fix: Use JDK path, not JRE path

**Problem**
There are a few places where javaHome or java path is set,
using java.home system property. The problem is that it points to JRE,
not JDK, so it would break on Java compilation etc.

**Solution**
If the path ends with jre, go up one directory.

internal/util-core/src/main/scala/sbt/internal/util/Util.scala

@@ -8,11 +8,12 @@
 
 package sbt.internal.util
 
+import java.nio.file.{ Path, Paths }
 import java.util.Locale
 
 import scala.collection.concurrent.TrieMap
 
-object Util {
+object Util:
   def makeList[T](size: Int, value: T): List[T] = List.fill(size)(value)
 
   def separate[T, A, B](ps: Seq[T])(f: T => Either[A, B]): (Seq[A], Seq[B]) = {
@@ -78,4 +79,8 @@ object Util {
     val cache = TrieMap.empty[A1, A2]
     x => cache.getOrElseUpdate(x, f(x))
   }
-}
+
+  lazy val javaHome: Path =
+    if sys.props("java.home").endsWith("jre") then Paths.get(sys.props("java.home")).getParent()
+    else Paths.get(sys.props("java.home"))
+end Util

main/src/main/scala/sbt/Defaults.scala

@@ -9,7 +9,7 @@
 package sbt
 
 import java.io.{ File, PrintWriter }
-import java.nio.file.{ Files, Paths, Path => NioPath }
+import java.nio.file.{ Files, Path => NioPath }
 import java.util.Optional
 import java.util.concurrent.TimeUnit
 import lmcoursier.CoursierDependencyResolution
@@ -245,13 +245,12 @@ object Defaults extends BuildCommon {
     val base = app.baseDirectory.getCanonicalFile.toPath
     val boot = app.provider.scalaProvider.launcher.bootDirectory.toPath
     val ih = app.provider.scalaProvider.launcher.ivyHome.toPath
-    val javaHome = Paths.get(sys.props("java.home"))
     ListMap(
       "OUT" -> out,
       "BASE" -> base,
       "SBT_BOOT" -> boot,
       "IVY_HOME" -> ih,
-      "JAVA_HOME" -> javaHome
+      "JAVA_HOME" -> Util.javaHome,
     )
 
   private[sbt] lazy val globalIvyCore: Seq[Setting[?]] =

main/src/main/scala/sbt/internal/Load.scala

@@ -33,7 +33,7 @@ import xsbti.{ FileConverter, HashedVirtualFileRef, VirtualFile }
 import xsbti.compile.{ ClasspathOptionsUtil, Compilers }
 import java.io.File
 import java.net.URI
-import java.nio.file.{ Path, Paths }
+import java.nio.file.Path
 import scala.annotation.tailrec
 import scala.collection.mutable
 import sbt.internal.util.Util
@@ -71,7 +71,7 @@ private[sbt] object Load {
     val scalaProvider = app.provider.scalaProvider
     val launcher = scalaProvider.launcher
     val stagingDirectory = getStagingDirectory(state, globalBase).getCanonicalFile
-    val javaHome = Paths.get(sys.props("java.home"))
+    val javaHome = Util.javaHome
     val out = baseDirectory.toPath.resolve("target").resolve("out")
     val rootPaths = Map(
       "OUT" -> out,

protocol/src/main/scala/sbt/internal/bsp/BuildServerConnection.scala

@@ -9,6 +9,7 @@
 package sbt.internal.bsp
 
 import sbt.internal.bsp.codec.JsonProtocol.BspConnectionDetailsFormat
+import sbt.internal.util.Util
 import sbt.io.IO
 import sjsonnew.support.scalajson.unsafe.{ CompactPrinter, Converter }
 
@@ -25,7 +26,7 @@ object BuildServerConnection {
 
   private[sbt] def writeConnectionFile(sbtVersion: String, baseDir: File): Unit = {
     val bspConnectionFile = new File(baseDir, ".bsp/sbt.json")
-    val javaHome = System.getProperty("java.home")
+    val javaHome = Util.javaHome
     val classPath = System.getProperty("java.class.path")
 
     val sbtScript = Option(System.getProperty("sbt.script"))

server-test/src/test/scala/testpkg/BuildServerTest.scala

@@ -234,7 +234,6 @@ class BuildServerTest extends AbstractServerTest {
     val buildTarget = buildTargetUri("javaProj", "Compile")
 
     compile(buildTarget)
-
     assertMessage(
       "build/publishDiagnostics",
       "Hello.java",
@@ -257,16 +256,17 @@ class BuildServerTest extends AbstractServerTest {
     val testFile = new File(svr.baseDirectory, s"java-proj/src/main/java/example/Hello.java")
 
     val otherBuildFile = new File(svr.baseDirectory, "force-java-out-of-process-compiler.sbt")
-    // Setting `javaHome` will force SBT to shell out to an external Java compiler instead
+    // Setting `javaHome` will force sbt to shell out to an external Java compiler instead
     // of using the local compilation service offered by the JVM running this SBT instance.
     IO.write(
       otherBuildFile,
       """
+        |def jdk: File = sbt.internal.Util.javaHome.toFile()
         |lazy val javaProj = project
         |  .in(file("java-proj"))
         |  .settings(
         |    javacOptions += "-Xlint:all",
-        |    javaHome := Some(file(System.getProperty("java.home")))
+        |    javaHome := Some(jdk)
         |  )
         |""".stripMargin
     )
@@ -278,16 +278,17 @@ class BuildServerTest extends AbstractServerTest {
       "build/publishDiagnostics",
       "Hello.java",
       """"severity":2""",
-      """found raw type: List"""
-    )(message = "should send publishDiagnostics with severity 2 for Hello.java")
+      """found raw type"""
+    )(message = "should send publishDiagnostics with severity 2 for Hello.java", debug = false)
 
     assertMessage(
       "build/publishDiagnostics",
       "Hello.java",
       """"severity":1""",
-      """incompatible types: int cannot be converted to String"""
+      """incompatible types: int cannot be converted"""
     )(
-      message = "should send publishDiagnostics with severity 1 for Hello.java"
+      message = "should send publishDiagnostics with severity 1 for Hello.java",
+      debug = true
     )
     // Note the messages changed slightly in both cases. That's interesting…
 
@@ -690,9 +691,14 @@ class BuildServerTest extends AbstractServerTest {
     def assertion =
       svr.waitForString(duration) { msg =>
         if (debug) println(msg)
+        if debug then
+          parts.foreach: p =>
+            if msg.contains(p) then println(s"> $msg contains $p")
+            else ()
         parts.forall(msg.contains)
       }
-    if (message.nonEmpty) assert(assertion, message) else assert(assertion)
+    if message.nonEmpty then assert(assertion, message)
+    else assert(assertion)
   }
 
   private def reloadWorkspace(): Int =