8350649: Class unloading accesses/resurrects dead Java mirror after JDK-8346567

Reviewed-by: coleenp, egahlin

Author: shipilev

src/hotspot/share/jfr/recorder/checkpoint/types/jfrTypeSet.cpp

@@ -347,7 +347,7 @@ static void do_write_klass(JfrCheckpointWriter* writer, CldPtr cld, KlassPtr kla
   writer->write(cld != nullptr ? cld_id(cld, leakp) : 0);
   writer->write(mark_symbol(klass, leakp));
   writer->write(package_id(klass, leakp));
-  writer->write(klass->modifier_flags());
+  writer->write(klass->compute_modifier_flags());
   writer->write<bool>(klass->is_hidden());
   if (leakp) {
     assert(IS_LEAKP(klass), "invariant");

src/hotspot/share/oops/klass.hpp

@@ -749,9 +749,14 @@ class Klass : public Metadata {
   virtual void release_C_heap_structures(bool release_constant_pool = true);
 
  public:
-  virtual u2 compute_modifier_flags() const = 0;
+  // Get modifier flags from Java mirror cache.
   int modifier_flags() const;
 
+  // Compute modifier flags from the original data. This also allows
+  // accessing flags when Java mirror is already dead, e.g. during class
+  // unloading.
+  virtual u2 compute_modifier_flags() const = 0;
+
   // JVMTI support
   virtual jint jvmti_class_status() const;