Option to target system JDK instead of JDK8 (#27083)

Inspired by the proposal by @nvollmar in 0940dfc

build.sbt

@@ -23,7 +23,6 @@ import spray.boilerplate.BoilerplatePlugin
 initialize := {
   // Load system properties from a file to make configuration from Jenkins easier
   loadSystemProperties("project/akka-build.properties")
-  assert(CrossJava.Keys.fullJavaHomes.value.contains("8"), "JDK 8 is not installed but required to build akka")
   initialize.value
 }
 

project/AkkaBuild.scala

@@ -11,8 +11,11 @@ import java.time.format.DateTimeFormatter
 import java.time.ZonedDateTime
 import java.time.ZoneOffset
 
-import sbt.Keys._
+// Overriding CrossJava imports #26935
+import sbt.Keys.{fullJavaHomes=>_,_}
 import sbt._
+
+import CrossJava.autoImport._
 import org.scalafmt.sbt.ScalafmtPlugin.autoImport._
 import scala.collection.breakOut
 
@@ -128,21 +131,15 @@ object AkkaBuild {
 
     // compile options
     scalacOptions in Compile ++= DefaultScalacOptions,
-    // Makes sure that, even when compiling with a jdk version greater than 8, the resulting jar will not refer to
-    // methods not found in jdk8. To test whether this has the desired effect, compile akka-remote and check the
-    // invocation of 'ByteBuffer.clear()' in EnvelopeBuffer.class with 'javap -c': it should refer to
-    // "java/nio/ByteBuffer.clear:()Ljava/nio/Buffer" and not "java/nio/ByteBuffer.clear:()Ljava/nio/ByteBuffer":
-    scalacOptions in Compile ++= (
-      if (JavaVersion.isJdk8)
-        Seq("-target:jvm-1.8")
-      else
-        // -release 8 is not enough, for some reason we need the 8 rt.jar explicitly #25330
-        Seq("-release", "8", "-javabootclasspath", CrossJava.Keys.fullJavaHomes.value("8") + "/jre/lib/rt.jar")),
+    scalacOptions in Compile ++=
+      CrossJava.targetJdkScalacOptions(targetSystemJdk.value, fullJavaHomes.value),
     scalacOptions in Compile ++= (if (allWarnings) Seq("-deprecation") else Nil),
     scalacOptions in Test := (scalacOptions in Test).value.filterNot(opt =>
       opt == "-Xlog-reflective-calls" || opt.contains("genjavadoc")),
-    javacOptions in compile ++= DefaultJavacOptions ++ JavaVersion.sourceAndTarget(CrossJava.Keys.fullJavaHomes.value("8")),
-    javacOptions in test ++= DefaultJavacOptions ++ JavaVersion.sourceAndTarget(CrossJava.Keys.fullJavaHomes.value("8")),
+    javacOptions in compile ++= DefaultJavacOptions ++
+      CrossJava.targetJdkJavacOptions(targetSystemJdk.value, fullJavaHomes.value),
+    javacOptions in test ++= DefaultJavacOptions ++
+      CrossJava.targetJdkJavacOptions(targetSystemJdk.value, fullJavaHomes.value),
     javacOptions in compile ++= (if (allWarnings) Seq("-Xlint:deprecation") else Nil),
     javacOptions in doc ++= Seq(),
 

project/CrossJava.scala

@@ -15,8 +15,9 @@ import akka.CrossJava.nullBlank
 
 /*
  * Tools for discovering different Java versions,
- * will be in sbt 1.3.0 (https://github.com/sbt/sbt/pull/4139 et al)
- * but until that time replicated here
+ *
+ * Some of these will be in sbt 1.3.0 (https://github.com/sbt/sbt/pull/4139 et al)
+ * but are replicated here until that time
  */
 case class JavaVersion(numbers: Vector[Long], vendor: Option[String]) {
   def numberStr: String = numbers.mkString(".")
@@ -49,21 +50,59 @@ object JavaVersion {
   def sourceAndTarget(fullJavaHome: File): Seq[String] =
     if (isJdk8) Seq.empty
     else Seq("-source", "8", "-target", "8", "-bootclasspath", fullJavaHome + "/jre/lib/rt.jar")
+
 }
 
-object CrossJava {
-  object Keys {
+object CrossJava extends AutoPlugin {
+  object autoImport {
     val discoveredJavaHomes = settingKey[Map[String, File]]("Discovered Java home directories")
     val javaHomes = settingKey[Map[String, File]]("The user-defined additional Java home directories")
     val fullJavaHomes = settingKey[Map[String, File]]("Combines discoveredJavaHomes and custom javaHomes.")
+    val targetSystemJdk = settingKey[Boolean]("Target the system JDK instead of building against JDK 8. When this is enabled resulting artifacts may not work on JDK 8!")
   }
+  import autoImport._
 
-  import Keys._
+  def targetJdkScalacOptions(targetSystemJdk: Boolean, fullJavaHomes: Map[String, File]): Seq[String] =
+    selectOptions(
+      targetSystemJdk,
+      fullJavaHomes,
+      Seq("-target:jvm-1.8"),
+      // '-release 8' is not enough, for some reason we need the 8 rt.jar
+      // explicitly. To test whether this has the desired effect, compile
+      // akka-remote and check the invocation of 'ByteBuffer.clear()' in
+      // EnvelopeBuffer.class with 'javap -c': it should refer to
+      //""java/nio/ByteBuffer.clear:()Ljava/nio/Buffer" and not
+      // "java/nio/ByteBuffer.clear:()Ljava/nio/ByteBuffer". Issue #27079
+      (java8home: File) => Seq("-release", "8", "-javabootclasspath", java8home + "/jre/lib/rt.jar")
+    )
+  def targetJdkJavacOptions(targetSystemJdk: Boolean, fullJavaHomes: Map[String, File]): Seq[String] =
+    selectOptions(
+      targetSystemJdk,
+      fullJavaHomes,
+      Nil,
+      // '-release 8' would be a neater option here, but is currently not an
+      // option because it doesn't provide access to `sun.misc.Unsafe` #27079
+      (java8home: File) => Seq("-source", "8", "-target", "8", "-bootclasspath", java8home + "/jre/lib/rt.jar")
+  )
+
+  private def selectOptions(targetSystemJdk: Boolean, fullJavaHomes: Map[String, File], jdk8options: Seq[String], jdk11options: File => Seq[String]): Seq[String] =
+    if (targetSystemJdk)
+      Nil
+    else if (JavaVersion.isJdk8)
+      jdk8options
+    else fullJavaHomes.get("8") match {
+      case Some(java8home) =>
+        jdk11options(java8home)
+      case None =>
+        throw new MessageOnlyException("A JDK 8 installation was not found, but is required to build Akka. To target your system JDK, use the \"set every targetSystemJdk := true\" sbt command. Resulting artifacts may not work on JDK 8")
+    }
 
   val crossJavaSettings = Seq(
     discoveredJavaHomes := CrossJava.discoverJavaHomes,
     javaHomes := ListMap.empty,
-    fullJavaHomes := CrossJava.expandJavaHomes(discoveredJavaHomes.value ++ javaHomes.value))
+    fullJavaHomes := CrossJava.expandJavaHomes(discoveredJavaHomes.value ++ javaHomes.value),
+    targetSystemJdk := false,
+  )
 
   // parses jabaa style version number adopt@1.8
   def parseJavaVersion(version: String): JavaVersion = {