8177107: Reduce memory footprint of java.lang.reflect.Constructor/Method

Reviewed-by: darcy, shade, coleenp

src/hotspot/share/runtime/reflection.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -728,8 +728,15 @@ static objArrayHandle get_parameter_types(const methodHandle& method,
                                           int parameter_count,
                                           oop* return_type,
                                           TRAPS) {
-  // Allocate array holding parameter types (java.lang.Class instances)
-  objArrayOop m = oopFactory::new_objArray(vmClasses::Class_klass(), parameter_count, CHECK_(objArrayHandle()));
+  objArrayOop m;
+  if (parameter_count == 0) {
+    // Avoid allocating an array for the empty case
+    // Still need to parse the signature for the return type below
+    m = Universe::the_empty_class_array();
+  } else {
+    // Allocate array holding parameter types (java.lang.Class instances)
+    m = oopFactory::new_objArray(vmClasses::Class_klass(), parameter_count, CHECK_(objArrayHandle()));
+  }
   objArrayHandle mirrors(THREAD, m);
   int index = 0;
   // Collect parameter types

src/java.base/share/classes/java/lang/reflect/Constructor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -35,6 +35,7 @@
 import sun.reflect.annotation.TypeAnnotation;
 import sun.reflect.annotation.TypeAnnotationParser;
 import sun.reflect.generics.repository.ConstructorRepository;
+import sun.reflect.generics.repository.GenericDeclRepository;
 import sun.reflect.generics.factory.CoreReflectionFactory;
 import sun.reflect.generics.factory.GenericsFactory;
 import sun.reflect.generics.scope.ConstructorScope;
@@ -244,7 +245,7 @@ public TypeVariable<Constructor<T>>[] getTypeParameters() {
       if (getSignature() != null) {
         return (TypeVariable<Constructor<T>>[])getGenericInfo().getTypeParameters();
       } else
-          return (TypeVariable<Constructor<T>>[])new TypeVariable[0];
+          return (TypeVariable<Constructor<T>>[])GenericDeclRepository.EMPTY_TYPE_VARS;
     }
 
 

src/java.base/share/classes/java/lang/reflect/Method.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1996, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1996, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -36,6 +36,7 @@
 import jdk.internal.vm.annotation.Stable;
 import sun.reflect.annotation.ExceptionProxy;
 import sun.reflect.annotation.TypeNotPresentExceptionProxy;
+import sun.reflect.generics.repository.GenericDeclRepository;
 import sun.reflect.generics.repository.MethodRepository;
 import sun.reflect.generics.factory.CoreReflectionFactory;
 import sun.reflect.generics.factory.GenericsFactory;
@@ -254,7 +255,7 @@ public TypeVariable<Method>[] getTypeParameters() {
         if (getGenericSignature() != null)
             return (TypeVariable<Method>[])getGenericInfo().getTypeParameters();
         else
-            return (TypeVariable<Method>[])new TypeVariable[0];
+            return (TypeVariable<Method>[])GenericDeclRepository.EMPTY_TYPE_VARS;
     }
 
     /**

src/java.base/share/classes/sun/reflect/generics/repository/GenericDeclRepository.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -42,6 +42,8 @@
 public abstract class GenericDeclRepository<S extends Signature>
     extends AbstractRepository<S> {
 
+    public static final TypeVariable<?>[] EMPTY_TYPE_VARS = new TypeVariable<?>[0];
+
     /** The formal type parameters.  Lazily initialized. */
     private volatile TypeVariable<?>[] typeParameters;
 
@@ -77,6 +79,9 @@ private TypeVariable<?>[] computeTypeParameters() {
         FormalTypeParameter[] ftps = getTree().getFormalTypeParameters();
         // create array to store reified subtree(s)
         int length = ftps.length;
+        if (length == 0) {
+            return EMPTY_TYPE_VARS;
+        }
         TypeVariable<?>[] typeParameters = new TypeVariable<?>[length];
         // reify all subtrees
         for (int i = 0; i < length; i++) {