How SBT search for dependencies for com.typesafe.play:sbt-plugin

[ppllaxxa]

Hello, I am curious how SBT understand which dependencies should be downloaded for com.typesafe.play:sbt-plugin.
Per my understanding by adding addSbtPlugin("com.typesafe.play" % "sbt-plugin" % playVersion) in plugins file for SBT project we instruct SBT to download play sbt plugin and all downstrem dependencies (e.g. https://scala.jfrog.io/ui/native/sbt-plugin-releases/com.typesafe.play/sbt-plugin/scala_2.12/sbt_1.0/2.7.9/). But in ivy.xml I could see only 2 dependnecies related to playframework:

Could someone help to understand where SBT takes info that it should download all other playframework dependencies?

[mkurz]

Lets put aside Play for a moment an lets talk about a plain sbt project: When you want to add a dependency to a sbt project you do that by adding it to the build.sbt file. For example if you want to the slf4j-api dependency to your project you do that by:

// in build.sbt
libraryDependencies += "org.slf4j" % "slf4j-api" % "2.0.6"

Now back to Play. Here, like you are correctly said, you add a sbt plugin to your project/plugins.sbt file. What you didn't mention is that you have to enable the play plugin in the build.sbt as well doing e.g.:

lazy val root = (project in file(".")).enablePlugins(PlayScala) // or PlayJava

This enabled plugin now modifies your project settings by e.g. adding dependencies to your libraryDependencies list. Lets take a deeper look at the PlayScala plugin I just mentioned in the code snippet above:

playframework/dev-mode/sbt-plugin/src/main/scala/play/sbt/Play.scala

Lines 89 to 92 in 9b86046

	object PlayScala extends AutoPlugin {
	override def requires = PlayWeb
	override def projectSettings = PlaySettings.defaultScalaSettings
	}

As you can see it requires another plugin to be loaded: PlayWeb (so you see sbtplugins can depend on other sbt plugins):

playframework/dev-mode/sbt-plugin/src/main/scala/play/sbt/Play.scala

Lines 41 to 44 in 9b86046

	object PlayWeb extends AutoPlugin {
	override def requires = PlayService && SbtTwirl && SbtJsTask && RoutesCompiler
	override def projectSettings = PlaySettings.webSettings
	}

PlayWeb now defined projectSettings which are the settings that will be applied onto your project!
And this is how these settings look like:

playframework/dev-mode/sbt-plugin/src/main/scala/play/sbt/PlaySettings.scala

Lines 237 to 271 in 9b86046

	lazy val webSettings = Seq[Setting[_]](
	constructorAnnotations += "@javax.inject.Inject()",
	playMonitoredFiles ++= (Compile / compileTemplates / sourceDirectories).value,
	routesImport ++= Seq("controllers.Assets.Asset"),
	// sbt-web
	Assets / jsFilter := new PatternFilter("""[^_].*\.js""".r.pattern),
	WebKeys.stagingDirectory := WebKeys.stagingDirectory.value / "public",
	playAssetsWithCompilation := (Compile / compile).value.asInstanceOf[Analysis],
	playAssetsWithCompilation := playAssetsWithCompilation.dependsOn((Assets / assets).?).value,
	// Assets for run mode
	PlayRun.playPrefixAndAssetsSetting,
	PlayRun.playAllAssetsSetting,
	assetsPrefix := "public/",
	// Assets for distribution
	Assets / WebKeys.packagePrefix := assetsPrefix.value,
	playPackageAssets := (Assets / packageBin).value,
	scriptClasspathOrdering := Def.taskDyn {
	val oldValue = scriptClasspathOrdering.value
	// only create a assets-jar if the task is active
	// this actually disables calling playPackageAssets, which in turn would call packageBin in Assets
	// without these calls no assets jar will be created
	if (generateAssetsJar.value) {
	Def.task {
	val (id, art) = (projectID.value, (Assets / packageBin / artifact).value)
	val jarName = JavaAppPackaging.makeJarName(id.organization, id.name, id.revision, art.name, Some("assets"))
	oldValue :+ playPackageAssets.value -> ("lib/" + jarName)
	}
	} else {
	Def.task(oldValue)
	}
	}.value,
	// Assets for testing
	TestAssets / public := (TestAssets / public).value / assetsPrefix.value,
	Test / fullClasspath += Attributed.blank((TestAssets / assets).value.getParentFile)
	)

These settings are applied to your project like if you would write it yourself into the build.sbt file.
In the example above there is no libraryDependencies settings, but e.g. the PlayService plugin you see in the snippet above does have one:

playframework/dev-mode/sbt-plugin/src/main/scala/play/sbt/PlaySettings.scala

Lines 102 to 108 in 9b86046

	libraryDependencies += {
	if (playPlugin.value)
	"com.typesafe.play" %% "play" % PlayVersion.current % "provided"
	else
	"com.typesafe.play" %% "play-server" % PlayVersion.current
	},
	libraryDependencies += "com.typesafe.play" %% "play-test" % PlayVersion.current % "test",

So the sbt plugin does the job of configuring the project for you, including setting libraryDependencies.

I hope that explains what you are asking for.

[ppllaxxa]

Wow, now it is absolutely clear. Thank you a lot!