fix: restrict -XX:ActiveProcessorCount to HotSpot JREs to match Ruby buildpack

Ruby reference (open_jdk_like_jre.rb): -XX:ActiveProcessorCount is added
in the OpenJDK-like JRE release step only. IBM JRE initializer never sets
it — IBM JRE already had its own extraFinalizeOpts with -Xtune:virtualized
and -Xshareclasses:none (ibm.go, unchanged).

Previously -XX:ActiveProcessorCount was in the universal baseOpts, so IBM
received it on top of its own opts. Fix: move it out of baseOpts into
extraFinalizeOpts on each HotSpot JRE (OpenJDK, Oracle, SapMachine, Zulu,
GraalVM). IBM JRE and its existing opts are untouched.
Only -Djava.io.tmpdir=$TMPDIR remains in baseOpts — universal in Ruby too.

Author: stokpop

src/java/jres/base_jre.go

@@ -156,7 +156,6 @@ func (b *BaseJRE) Finalize() error {
 
 	baseOpts := []string{
 		"-Djava.io.tmpdir=$TMPDIR",
-		"-XX:ActiveProcessorCount=$(nproc)",
 	}
 	if err := WriteJavaOpts(b.ctx, strings.Join(baseOpts, " ")); err != nil {
 		b.ctx.Log.Warning("Failed to write base JAVA_OPTS: %s", err.Error())
@@ -189,6 +188,16 @@ func (b *BaseJRE) Version() string {
 	return b.installedVersion
 }
 
+// ExtraFinalizeOpts returns the JRE-specific JVM options written during Finalize.
+// These are opts beyond the universal base opts (-Djava.io.tmpdir) — e.g. HotSpot
+// flags for OpenJDK-like JREs, or J9 tuning flags for IBM JRE.
+func (b *BaseJRE) ExtraFinalizeOpts() string {
+	if b.extraFinalizeOpts == nil {
+		return ""
+	}
+	return b.extraFinalizeOpts()
+}
+
 func (b *BaseJRE) MemoryCalculatorCommand() string {
 	if b.memoryCalc == nil {
 		return ""

src/java/jres/graalvm.go

@@ -7,5 +7,7 @@ type GraalVMJRE struct{ BaseJRE }
 
 // NewGraalVMJRE creates a new GraalVM JRE provider.
 func NewGraalVMJRE(ctx *common.Context) *GraalVMJRE {
-	return &GraalVMJRE{newBaseJRE(ctx, "GraalVM", "graalvm", []string{"graalvm"}, nil, "(ensure repository_root is configured)")}
+	b := newBaseJRE(ctx, "GraalVM", "graalvm", []string{"graalvm"}, nil, "(ensure repository_root is configured)")
+	b.extraFinalizeOpts = func() string { return "-XX:ActiveProcessorCount=$(nproc)" }
+	return &GraalVMJRE{b}
 }

src/java/jres/jre_test.go

@@ -616,6 +616,20 @@ IMPLEMENTOR="Eclipse Adoptium"`
 		})
 	})
 
+	Describe("JRE-specific finalize opts (Ruby parity)", func() {
+		It("OpenJDK includes -XX:ActiveProcessorCount (HotSpot flag)", func() {
+			Expect(jres.NewOpenJDKJRE(ctx).ExtraFinalizeOpts()).To(ContainSubstring("-XX:ActiveProcessorCount"))
+		})
+
+		It("IBM JRE does not include -XX:ActiveProcessorCount (J9 incompatible)", func() {
+			Expect(jres.NewIBMJRE(ctx).ExtraFinalizeOpts()).NotTo(ContainSubstring("-XX:ActiveProcessorCount"))
+		})
+
+		It("IBM JRE includes J9-specific tuning opts", func() {
+			Expect(jres.NewIBMJRE(ctx).ExtraFinalizeOpts()).To(ContainSubstring("-Xtune:virtualized"))
+		})
+	})
+
 	Describe("JRE Detection with Environment Variables", func() {
 		var testLogBuffer *bytes.Buffer
 		var testCtx *common.Context

src/java/jres/openjdk.go

@@ -7,5 +7,7 @@ type OpenJDKJRE struct{ BaseJRE }
 
 // NewOpenJDKJRE creates a new OpenJDK JRE provider.
 func NewOpenJDKJRE(ctx *common.Context) *OpenJDKJRE {
-	return &OpenJDKJRE{newBaseJRE(ctx, "OpenJDK", "openjdk", []string{"jdk", "jre"}, nil, "")}
+	b := newBaseJRE(ctx, "OpenJDK", "openjdk", []string{"jdk", "jre"}, nil, "")
+	b.extraFinalizeOpts = func() string { return "-XX:ActiveProcessorCount=$(nproc)" }
+	return &OpenJDKJRE{b}
 }

src/java/jres/oracle.go

@@ -7,5 +7,7 @@ type OracleJRE struct{ BaseJRE }
 
 // NewOracleJRE creates a new Oracle JRE provider.
 func NewOracleJRE(ctx *common.Context) *OracleJRE {
-	return &OracleJRE{newBaseJRE(ctx, "Oracle JRE", "oracle", []string{"jdk", "jre"}, nil, "")}
+	b := newBaseJRE(ctx, "Oracle JRE", "oracle", []string{"jdk", "jre"}, nil, "")
+	b.extraFinalizeOpts = func() string { return "-XX:ActiveProcessorCount=$(nproc)" }
+	return &OracleJRE{b}
 }

src/java/jres/sapmachine.go

@@ -7,5 +7,7 @@ type SapMachineJRE struct{ BaseJRE }
 
 // NewSapMachineJRE creates a new SAP Machine JRE provider.
 func NewSapMachineJRE(ctx *common.Context) *SapMachineJRE {
-	return &SapMachineJRE{newBaseJRE(ctx, "SapMachine", "sapmachine", []string{"sapmachine"}, nil, "")}
+	b := newBaseJRE(ctx, "SapMachine", "sapmachine", []string{"sapmachine"}, nil, "")
+	b.extraFinalizeOpts = func() string { return "-XX:ActiveProcessorCount=$(nproc)" }
+	return &SapMachineJRE{b}
 }

src/java/jres/zulu.go

@@ -7,5 +7,7 @@ type ZuluJRE struct{ BaseJRE }
 
 // NewZuluJRE creates a new Zulu JRE provider.
 func NewZuluJRE(ctx *common.Context) *ZuluJRE {
-	return &ZuluJRE{newBaseJRE(ctx, "Zulu", "zulu", []string{"zulu"}, nil, "")}
+	b := newBaseJRE(ctx, "Zulu", "zulu", []string{"zulu"}, nil, "")
+	b.extraFinalizeOpts = func() string { return "-XX:ActiveProcessorCount=$(nproc)" }
+	return &ZuluJRE{b}
 }