[2.x] fix: Honor JAVA_HOME in the Unix launcher (#9825)

Problem: The Unix launcher ignores JAVA_HOME and selects Java from PATH when JAVACMD is absent.

Solution: Check JAVA_HOME/bin/java before falling back to PATH while preserving explicit --java-home and JAVACMD precedence.

Fixes #9794

launcher-package/integration-test/src/test/scala/JavaHomeScriptTest.scala

@@ -9,6 +9,8 @@
 package example.test
 
 import java.io.File
+import sbt.io.IO
+import scala.sys.process.{ Process, ProcessLogger }
 
 /**
  * First-hop propagation guard: an explicit `-java-home` must be the JDK the Windows launcher runs
@@ -33,4 +35,55 @@ object JavaHomeScriptTest extends verify.BasicTestSuite with ShellScriptUtil:
         out.exists(_.contains(javaExe)),
         s"launcher should run $javaExe; command echo was: ${out.mkString(" | ")}"
       )
+  List(
+    ("JAVA_HOME overrides PATH", "", "home", "", "home"),
+    ("JAVACMD overrides JAVA_HOME", "command", "home", "", "command"),
+    ("invalid JAVACMD falls back to JAVA_HOME", "missing", "home", "", "home"),
+    ("empty JAVA_HOME falls back to PATH", "", "", "", "path"),
+    ("invalid JAVA_HOME falls back to PATH", "", "missing", "", "path"),
+    ("invalid JAVACMD and JAVA_HOME fall back to PATH", "missing", "missing", "", "path"),
+    ("--java-home overrides environment", "command", "home", "option", "option"),
+  ).foreach: (name, command, home, option, expected) =>
+    test(name):
+      checkJavaSelection(command, home, option, expected)
+
+  private def checkJavaSelection(
+      command: String,
+      home: String,
+      option: String,
+      expected: String
+  ): Unit =
+    if isWindows then cancel("Bash launcher Java selection")
+    else
+      IO.withTemporaryDirectory: dir =>
+        def javaHome(name: String): File = new File(dir, s"$name jdk")
+        def java(name: String): File = new File(javaHome(name), "bin/java")
+        List("path", "home", "command", "option").foreach: name =>
+          IO.write(
+            java(name),
+            IO.read(new File(javaBinDir, "java"))
+              .replace("else:\n", s"else:\n    print('SELECTED: $name')\n")
+          )
+          Predef.assert(java(name).setExecutable(true))
+        IO.write(new File(dir, "build.sbt"), "")
+        val env = Seq(
+          "PATH" -> s"${java("path").getParent}${File.pathSeparator}${sys.env("PATH")}",
+          "JAVACMD" -> (if command.isEmpty then "" else java(command).getAbsolutePath),
+          "JAVA_HOME" -> (if home.isEmpty then "" else javaHome(home).getAbsolutePath),
+          "JAVA_OPTS" -> "",
+          "SBT_OPTS" -> "",
+          "JAVA_TOOL_OPTIONS" -> "",
+          "XDG_CONFIG_HOME" -> dir.getAbsolutePath,
+          "SBT_ETC_FILE" -> new File(dir, "missing-sbtopts").getAbsolutePath,
+        )
+        val args = if option.isEmpty then Seq.empty
+        else Seq("--java-home", javaHome(option).getAbsolutePath)
+        val output = scala.collection.mutable.ListBuffer.empty[String]
+        val exit = Process(
+          Seq(sbtScript.getAbsolutePath, "--server") ++ args ++ Seq("compile"),
+          dir,
+          env*
+        ).!(ProcessLogger(line => output += line, line => output += line))
+        Predef.assert(exit == 0, output.mkString("\n"))
+        Predef.assert(output.contains(s"SELECTED: $expected"), output.mkString("\n"))
 end JavaHomeScriptTest

launcher-package/integration-test/src/test/scala/ShellScriptUtil.scala

@@ -173,6 +173,7 @@ trait ShellScriptUtil extends BasicTestSuite:
             envVars("JAVACMD") = new File(javaBinDir, "java").getAbsolutePath()
             envVars("JAVA_HOME") = javaHomeEnv
           else
+            envVars("JAVACMD") = new File(javaBinDir, "java").getAbsolutePath()
             envVars("PATH") = javaBinDir + File.pathSeparator + path
             envVars("JAVA_HOME") = javaHomeEnv
           val cmd =

sbt

@@ -995,6 +995,8 @@ sbt_file_opts=()
 
 if [[ -f "$JAVACMD" ]]; then
   java_cmd="$JAVACMD"
+elif [[ -n "$JAVA_HOME" && -f "$JAVA_HOME/bin/java" ]]; then
+  java_cmd="$JAVA_HOME/bin/java"
 fi
 
 # Pull in the machine-wide settings configuration.