Text and charsets

The text flows allow to translate a stream of text data according to the used character sets. It supports conversion between ByteString and String, as well as conversion of the character set in binary text data in the form of ByteStrings.

The main use case for these flows is the transcoding of text read from a source with a certain character set, which may not be usable with other flows or sinks. For example may CSV data arrive in UTF-16 encoding, but the Alpakka CSV parser does only support UTF-8.

Artifacts

sbt

val AkkaVersion = "2.6.14"
libraryDependencies ++= Seq(
  "com.lightbend.akka" %% "akka-stream-alpakka-text" % "3.0.4",
  "com.typesafe.akka" %% "akka-stream" % AkkaVersion
)

Maven

<properties>
  <akka.version>2.6.14</akka.version>
  <scala.binary.version>2.12</scala.binary.version>
</properties>
<dependencies>
  <dependency>
    <groupId>com.lightbend.akka</groupId>
    <artifactId>akka-stream-alpakka-text_${scala.binary.version}</artifactId>
    <version>3.0.4</version>
  </dependency>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-stream_${scala.binary.version}</artifactId>
    <version>${akka.version}</version>
  </dependency>
</dependencies>

Gradle

def versions = [
  AkkaVersion: "2.6.14",
  ScalaBinary: "2.12"
]
dependencies {
  implementation "com.lightbend.akka:akka-stream-alpakka-text_${versions.ScalaBinary}:3.0.4"
  implementation "com.typesafe.akka:akka-stream_${versions.ScalaBinary}:${versions.AkkaVersion}"
}

The table below shows direct dependencies of this module and the second tab shows all libraries it depends on transitively.

Direct dependencies

Organization	Artifact	Version
com.typesafe.akka	akka-stream_2.12	2.6.14
org.scala-lang	scala-library	2.12.11

Dependency tree

com.typesafe.akka    akka-stream_2.12    2.6.14    Apache-2.0
    com.typesafe.akka    akka-actor_2.12    2.6.14    Apache-2.0
        com.typesafe    config    1.4.0    Apache-2.0
        org.scala-lang.modules    scala-java8-compat_2.12    0.8.0    BSD 3-clause
            org.scala-lang    scala-library    2.12.11    Apache-2.0
        org.scala-lang    scala-library    2.12.11    Apache-2.0
    com.typesafe.akka    akka-protobuf-v3_2.12    2.6.14    Apache-2.0
    com.typesafe    ssl-config-core_2.12    0.4.2    Apache-2.0
        com.typesafe    config    1.4.0    Apache-2.0
        org.scala-lang.modules    scala-parser-combinators_2.12    1.1.2    Apache-2.0
            org.scala-lang    scala-library    2.12.11    Apache-2.0
        org.scala-lang    scala-library    2.12.11    Apache-2.0
    org.reactivestreams    reactive-streams    1.0.3    CC0
    org.scala-lang    scala-library    2.12.11    Apache-2.0
org.scala-lang    scala-library    2.12.11    Apache-2.0

Text transcoding

The text transcoding flow converts incoming binary text data (ByteString) to binary text data of another character encoding.

The flow fails with an UnmappableCharacterException, if a character is not representable in the targeted character set.

Scala

copysourceimport java.nio.charset.StandardCharsets

import akka.stream.alpakka.text.scaladsl.TextFlow
import akka.stream.scaladsl.FileIO

val byteStringSource: Source[ByteString, _] = // ...

byteStringSource
  .via(TextFlow.transcoding(StandardCharsets.UTF_16, StandardCharsets.UTF_8))
  .runWith(FileIO.toPath(targetFile))

Java

copysourceSource<ByteString, ?> byteStringSource = // ...
    byteStringSource
        .via(TextFlow.transcoding(StandardCharsets.UTF_16, StandardCharsets.UTF_8))
        .runWith(FileIO.toPath(targetFile), system);

Text encoding

The text encoding flow converts incoming Strings to binary text data (ByteString) with the given character encoding.

The flow fails with an UnmappableCharacterException, if a character is not representable in the targeted character set.

Scala

copysourceimport java.nio.charset.StandardCharsets

import akka.stream.alpakka.text.scaladsl.TextFlow
import akka.stream.scaladsl.FileIO

val stringSource: Source[String, _] = // ...

stringSource
  .via(TextFlow.encoding(StandardCharsets.US_ASCII))
  .intersperse(ByteString("\n"))
  .runWith(FileIO.toPath(targetFile))

Java

copysourceimport akka.stream.alpakka.testkit.javadsl.LogCapturingJunit4;
import akka.stream.alpakka.text.javadsl.TextFlow;
import akka.stream.IOResult;
import akka.stream.javadsl.FileIO;
import akka.stream.javadsl.Sink;
import akka.stream.javadsl.Source;
import akka.util.ByteString;

import java.nio.charset.StandardCharsets;

Source<String, ?> stringSource = // ...
    stringSource
        .via(TextFlow.encoding(StandardCharsets.US_ASCII))
        .intersperse(ByteString.fromString("\n"))
        .runWith(FileIO.toPath(targetFile), system);

Text decoding

The text decoding flow converts incoming ByteStrings to Strings using the given character encoding.

Scala

copysourceimport java.nio.charset.StandardCharsets

import akka.stream.alpakka.text.scaladsl.TextFlow

val byteStringSource: Source[ByteString, _] = // ...

val result: Future[immutable.Seq[String]] =
  byteStringSource
    .via(TextFlow.decoding(StandardCharsets.UTF_16))
    .runWith(Sink.seq)

Java

copysourceSource<ByteString, ?> byteStringSource = // ...
    byteStringSource
        .via(TextFlow.decoding(StandardCharsets.UTF_16))
        .runWith(Sink.seq(), system);