 7 | | String.length | 7 | 8 | | Utf8.size | 10 | 11 |

For the most part Okio lets you ignore these problems and focus on your data. But when you need them, there are convenient APIs for dealing with low-level UTF-8 strings.

Use

Utf8.size()
to count the number of bytes required to encode a string as UTF-8 without actually encoding it. This is handy in length-prefixed encodings like protocol buffers.

Use

BufferedSource.readUtf8CodePoint()
to read a single variable-length code point, and
BufferedSink.writeUtf8CodePoint()
to write one.

Golden Values (Java/Kotlin)

Okio likes testing. The library itself is heavily tested, and it has features that are often helpful when testing application code. One pattern we’ve found to be quite useful is “golden value” testing. The goal of such tests is to confirm that data encoded with earlier versions of a program can safely be decoded by the current program.

We’ll illustrate this by encoding a value using Java Serialization. Though we must disclaim that Java Serialization is an awful encoding system and most programs should prefer other formats like JSON or protobuf! In any case, here’s a method that takes an object, serializes it, and returns the result as a

ByteString
:

=== "Java"

```Java
private ByteString serialize(Object o) throws IOException {
  Buffer buffer = new Buffer();
  try (ObjectOutputStream objectOut = new ObjectOutputStream(buffer.outputStream())) {
    objectOut.writeObject(o);
  }
  return buffer.readByteString();
}
```

=== "Kotlin"

```Kotlin
@Throws(IOException::class)
private fun serialize(o: Any?): ByteString {
  val buffer = Buffer()
  ObjectOutputStream(buffer.outputStream()).use { objectOut ->
    objectOut.writeObject(o)
  }
  return buffer.readByteString()
}
```

There’s a lot going on here.

  1. We create a buffer as a holding space for our serialized data. It’s a convenient replacement for

    ByteArrayOutputStream
    .
  2. We ask the buffer for its output stream. Writes to a buffer or its output stream always append data to the end of the buffer.

  3. We create an

    ObjectOutputStream
    (the encoding API for Java serialization) and write our object. The try block takes care of closing the stream for us. Note that closing a buffer has no effect.
  4. Finally we read a byte string from the buffer. The

    readByteString()
    method allows us to specify how many bytes to read; here we don’t specify a count in order to read the entire thing. Reads from a buffer always consume data from the front of the buffer.

With our

serialize()
method handy we are ready to compute and print a golden value.

=== "Java"

```Java
Point point = new Point(8.0, 15.0);
ByteString pointBytes = serialize(point);
System.out.println(pointBytes.base64());
```

=== "Kotlin"

```Kotlin
val point = Point(8.0, 15.0)
val pointBytes = serialize(point)
println(pointBytes.base64())
```

We print the

ByteString
as base64 because it’s a compact format that’s suitable for embedding in a test case. The program prints this:
rO0ABXNyAB5va2lvLnNhbXBsZXMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA

That’s our golden value! We can embed it in our test case using base64 again to convert it back into a

ByteString
:

=== "Java"

```Java
ByteString goldenBytes = ByteString.decodeBase64("rO0ABXNyAB5va2lvLnNhbXBsZ"
    + "XMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuA"
    + "AAAAAAA");
```

=== "Kotlin"

```Kotlin
val goldenBytes = ("rO0ABXNyACRva2lvLnNhbXBsZXMuS290bGluR29sZGVuVmFsdWUkUG9pbnRF9yaY7cJ9EwIAA" +
  "kQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA").decodeBase64()
```

The next step is to deserialize the

ByteString
back into our value class. This method reverses the
serialize()
method above: we append a byte string to a buffer then consume it using an
ObjectInputStream
:

=== "Java"

```Java
private Object deserialize(ByteString byteString) throws IOException, ClassNotFoundException {
  Buffer buffer = new Buffer();
  buffer.write(byteString);
  try (ObjectInputStream objectIn = new ObjectInputStream(buffer.inputStream())) {
    return objectIn.readObject();
  }
}
```

=== "Kotlin"

```Kotlin
@Throws(IOException::class, ClassNotFoundException::class)
private fun deserialize(byteString: ByteString): Any? {
  val buffer = Buffer()
  buffer.write(byteString)
  ObjectInputStream(buffer.inputStream()).use { objectIn ->
    return objectIn.readObject()
  }
}
```

Now we can test the decoder against the golden value:

=== "Java"

```Java
ByteString goldenBytes = ByteString.decodeBase64("rO0ABXNyAB5va2lvLnNhbXBsZ"
    + "XMuR29sZGVuVmFsdWUkUG9pbnTdUW8rMji1IwIAAkQAAXhEAAF5eHBAIAAAAAAAAEAuA"
    + "AAAAAAA");
Point decoded = (Point) deserialize(goldenBytes);
assertEquals(new Point(8.0, 15.0), decoded);
```

=== "Kotlin"

```Kotlin
val goldenBytes = ("rO0ABXNyACRva2lvLnNhbXBsZXMuS290bGluR29sZGVuVmFsdWUkUG9pbnRF9yaY7cJ9EwIAA" +
  "kQAAXhEAAF5eHBAIAAAAAAAAEAuAAAAAAAA").decodeBase64()!!
val decoded = deserialize(goldenBytes) as Point
assertEquals(point, decoded)
```

With this test we can change the serialization of the

Point
class without breaking compatibility.

Write a binary file (Java/Kotlin)

Encoding a binary file is not unlike encoding a text file. Okio uses the same

BufferedSink
and
BufferedSource
bytes for both. This is handy for binary formats that include both byte and character data.

Writing binary data is more hazardous than text because if you make a mistake it is often quite difficult to diagnose. Avoid such mistakes by being careful around these traps:

  • The width of each field. This is the number of bytes used. Okio doesn't include a mechanism to emit partial bytes. If you need that, you’ll need to do your own bit shifting and masking before writing.

  • The endianness of each field. All fields that have more than one byte have endianness: whether the bytes are ordered most-significant to least (big endian) or least-significant to most (little endian). Okio uses the

    Le
    suffix for little-endian methods; methods without a suffix are big-endian.
  • Signed vs. Unsigned. Java doesn’t have unsigned primitive types (except for

    char
    !) so coping with this is often something that happens at the application layer. To make this a little easier Okio accepts
    int
    types for
    writeByte()
    and
    writeShort()
    . You can pass an “unsigned” byte like 255 and Okio will do the right thing.

| Method | Width | Endianness | Value | Encoded Value | | :----------- | ----: | :--------- | --------------: | :------------------------ | | writeByte | 1 | | 3 |

03
| | writeShort | 2 | big | 3 |
00 03
| | writeInt | 4 | big | 3 |
00 00 00 03
| | writeLong | 8 | big | 3 |
00 00 00 00 00 00 00 03
| | writeShortLe | 2 | little | 3 |
03 00
| | writeIntLe | 4 | little | 3 |
03 00 00 00
| | writeLongLe | 8 | little | 3 |
03 00 00 00 00 00 00 00
| | writeByte | 1 | | Byte.MAXVALUE |
7f
| | writeShort | 2 | big | Short.MAX
VALUE |
7f ff
| | writeInt | 4 | big | Int.MAXVALUE |
7f ff ff ff
| | writeLong | 8 | big | Long.MAX
VALUE |
7f ff ff ff ff ff ff ff
| | writeShortLe | 2 | little | Short.MAXVALUE |
ff 7f
| | writeIntLe | 4 | little | Int.MAX
VALUE |
ff ff ff 7f
| | writeLongLe | 8 | little | Long.MAX_VALUE |
ff ff ff ff ff ff ff 7f
|

This code encodes a bitmap following the BMP file format.

=== "Java"

```Java
void encode(Bitmap bitmap, BufferedSink sink) throws IOException {
  int height = bitmap.height();
  int width = bitmap.width();

int bytesPerPixel = 3; int rowByteCountWithoutPadding = (bytesPerPixel * width); int rowByteCount = ((rowByteCountWithoutPadding + 3) / 4) * 4; int pixelDataSize = rowByteCount * height; int bmpHeaderSize = 14; int dibHeaderSize = 40;

// BMP Header sink.writeUtf8("BM"); // ID. sink.writeIntLe(bmpHeaderSize + dibHeaderSize + pixelDataSize); // File size. sink.writeShortLe(0); // Unused. sink.writeShortLe(0); // Unused. sink.writeIntLe(bmpHeaderSize + dibHeaderSize); // Offset of pixel data.

// DIB Header sink.writeIntLe(dibHeaderSize); sink.writeIntLe(width); sink.writeIntLe(height); sink.writeShortLe(1); // Color plane count. sink.writeShortLe(bytesPerPixel * Byte.SIZE); sink.writeIntLe(0); // No compression. sink.writeIntLe(16); // Size of bitmap data including padding. sink.writeIntLe(2835); // Horizontal print resolution in pixels/meter. (72 dpi). sink.writeIntLe(2835); // Vertical print resolution in pixels/meter. (72 dpi). sink.writeIntLe(0); // Palette color count. sink.writeIntLe(0); // 0 important colors.

// Pixel data. for (int y = height - 1; y >= 0; y--) { for (int x = 0; x < width; x++) { sink.writeByte(bitmap.blue(x, y)); sink.writeByte(bitmap.green(x, y)); sink.writeByte(bitmap.red(x, y)); }

// Padding for 4-byte alignment.
for (int p = rowByteCountWithoutPadding; p &lt; rowByteCount; p++) {
  sink.writeByte(0);
}

} }

</pre>
<p>=== "Kotlin"</p>
<pre>```Kotlin
@Throws(IOException::class)
fun encode(bitmap: Bitmap, sink: BufferedSink) {
  val height = bitmap.height
  val width = bitmap.width
  val bytesPerPixel = 3
  val rowByteCountWithoutPadding = bytesPerPixel * width
  val rowByteCount = (rowByteCountWithoutPadding + 3) / 4 * 4
  val pixelDataSize = rowByteCount * height
  val bmpHeaderSize = 14
  val dibHeaderSize = 40

  // BMP Header
  sink.writeUtf8("BM") // ID.
  sink.writeIntLe(bmpHeaderSize + dibHeaderSize + pixelDataSize) // File size.
  sink.writeShortLe(0) // Unused.
  sink.writeShortLe(0) // Unused.
  sink.writeIntLe(bmpHeaderSize + dibHeaderSize) // Offset of pixel data.

  // DIB Header
  sink.writeIntLe(dibHeaderSize)
  sink.writeIntLe(width)
  sink.writeIntLe(height)
  sink.writeShortLe(1) // Color plane count.
  sink.writeShortLe(bytesPerPixel * Byte.SIZE_BITS)
  sink.writeIntLe(0) // No compression.
  sink.writeIntLe(16) // Size of bitmap data including padding.
  sink.writeIntLe(2835) // Horizontal print resolution in pixels/meter. (72 dpi).
  sink.writeIntLe(2835) // Vertical print resolution in pixels/meter. (72 dpi).
  sink.writeIntLe(0) // Palette color count.
  sink.writeIntLe(0) // 0 important colors.

  // Pixel data.
  for (y in height - 1 downTo 0) {
    for (x in 0 until width) {
      sink.writeByte(bitmap.blue(x, y))
      sink.writeByte(bitmap.green(x, y))
      sink.writeByte(bitmap.red(x, y))
    }

    // Padding for 4-byte alignment.
    for (p in rowByteCountWithoutPadding until rowByteCount) {
      sink.writeByte(0)
    }
  }
}

The trickiest part of this program is the format’s required padding. The BMP format expects each row to begin on a 4-byte boundary so it is necessary to add zeros to maintain the alignment.

Encoding other binary formats is usually quite similar. Some tips:

  • Write tests with golden values! Confirming that your program emits the expected result can make debugging easier.
  • Use
    Utf8.size()
    to compute the number of bytes of an encoded string. This is essential for length-prefixed formats.
  • Use
    Float.floatToIntBits()
    and
    Double.doubleToLongBits()
    to encode floating point values.

Communicate on a Socket (Java/Kotlin)

Sending and receiving data over the network is a bit like writing and reading files. We use

BufferedSink
to encode output and
BufferedSource
to decode input. Like files, network protocols can be text, binary, or a mix of both. But there are also some substantial differences between the network and the filesystem.

With a file you’re either reading or writing but with the network you can do both! Some protocols handle this by taking turns: write a request, read a response, repeat. You can implement this kind of protocol with a single thread. In other protocols you may read and write simultaneously. Typically you’ll want one dedicated thread for reading. For writing you can use either a dedicated thread or use

synchronized
so that multiple threads can share a sink. Okio’s streams are not safe for concurrent use.

Sinks buffer outbound data to minimize I/O operations. This is efficient but it means you must manually call

flush()
to transmit data. Typically message-oriented protocols flush after each message. Note that Okio will automatically flush when the buffered data exceeds some threshold. This is intended to save memory and you shouldn’t rely on it for interactive protocols.

Okio builds on

java.io.Socket
for connectivity. Create your socket as a server or as a client, then use
Okio.source(Socket)
to read and
Okio.sink(Socket)
to write. These APIs also work with
SSLSocket
. You should use SSL unless you have a very good reason not to!

Cancel a socket from any thread by calling

Socket.close()
; this will cause its sources and sinks to immediately fail with an
IOException
. You can also configure timeouts for all socket operations. You don’t need a reference to the socket to adjust timeouts:
Source
and
Sink
expose timeouts directly. This API works even if the streams are decorated.

As a complete example of networking with Okio we wrote a basic SOCKS proxy server. Some highlights:

=== "Java"

```Java
Socket fromSocket = ...
BufferedSource fromSource = Okio.buffer(Okio.source(fromSocket));
BufferedSink fromSink = Okio.buffer(Okio.sink(fromSocket));
```

=== "Kotlin"

```Kotlin
val fromSocket: Socket = ...
val fromSource = fromSocket.source().buffer()
val fromSink = fromSocket.sink().buffer()
```

Creating sources and sinks for sockets is the same as creating them for files. Once you create a

Source
or
Sink
for a socket you must not use its
InputStream
or
OutputStream
, respectively.

=== "Java"

```Java
Buffer buffer = new Buffer();
for (long byteCount; (byteCount = source.read(buffer, 8192L)) != -1; ) {
  sink.write(buffer, byteCount);
  sink.flush();
}
```

=== "Kotlin"

```Kotlin
val buffer = Buffer()
var byteCount: Long
while (source.read(buffer, 8192L).also { byteCount = it } != -1L) {
  sink.write(buffer, byteCount)
  sink.flush()
}
```

The above loop copies data from the source to the sink, flushing after each read. If we didn’t need the flushing we could replace this loop with a single call to

BufferedSink.writeAll(Source)
.

The

8192
argument to
read()
is the maximum number of bytes to read before returning. We could have passed any value here, but we like 8 KiB because that’s the largest value Okio can do in a single system call. Most of the time application code doesn’t need to deal with such limits!

=== "Java"

```Java
int addressType = fromSource.readByte() & 0xff;
int port = fromSource.readShort() & 0xffff;
```

=== "Kotlin"

```Kotlin
val addressType = fromSource.readByte().toInt() and 0xff
val port = fromSource.readShort().toInt() and 0xffff
```

Okio uses signed types like

byte
and
short
, but often protocols want unsigned values. The bitwise
&
operator is Java’s preferred idiom to convert a signed value into an unsigned value. Here’s a cheat sheet for bytes, shorts, and ints:

| Type | Signed Range | Unsigned Range | Signed to Unsigned | | :---- | :---------------------------: | :--------------- | :-------------------------- | | byte | -128..127 | 0..255 |

int u = s & 0xff;
| | short | -32,768..32,767 | 0..65,535 |
int u = s & 0xffff;
| | int | -2,147,483,648..2,147,483,647 | 0..4,294,967,295 |
long u = s & 0xffffffffL;
|

Java has no primitive type that can represent unsigned longs.

Hashing (Java/Kotlin)

We’re bombarded by hashing in our lives as Java programmers. Early on we're introduced to the

hashCode()
method, something we know we need to override otherwise unforeseen bad things happen. Later we’re shown
LinkedHashMap
and its friends. These build on that
hashCode()
method to organize data for fast retrieval.

Elsewhere we have cryptographic hash functions. These get used all over the place. HTTPS certificates, Git commits, BitTorrent integrity checking, and Blockchain blocks all use cryptographic hashes. Good use of hashes can improve the performance, privacy, security, and simplicity of an application.

Each cryptographic hash function accepts a variable-length stream of input bytes and produces a fixed-length byte string value called the “hash”. Hash functions have these important qualities:

  • Deterministic: each input always produces the same output.
  • Uniform: each output byte string is equally likely. It is very difficult to find or create pairs of different inputs that yield the same output. This is called a “collision”.
  • Non-reversible: knowing an output doesn't help you to find the input. Note that if you know some possible inputs you can hash them to see if their hashes match.
  • Well-known: the hash is implemented everywhere and rigorously understood.

Good hash functions are very cheap to compute (dozens of microseconds) and expensive to reverse (quintillions of millenia). Steady advances in computing and mathematics have caused once-great hash functions to become inexpensive to reverse. When choosing a hash function, beware that not all are created equal! Okio supports these well-known cryptographic hash functions:

  • MD5: a 128-bit (16 byte) cryptographic hash. It is both insecure and obsolete because it is inexpensive to reverse! This hash is offered because it is popular and convenient for use in legacy systems that are not security-sensitive.
  • SHA-1: a 160-bit (20 byte) cryptographic hash. It was recently demonstrated that it is feasible to create SHA-1 collisions. Consider upgrading from SHA-1 to SHA-256.
  • SHA-256: a 256-bit (32 byte) cryptographic hash. SHA-256 is widely understood and expensive to reverse. This is the hash most systems should use.
  • SHA-512: a 512-bit (64 byte) cryptographic hash. It is expensive to reverse.

Each hash creates a

ByteString
of the specified length. Use
hex()
to get the conventional human-readable form. Or leave it as a
ByteString
because that’s a convenient model type!

Okio can produce cryptographic hashes from byte strings:

=== "Java"

```Java
ByteString byteString = readByteString(new File("README.md"));
System.out.println("   md5: " + byteString.md5().hex());
System.out.println("  sha1: " + byteString.sha1().hex());
System.out.println("sha256: " + byteString.sha256().hex());
System.out.println("sha512: " + byteString.sha512().hex());
```

=== "Kotlin"

```Kotlin
val byteString = readByteString(File("README.md"))
println("       md5: " + byteString.md5().hex())
println("      sha1: " + byteString.sha1().hex())
println("    sha256: " + byteString.sha256().hex())
println("    sha512: " + byteString.sha512().hex())
```

From buffers:

=== "Java"

```Java
Buffer buffer = readBuffer(new File("README.md"));
System.out.println("   md5: " + buffer.md5().hex());
System.out.println("  sha1: " + buffer.sha1().hex());
System.out.println("sha256: " + buffer.sha256().hex());
System.out.println("sha512: " + buffer.sha512().hex());
```

=== "Kotlin"

```Kotlin
val buffer = readBuffer(File("README.md"))
println("       md5: " + buffer.md5().hex())
println("      sha1: " + buffer.sha1().hex())
println("    sha256: " + buffer.sha256().hex())
println("    sha512: " + buffer.sha512().hex())
```

While streaming from a source:

=== "Java"

```Java
try (HashingSink hashingSink = HashingSink.sha256(Okio.blackhole());
     BufferedSource source = Okio.buffer(Okio.source(file))) {
  source.readAll(hashingSink);
  System.out.println("sha256: " + hashingSink.hash().hex());
}
```

=== "Kotlin"

```Kotlin
sha256(blackholeSink()).use { hashingSink ->
  file.source().buffer().use { source ->
    source.readAll(hashingSink)
    println("    sha256: " + hashingSink.hash.hex())
  }
}
```

While streaming to a sink:

=== "Java"

```Java
try (HashingSink hashingSink = HashingSink.sha256(Okio.blackhole());
     BufferedSink sink = Okio.buffer(hashingSink);
     Source source = Okio.source(file)) {
  sink.writeAll(source);
  sink.close(); // Emit anything buffered.
  System.out.println("sha256: " + hashingSink.hash().hex());
}
```

=== "Kotlin"

```Kotlin
sha256(blackholeSink()).use { hashingSink ->
  hashingSink.buffer().use { sink ->
    file.source().use { source ->
      sink.writeAll(source)
      sink.close() // Emit anything buffered.
      println("    sha256: " + hashingSink.hash.hex())
    }
  }
}
```

Okio also supports HMAC (Hash Message Authentication Code) which combines a secret and a hash. Applications use HMAC for data integrity and authentication.

=== "Java"

```Java
ByteString secret = ByteString.decodeHex("7065616e7574627574746572");
System.out.println("hmacSha256: " + byteString.hmacSha256(secret).hex());
```

=== "Kotlin"

```Kotlin
val secret = "7065616e7574627574746572".decodeHex()
println("hmacSha256: " + byteString.hmacSha256(secret).hex())
```

As with hashing, you can generate an HMAC from a

ByteString
,
Buffer
,
HashingSource
, and
HashingSink
. Note that Okio doesn’t implement HMAC for MD5. Okio uses Java’s
java.security.MessageDigest
for cryptographic hashes and
javax.crypto.Mac
for HMAC.

Encryption and Decryption

Use

Okio.cipherSink(Sink, Cipher)
or
Okio.cipherSource(Source, Cipher)
to encrypt or decrypt a stream using a block cipher.

Callers are responsible for the initialization of the encryption or decryption cipher with the chosen algorithm, the key, and algorithm-specific additional parameters like the initialization vector. The following example shows a typical usage with AES encryption, in which

key
and
iv
parameters should both be 16 bytes long.
void encryptAes(ByteString bytes, File file, byte[] key, byte[] iv)
    throws GeneralSecurityException, IOException {
  Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
  cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key, "AES"), new IvParameterSpec(iv));
  try (BufferedSink sink = Okio.buffer(Okio.cipherSink(Okio.sink(file), cipher))) {
    sink.write(bytes);
  }
}

ByteString decryptAesToByteString(File file, byte[] key, byte[] iv) throws GeneralSecurityException, IOException { Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"), new IvParameterSpec(iv)); try (BufferedSource source = Okio.buffer(Okio.cipherSource(Okio.source(file), cipher))) { return source.readByteString(); } }

In Kotlin, these encryption and decryption methods are extensions on

Cipher
:
fun encryptAes(bytes: ByteString, file: File, key: ByteArray, iv: ByteArray) {
  val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
  cipher.init(Cipher.ENCRYPT_MODE, SecretKeySpec(key, "AES"), IvParameterSpec(iv))
  val cipherSink = file.sink().cipherSink(cipher)
  cipherSink.buffer().use { 
    it.write(bytes) 
  }
}

fun decryptAesToByteString(file: File, key: ByteArray, iv: ByteArray): ByteString { val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding") cipher.init(Cipher.DECRYPT_MODE, SecretKeySpec(key, "AES"), IvParameterSpec(iv)) val cipherSource = file.source().cipherSource(cipher) return cipherSource.buffer().use { it.readByteString() } }

Releases

Our change log has release history.

implementation("com.squareup.okio:okio:2.9.0")
Snapshot builds are also available
repositories {
    maven {
        url = uri("https://oss.sonatype.org/content/repositories/snapshots/")
    }
}

dependencies {
   implementation("com.squareup.okio:okio:2.9.0")
}

R8 / ProGuard

If you are using R8 or ProGuard add the options from this file.

License

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

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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