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Parambir Singh
Parambir Singh

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Easily distributable Scala CLIs using jlink

#scala #jlink #cli #jvm

Final code for this project is available at github.com/parambirs/simple-scala-cli

JDK 9 added the jlink tool that allows us to create a distributable custom JRE that containing only specific JDK modules our application needs. We no longer need to expect customers to have Java pre-installed or bundle the complete JRE (a few hundred MBs) with our app. I've created a few example apps where the uncompressed runtime is between 40MB to 90MB depending upon external libraries used by the app. After zipping, the package is around 20-25 MB. For me, this makes JVM a great platform for writing and distributing command-line apps comparable to Python or Ruby.

In this post, we'll see how to create a simple "hello world" CLI written in Scala (although it's possible to write apps using any JVM language). This app will make an HTTP call using Java's new java.net.http module.

Note: You'll need JDK 9 or later to follow along this tutorial.

Create a new sbt project

I created a new sbt project using IntelliJ:

New IntelliJ SBT Project

Add src/main/scala/App.scala file that prints a message to the terminal when run:

object App {
  def main(args: Array[String]): Unit = {
    println("Hello, from scala command-line app")
  }
}
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Let's test that our basic app works from the command line by running it via sbt:

~/t/simple-scala-cli> sbt run
......
[info] running App 
Hello, from scala command-line app
[success] Total time: 4 s
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Add sbt-native-packager plugin

sbt-native-packager is an sbt plugin that makes it easy to build packages for different operating systems. We'll use its Jlink plugin to generate a custom JRE for our application.

Add or edit the project/plugins.sbt file to add the following:

addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.7.0")
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We need to enable the JlinkPlugin in build.sbt:

enablePlugins(JlinkPlugin)
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Now, we can create a custom distribution via the following command:

~/t/simple-scala-cli> sbt stage
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Our CLI app with a custom JRE is available in target/universal/stage directory:

Custom JRE

Let's run our app from the stage directory:

~/t/simple-scala-cli> ./target/universal/stage/bin/simple-scala-cli 
Hello, from scala command-line app
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Great! It works. But how can we be sure that it's not using the system wide Java installation on my machine? We can rename the stage/jre directory to something else and try again:

 ~/t/simple-scala-cli> mv ./target/universal/stage/jre ./target/universal/stage/xyz

~/t/simple-scala-cli> ./target/universal/stage/bin/simple-scala-cli
No java installations was detected.
Please go to http://www.java.com/getjava/ and download


~/t/simple-scala-cli> mv ./target/universal/stage/xyz ./target/universal/stage/jre

~/t/simple-scala-cli> ./target/universal/stage/bin/simple-scala-cli
Hello, from scala command-line app
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Cool! It is indeed using the custom JRE image. On my machine the size of the stage directory is 46.9 MB. After zipping it up, the size gets reduced to 19.3 MB.

Let me reiterate - This 20MB zip file contains everything that your clients need to run your app. JVM apps aren't that verbose anymore compared to standalone ruby/python apps that bundle their runtime!

What does jlink plugin do?

The Jlink plugin performs two important steps in the build process. Here're the two relevant lines from the sbt stage command output:

[info] Running: jdeps --multi-release 13 -R /Users/parambirs/tmp/simple-scala-cli/target/scala-2.13/classes /Users/parambirs/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.1/scala-library-2.13.1.jar
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It first runs jdeps to determine which java modules are being used by our codebase.

[info] Running: jlink --output /Users/parambirs/tmp/simple-scala-cli/target/jlink/output --add-modules java.base
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It then runs jlink to build a custom JRE that includes all the Java modules our app will need. In this simple application, we are only using the java.base module.

Making an HTTP call

Let's make our app do something more than just printing hello. We'll make the app fetch and print google.ca home page HTML to the console. I'm not using any 3rd party HTTP client as Java now comes with a built-in easy-to-use one. Here's the Main class after the modifications:

import java.net.URI
import java.net.http.HttpResponse.BodyHandlers
import java.net.http.{HttpClient, HttpRequest}

object App {
  def main(args: Array[String]): Unit = {
    val uri = new URI("https://google.ca")

    val client = HttpClient.newHttpClient()
    val request = HttpRequest.newBuilder(uri).build()
    val response = client.send(request, BodyHandlers.ofString())
    println(Console.YELLOW + response.body() + Console.RESET)
  }
}
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Let's test our App :

sbt:simple-scala-cli> run
[info] Compiling 1 Scala source to /Users/parambirs/tmp/simple-scala-cli/target/scala-2.13/classes ...
[info] running App 
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="https://www.google.ca/">here</A>.
</BODY></HTML>
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Okay, looks good to ship. Let's package it up:

sbt:simple-scala-cli> stage
...
[info] Running: jdeps --multi-release 13 -R /Users/parambirs/tmp/simple-scala-cli/target/scala-2.13/classes /Users/parambirs/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.1/scala-library-2.13.1.jar
...
[info] Running: jlink --output /Users/parambirs/tmp/simple-scala-cli/target/jlink/output --add-modules java.base,java.net.http
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As you can see here, jdeps identified that we need both java.base and java.net.http modules at runtime and jlink added them to our runtime image.

Wrapping up

Seems like we're ready to ship our CLI to our clients. However, before we can do that, we need to make sure it works:

~/t/simple-scala-cli> ./target/universal/stage/bin/simple-scala-cli
Exception in thread "main" javax.net.ssl.SSLHandshakeException: Received fatal alert: handshake_failure
    at java.net.http/jdk.internal.net.http.HttpClientImpl.send(HttpClientImpl.java:568)
    at java.net.http/jdk.internal.net.http.HttpClientFacade.send(HttpClientFacade.java:119)
    at App$.main(App.scala:11)
    at App.main(App.scala)
Caused by: javax.net.ssl.SSLHandshakeException: Received fatal alert: handshake_failure
    at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:131)
    at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:117)
    at .......
        ......
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Oops! Looks like we are missing something in our runtime JRE. I had to spend some time googling to fix this. We need jdk.crypto.ec module for SSL to work correctly, however, jdeps isn't able to figure this out. The solution is to add this jlink dependency in our build.sbt file:

jlinkModules += "jdk.crypto.ec"
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Let's build and test our runtime image one more time:

~/t/simple-scala-cli [1]> sbt stage
...
[info] Running: jdeps --multi-release 13 -R /Users/parambirs/tmp/simple-scala-cli/target/scala-2.13/classes /Users/parambirs/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.1/scala-library-2.13.1.jar
[info] Running: jlink --output /Users/parambirs/tmp/simple-scala-cli/target/jlink/output --add-modules java.base,java.net.http,jdk.crypto.ec
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~/t/simple-scala-cli> ./target/universal/stage/bin/simple-scala-cli
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="https://www.google.ca/">here</A>.
</BODY></HTML>
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Great! Finally we have a working and distributable command line app. After adding two extra modules, the runtime image size became 48.8MB uncompressed and 19.9MB when zipped.

Hope you found this useful.

Top comments (1)

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nickfun profile image
Nick F •

Thanks for the writeup!

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