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

@@ -8,6 +8,7 @@
 
 package sbt.internal.util
 
+import java.nio.file.{ Path, Paths }
 import java.util.Locale
 
 import scala.reflect.macros.blackbox
@@ -121,4 +122,8 @@ object Util {
         case g: ThreadId @unchecked => g.threadId
       }
     }
+
+  lazy val javaHome: Path =
+    if (sys.props("java.home").endsWith("jre")) Paths.get(sys.props("java.home")).getParent()
+    else Paths.get(sys.props("java.home"))
 }

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

@@ -9,8 +9,7 @@
 package sbt
 
 import java.io.{ File, PrintWriter }
-import java.net.{ URI, URL }
-import java.nio.file.{ Paths, Path => NioPath }
+import java.nio.file.{ Path => NioPath }
 import java.util.Optional
 import java.util.concurrent.TimeUnit
 import lmcoursier.CoursierDependencyResolution
@@ -408,13 +407,12 @@ object Defaults extends BuildCommon {
       val boot = app.provider.scalaProvider.launcher.bootDirectory
       val ih = app.provider.scalaProvider.launcher.ivyHome
       val coursierCache = csrCacheDirectory.value
-      val javaHome = Paths.get(sys.props("java.home"))
       Map(
         "BASE" -> base.toPath,
         "SBT_BOOT" -> boot.toPath,
         "CSR_CACHE" -> coursierCache.toPath,
         "IVY_HOME" -> ih.toPath,
-        "JAVA_HOME" -> javaHome,
+        "JAVA_HOME" -> Util.javaHome,
       )
     },
     fileConverter := MappedFileConverter(rootPaths.value, allowMachinePath.value),

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

@@ -228,7 +228,6 @@ object BuildServerTest extends AbstractServerTest {
     val buildTarget = buildTargetUri("javaProj", "Compile")
 
     compile(buildTarget)
-
     assertMessage(
       "build/publishDiagnostics",
       "Hello.java",
@@ -251,16 +250,17 @@ object 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
     )
@@ -272,16 +272,17 @@ object 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…
 
@@ -685,6 +686,11 @@ object BuildServerTest extends AbstractServerTest {
     def assertion =
       svr.waitForString(duration) { msg =>
         if (debug) println(msg)
+        if (debug)
+          parts.foreach { p =>
+            if (msg.contains(p)) println(s"> $msg contains $p")
+            else ()
+          }
         parts.forall(msg.contains)
       }
     if (message.nonEmpty) assert.apply(assertion, message) else assert(assertion)