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testing sbt plugins

NOTE: Official docs:

Let’s talk about testing. Once you write a plugin, it turns into a long-term thing. To keep adding new features (or to keep fixing bugs), writing tests makes sense. But how does one go about testing a plugin to a build tool? We fly, of course.

scripted test framework

sbt comes with scripted test framework, which let’s you script a build scenario. It was written to test sbt itself on complex scenarios such as change detection and partial compilation:

Now, consider what happens if you were to delete B.scala but do not update A.scala. When you recompile, you should get an error because B no longer exists for A to reference. [… (really complicated stuff)]

The scripted test framework is used to verify that sbt handles cases such as that described above.

To be precise, the framework is made available via scripted-plugin ported by Artyom Olshevskiy known as siasia, which is part of the official codebase.

step 1: snapshot

Before you start, set your version to a -SNAPSHOT one because scripted-plugin will publish your plugin locally.

step 2: scripted-plugin

Add scripted-plugin to your plugin build. project/scripted.sbt:

libraryDependencies <+= (sbtVersion) { sv =>
  "org.scala-sbt" % "scripted-plugin" % sv

Then add the following to scripted.sbt:


scriptedLaunchOpts := { scriptedLaunchOpts.value ++
  Seq("-Xmx1024M", "-XX:MaxPermSize=256M", "-Dplugin.version=" + version.value)

scriptedBufferLog := false

step 3: src/sbt-test

Make dir structure src/sbt-test/<test-group>/<test-name>. For starters, try something like src/sbt-test/<your-plugin-name>/simple.

Now ready? Create an initial build in simple. Like a real build using your plugin. I’m sure you already have several of them to test manually. Here’s an example build.sbt:

import AssemblyKeys._

version := "0.1"

scalaVersion := "2.10.2"


jarName in assembly := "foo.jar"

In project/plugins.sbt:

  val pluginVersion = System.getProperty("plugin.version")
  if(pluginVersion == null)
    throw new RuntimeException("""|The system property 'plugin.version' is not defined.
                                  |Specify this property using the scriptedLaunchOpts -D.""".stripMargin)
  else addSbtPlugin("com.eed3si9n" % "sbt-assembly" % pluginVersion)

This a trick I picked up from JamesEarlDouglas/xsbt-web-plugin@feabb2, which allows us to pass version number into the test.

I also have src/main/scala/hello.scala:

object Main extends App {

step 4: write a script

Now, write a script to describe your scenario in a file called test located at the root dir of your test project.

# check if the file gets created
> assembly
$ exists target/scala-2.10/foo.jar

The syntax for the script is described in ChangeDetectionAndTesting, but let me break it down:

  1. # starts a one-line comment
  2. > name sends a task to sbt (and tests if it succeeds)
  3. $ name arg* performs a file command (and tests if it succeeds)
  4. -> name sends a task to sbt, but expects it to fail
  5. -$ name arg* performs a file command, but expects it to fail

File commands are:

So my script will run assembly task, and checks if foo.jar gets created. We’ll cover more complex tests later.

step 5: run the script

To run the scripts, go back to your plugin project, and run:

> scripted

This will copy your test build into a temporary dir, and executes the test script. If everything works out, you’d see publish-local running, then:

Running sbt-assembly / simple
[success] Total time: 18 s, completed Sep 17, 2011 3:00:58 AM

step 6: custom assertion

The file commands are great, but not nearly enough because none of them test the actual contents. An easy way to test the contents is to implement a custom task in your test build.

For my hello project, I’d like to check if the resulting jar prints out “hello”. I can take advantage of sbt.Process to run the jar. To express a failure, just throw an error. Here’s build.sbt:

import AssemblyKeys._

version := "0.1"

scalaVersion := "2.10.2"


jarName in assembly := "foo.jar"

TaskKey[Unit]("check") <<= (crossTarget) map { (crossTarget) =>
  val process = sbt.Process("java", Seq("-jar", (crossTarget / "foo.jar").toString))
  val out = (process!!)
  if (out.trim != "bye") error("unexpected output: " + out)

I am intentionally testing if it matches “bye”, to see how the test fails. Be careful not to include empty lines, since it’d be interpreted as the end of the block.

Here’s test:

# check if the file gets created
> assembly
$ exists target/foo.jar

# check if it says hello
> check

Running scripted fails the test as expected:

[info] [error] {file:/private/var/folders/Ab/AbC1EFghIj4LMNOPqrStUV+++XX/-Tmp-/sbt_cdd1b3c4/simple/}default-0314bd/*:check: unexpected output: hello
[info] [error] Total time: 0 s, completed Sep 21, 2011 8:43:03 PM
[error] x sbt-assembly / simple
[error]    {line 6}  Command failed: check failed
[error] {file:/Users/foo/work/sbt-assembly/}default-373f46/*:scripted: sbt-assembly / simple failed
[error] Total time: 14 s, completed Sep 21, 2011 8:00:00 PM

If you want to reuse the assertions among the test builds, you could use full configuration and inherit from a custom build class.

step 7: testing the test

Until you get the hang of it, it might take a while for the test itself to behave correctly. There are several techniques that may come in handy.

First place to start is turning off the log buffering.

> set scriptedBufferLog := false

This for example should print out the location of the temporary dir:

[info] [info] Set current project to default-c6500b (in build file:/private/var/folders/Ab/AbC1EFghIj4LMNOPqrStUV+++XX/-Tmp-/sbt_8d950687/simple/project/plugins/)

Add the following line to your test script to suspend the test until you hit the enter key:

$ pause

If you’re thinking about going down to the sbt/sbt-test/sbt-foo/simple and running sbt, don’t do it. The right way, as Mark told me in the comment bellow, is to copy the dir somewhere else and run it.

step 8: get inspired

There are literally 100+ scripted tests under sbt project itself. Browse around to get inspirations.

For example, here’s the one called by-name.

> compile

# change => Int to Function0
$ copy-file changes/A.scala A.scala

# Both A.scala and B.scala need to be recompiled because the type has changed
-> compile

xsbt-web-plugin and sbt-assemlby have some scripted tests too.

That’s it! Let me know about your experience in testing plugins!