--- a/src/share/vm/classfile/dictionary.cpp	Fri Feb 29 20:03:58 2008 -0800
+++ b/src/share/vm/classfile/dictionary.cpp	Wed Mar 12 18:06:50 2008 -0700
@@ -155,8 +155,8 @@
         for (int i = ik->previous_versions()->length() - 1; i >= 0; i--) {
           // check the previous versions array for GC'ed weak refs
           PreviousVersionNode * pv_node = ik->previous_versions()->at(i);
-          jweak cp_ref = pv_node->prev_constant_pool();
-          assert(cp_ref != NULL, "weak cp ref was unexpectedly cleared");
+          jobject cp_ref = pv_node->prev_constant_pool();
+          assert(cp_ref != NULL, "cp ref was unexpectedly cleared");
           if (cp_ref == NULL) {
             delete pv_node;
             ik->previous_versions()->remove_at(i);

--- a/src/share/vm/memory/compactingPermGenGen.cpp	Fri Feb 29 20:03:58 2008 -0800
+++ b/src/share/vm/memory/compactingPermGenGen.cpp	Wed Mar 12 18:06:50 2008 -0700
@@ -26,9 +26,27 @@
 #include "incls/_compactingPermGenGen.cpp.incl"
 
 
-// Recursively adjust all pointers in an object and all objects by
-// referenced it.  Clear marks on objects in order to prevent visiting
-// any object twice.
+// An ObjectClosure helper: Recursively adjust all pointers in an object
+// and all objects by referenced it. Clear marks on objects in order to
+// prevent visiting any object twice. This helper is used when the
+// RedefineClasses() API has been called.
+
+class AdjustSharedObjectClosure : public ObjectClosure {
+public:
+  void do_object(oop obj) {
+    if (obj->is_shared_readwrite()) {
+      if (obj->mark()->is_marked()) {
+        obj->init_mark();         // Don't revisit this object.
+        obj->adjust_pointers();   // Adjust this object's references.
+      }
+    }
+  }
+};
+
+
+// An OopClosure helper: Recursively adjust all pointers in an object
+// and all objects by referenced it. Clear marks on objects in order
+// to prevent visiting any object twice.
 
 class RecursiveAdjustSharedObjectClosure : public OopClosure {
 public:
@@ -274,15 +292,34 @@
 // objects in the space will page in more objects than we need.
 // Instead, use the system dictionary as strong roots into the read
 // write space.
+//
+// If a RedefineClasses() call has been made, then we have to iterate
+// over the entire shared read-write space in order to find all the
+// objects that need to be forwarded. For example, it is possible for
+// an nmethod to be found and marked in GC phase-1 only for the nmethod
+// to be freed by the time we reach GC phase-3. The underlying method
+// is still marked, but we can't (easily) find it in GC phase-3 so we
+// blow up in GC phase-4. With RedefineClasses() we want replaced code
+// (EMCP or obsolete) to go away (i.e., be collectible) once it is no
+// longer being executed by any thread so we keep minimal attachments
+// to the replaced code. However, we can't guarantee when those EMCP
+// or obsolete methods will be collected so they may still be out there
+// even after we've severed our minimal attachments.
 
 void CompactingPermGenGen::pre_adjust_pointers() {
   if (spec()->enable_shared_spaces()) {
-    RecursiveAdjustSharedObjectClosure blk;
-    Universe::oops_do(&blk);
-    StringTable::oops_do(&blk);
-    SystemDictionary::always_strong_classes_do(&blk);
-    TraversePlaceholdersClosure tpc;
-    SystemDictionary::placeholders_do(&tpc);
+    if (JvmtiExport::has_redefined_a_class()) {
+      // RedefineClasses() requires a brute force approach
+      AdjustSharedObjectClosure blk;
+      rw_space()->object_iterate(&blk);
+    } else {
+      RecursiveAdjustSharedObjectClosure blk;
+      Universe::oops_do(&blk);
+      StringTable::oops_do(&blk);
+      SystemDictionary::always_strong_classes_do(&blk);
+      TraversePlaceholdersClosure tpc;
+      SystemDictionary::placeholders_do(&tpc);
+    }
   }
 }
 

--- a/src/share/vm/oops/instanceKlass.cpp	Fri Feb 29 20:03:58 2008 -0800
+++ b/src/share/vm/oops/instanceKlass.cpp	Wed Mar 12 18:06:50 2008 -0700
@@ -2165,12 +2165,20 @@
   RC_TRACE(0x00000100, ("adding previous version ref for %s @%d, EMCP_cnt=%d",
     ikh->external_name(), _previous_versions->length(), emcp_method_count));
   constantPoolHandle cp_h(ikh->constants());
-  jweak cp_ref = JNIHandles::make_weak_global(cp_h);
+  jobject cp_ref;
+  if (cp_h->is_shared()) {
+    // a shared ConstantPool requires a regular reference; a weak
+    // reference would be collectible
+    cp_ref = JNIHandles::make_global(cp_h);
+  } else {
+    cp_ref = JNIHandles::make_weak_global(cp_h);
+  }
   PreviousVersionNode * pv_node = NULL;
   objArrayOop old_methods = ikh->methods();
 
   if (emcp_method_count == 0) {
-    pv_node = new PreviousVersionNode(cp_ref, NULL);
+    // non-shared ConstantPool gets a weak reference
+    pv_node = new PreviousVersionNode(cp_ref, !cp_h->is_shared(), NULL);
     RC_TRACE(0x00000400,
       ("add: all methods are obsolete; flushing any EMCP weak refs"));
   } else {
@@ -2190,7 +2198,8 @@
         }
       }
     }
-    pv_node = new PreviousVersionNode(cp_ref, method_refs);
+    // non-shared ConstantPool gets a weak reference
+    pv_node = new PreviousVersionNode(cp_ref, !cp_h->is_shared(), method_refs);
   }
 
   _previous_versions->append(pv_node);
@@ -2208,7 +2217,7 @@
     // check the previous versions array for a GC'ed weak refs
     pv_node = _previous_versions->at(i);
     cp_ref = pv_node->prev_constant_pool();
-    assert(cp_ref != NULL, "weak cp ref was unexpectedly cleared");
+    assert(cp_ref != NULL, "cp ref was unexpectedly cleared");
     if (cp_ref == NULL) {
       delete pv_node;
       _previous_versions->remove_at(i);
@@ -2281,7 +2290,7 @@
           // check the previous versions array for a GC'ed weak refs
           pv_node = _previous_versions->at(j);
           cp_ref = pv_node->prev_constant_pool();
-          assert(cp_ref != NULL, "weak cp ref was unexpectedly cleared");
+          assert(cp_ref != NULL, "cp ref was unexpectedly cleared");
           if (cp_ref == NULL) {
             delete pv_node;
             _previous_versions->remove_at(j);
@@ -2379,8 +2388,8 @@
     // been GC'ed
     PreviousVersionNode * pv_node = _previous_versions->at(i);
 
-    jweak cp_ref = pv_node->prev_constant_pool();
-    assert(cp_ref != NULL, "weak reference was unexpectedly cleared");
+    jobject cp_ref = pv_node->prev_constant_pool();
+    assert(cp_ref != NULL, "cp reference was unexpectedly cleared");
     if (cp_ref == NULL) {
       continue;  // robustness
     }
@@ -2440,10 +2449,11 @@
 
 // Construct a PreviousVersionNode entry for the array hung off
 // the instanceKlass.
-PreviousVersionNode::PreviousVersionNode(jweak prev_constant_pool,
-  GrowableArray<jweak>* prev_EMCP_methods) {
+PreviousVersionNode::PreviousVersionNode(jobject prev_constant_pool,
+  bool prev_cp_is_weak, GrowableArray<jweak>* prev_EMCP_methods) {
 
   _prev_constant_pool = prev_constant_pool;
+  _prev_cp_is_weak = prev_cp_is_weak;
   _prev_EMCP_methods = prev_EMCP_methods;
 }
 
@@ -2451,7 +2461,11 @@
 // Destroy a PreviousVersionNode
 PreviousVersionNode::~PreviousVersionNode() {
   if (_prev_constant_pool != NULL) {
-    JNIHandles::destroy_weak_global(_prev_constant_pool);
+    if (_prev_cp_is_weak) {
+      JNIHandles::destroy_weak_global(_prev_constant_pool);
+    } else {
+      JNIHandles::destroy_global(_prev_constant_pool);
+    }
   }
 
   if (_prev_EMCP_methods != NULL) {
@@ -2471,8 +2485,8 @@
   _prev_constant_pool_handle = constantPoolHandle();  // NULL handle
   _prev_EMCP_method_handles = NULL;
 
-  jweak cp_ref = pv_node->prev_constant_pool();
-  assert(cp_ref != NULL, "weak constant pool ref was unexpectedly cleared");
+  jobject cp_ref = pv_node->prev_constant_pool();
+  assert(cp_ref != NULL, "constant pool ref was unexpectedly cleared");
   if (cp_ref == NULL) {
     return;  // robustness
   }

--- a/src/share/vm/oops/instanceKlass.hpp	Fri Feb 29 20:03:58 2008 -0800
+++ b/src/share/vm/oops/instanceKlass.hpp	Wed Mar 12 18:06:50 2008 -0700
@@ -838,11 +838,20 @@
 // A collection point for interesting information about the previous
 // version(s) of an instanceKlass. This class uses weak references to
 // the information so that the information may be collected as needed
-// by the system. A GrowableArray of PreviousVersionNodes is attached
+// by the system. If the information is shared, then a regular
+// reference must be used because a weak reference would be seen as
+// collectible. A GrowableArray of PreviousVersionNodes is attached
 // to the instanceKlass as needed. See PreviousVersionWalker below.
 class PreviousVersionNode : public CHeapObj {
  private:
-  jweak _prev_constant_pool;
+  // A shared ConstantPool is never collected so we'll always have
+  // a reference to it so we can update items in the cache. We'll
+  // have a weak reference to a non-shared ConstantPool until all
+  // of the methods (EMCP or obsolete) have been collected; the
+  // non-shared ConstantPool becomes collectible at that point.
+  jobject _prev_constant_pool;  // regular or weak reference
+  bool    _prev_cp_is_weak;     // true if not a shared ConstantPool
+
   // If the previous version of the instanceKlass doesn't have any
   // EMCP methods, then _prev_EMCP_methods will be NULL. If all the
   // EMCP methods have been collected, then _prev_EMCP_methods can
@@ -850,10 +859,10 @@
   GrowableArray<jweak>* _prev_EMCP_methods;
 
 public:
-  PreviousVersionNode(jweak prev_constant_pool,
+  PreviousVersionNode(jobject prev_constant_pool, bool prev_cp_is_weak,
     GrowableArray<jweak>* prev_EMCP_methods);
   ~PreviousVersionNode();
-  jweak prev_constant_pool() const {
+  jobject prev_constant_pool() const {
     return _prev_constant_pool;
   }
   GrowableArray<jweak>* prev_EMCP_methods() const {