8035412: Cleanup ClassLoaderData::is_alive

Reviewed-by: coleenp, mgerdin

src/share/vm/classfile/classLoaderData.cpp

@@ -73,7 +73,11 @@ ClassLoaderData * ClassLoaderData::_the_null_class_loader_data = NULL;
 
 ClassLoaderData::ClassLoaderData(Handle h_class_loader, bool is_anonymous, Dependencies dependencies) :
   _class_loader(h_class_loader()),
-  _is_anonymous(is_anonymous), _keep_alive(is_anonymous), // initially
+  _is_anonymous(is_anonymous),
+  // An anonymous class loader data doesn't have anything to keep
+  // it from being unloaded during parsing of the anonymous class.
+  // The null-class-loader should always be kept alive.
+  _keep_alive(is_anonymous || h_class_loader.is_null()),
   _metaspace(NULL), _unloading(false), _klasses(NULL),
   _claimed(0), _jmethod_ids(NULL), _handles(NULL), _deallocate_list(NULL),
   _next(NULL), _dependencies(dependencies),
@@ -317,11 +321,15 @@ void ClassLoaderData::unload() {
   }
 }
 
+oop ClassLoaderData::keep_alive_object() const {
+  assert(!keep_alive(), "Don't use with CLDs that are artificially kept alive");
+  return is_anonymous() ? _klasses->java_mirror() : class_loader();
+}
+
 bool ClassLoaderData::is_alive(BoolObjectClosure* is_alive_closure) const {
-  bool alive =
-    is_anonymous() ?
-       is_alive_closure->do_object_b(_klasses->java_mirror()) :
-       class_loader() == NULL || is_alive_closure->do_object_b(class_loader());
+  bool alive = keep_alive() // null class loader and incomplete anonymous klasses.
+      || is_alive_closure->do_object_b(keep_alive_object());
+
   assert(!alive || claimed(), "must be claimed");
   return alive;
 }
@@ -601,8 +609,6 @@ void ClassLoaderDataGraph::keep_alive_oops_do(OopClosure* f, KlassClosure* klass
 
 void ClassLoaderDataGraph::always_strong_oops_do(OopClosure* f, KlassClosure* klass_closure, bool must_claim) {
   if (ClassUnloading) {
-    ClassLoaderData::the_null_class_loader_data()->oops_do(f, klass_closure, must_claim);
-    // keep any special CLDs alive.
     ClassLoaderDataGraph::keep_alive_oops_do(f, klass_closure, must_claim);
   } else {
     ClassLoaderDataGraph::oops_do(f, klass_closure, must_claim);
@@ -689,7 +695,7 @@ bool ClassLoaderDataGraph::do_unloading(BoolObjectClosure* is_alive_closure) {
   bool has_redefined_a_class = JvmtiExport::has_redefined_a_class();
   MetadataOnStackMark md_on_stack;
   while (data != NULL) {
-    if (data->keep_alive() || data->is_alive(is_alive_closure)) {
+    if (data->is_alive(is_alive_closure)) {
       if (has_redefined_a_class) {
         data->classes_do(InstanceKlass::purge_previous_versions);
       }

src/share/vm/classfile/classLoaderData.hpp

@@ -149,7 +149,7 @@ class ClassLoaderData : public CHeapObj<mtClass> {
                            // classes in the class loader are allocated.
   Mutex* _metaspace_lock;  // Locks the metaspace for allocations and setup.
   bool _unloading;         // true if this class loader goes away
-  bool _keep_alive;        // if this CLD can be unloaded for anonymous loaders
+  bool _keep_alive;        // if this CLD is kept alive without a keep_alive_object().
   bool _is_anonymous;      // if this CLD is for an anonymous class
   volatile int _claimed;   // true if claimed, for example during GC traces.
                            // To avoid applying oop closure more than once.
@@ -240,13 +240,16 @@ class ClassLoaderData : public CHeapObj<mtClass> {
 
   oop class_loader() const      { return _class_loader; }
 
+  // The object the GC is using to keep this ClassLoaderData alive.
+  oop keep_alive_object() const;
+
   // Returns true if this class loader data is for a loader going away.
   bool is_unloading() const     {
     assert(!(is_the_null_class_loader_data() && _unloading), "The null class loader can never be unloaded");
     return _unloading;
   }
-  // Anonymous class loader data doesn't have anything to keep them from
-  // being unloaded during parsing the anonymous class.
+
+  // Used to make sure that this CLD is not unloaded.
   void set_keep_alive(bool value) { _keep_alive = value; }
 
   unsigned int identity_hash() {