natReference.cc   [plain text]

// natReference.cc - Native code for References

/* Copyright (C) 2001  Free Software Foundation

   This file is part of libgcj.

This software is copyrighted work licensed under the terms of the
Libgcj License.  Please consult the file "LIBGCJ_LICENSE" for
details.  */

// Written by Tom Tromey <tromey@redhat.com>

#include <config.h>

#include <gcj/cni.h>
#include <jvm.h>
#include <java/lang/Throwable.h>
#include <java/lang/ref/Reference.h>
#include <java/lang/ref/SoftReference.h>
#include <java/lang/ref/WeakReference.h>
#include <java/lang/ref/PhantomReference.h>
#include <java/lang/ref/ReferenceQueue.h>

static void finalize_reference (jobject ref);
static void finalize_referred_to_object (jobject obj);



enum weight
{
  SOFT = 0,
  WEAK = 1,
  FINALIZE = 2,
  PHANTOM = 3,

  // This is used to mark the head of a list.
  HEAD = 4,

  // This is used to mark a deleted item.
  DELETED = 5
};

// Objects of this type are used in the hash table to keep track of
// the mapping between a finalizable object and the various References
// which refer to it.
struct object_list
{
  // The reference object.  This is NULL for FINALIZE weight.
  jobject reference;

  // The weight of this object.
  enum weight weight;

  // Next in list.
  object_list *next;
};

// Hash table used to hold mapping from object to References.  The
// object_list item in the hash holds the object itself in the
// reference field; chained to it are all the references sorted in
// order of weight (lowest first).
static object_list *hash = NULL;

// Number of slots used in HASH.
static int hash_count = 0;

// Number of slots total in HASH.  Must be power of 2.
static int hash_size = 0;

static object_list *
find_slot (jobject key)
{
  jint hcode = _Jv_HashCode (key);
  /* step must be non-zero, and relatively prime with hash_size. */
  jint step = (hcode ^ (hcode >> 16)) | 1;
  int start_index = hcode & (hash_size - 1);
  int index = start_index;
  int deleted_index = -1;
  for (;;)
    {
      object_list *ptr = &hash[index];
      if (ptr->reference == key)
	return ptr;
      else if (ptr->reference == NULL)
	{
	  if (deleted_index == -1)
	    return ptr;
	  else
	    return &hash[deleted_index];
	}
      else if (ptr->weight == DELETED)
	deleted_index = index;
      index = (index + step) & (hash_size - 1);
      JvAssert (index != start_index);
    }
}

static void
rehash ()
{
  if (hash == NULL)
    {
      hash_size = 1024;
      hash = (object_list *) _Jv_Malloc (hash_size * sizeof (object_list));
      memset (hash, 0, hash_size * sizeof (object_list));
    }
  else
    {
      object_list *old = hash;
      int i = hash_size;

      hash_size *= 2;
      hash = (object_list *) _Jv_Malloc (hash_size * sizeof (object_list));
      memset (hash, 0, hash_size * sizeof (object_list));

      while (--i >= 0)
	{
	  if (old[i].reference == NULL || old[i].weight == DELETED)
	    continue;
	  object_list *newslot = find_slot (old[i].reference);
	  *newslot = old[i];
	}

      _Jv_Free (old);
    }
}

// Remove a Reference.
static void
remove_from_hash (jobject obj)
{
  java::lang::ref::Reference *ref
    = reinterpret_cast<java::lang::ref::Reference *> (obj);
  object_list *head = find_slot (ref->copy);
  object_list **link = &head->next;
  head = head->next;

  while (head && head->reference != ref)
    {
      link = &head->next;
      head = head->next;
    }

  // Remove the slot.
  if (head)
    {
      *link = head->next;
      _Jv_Free (head);
    }
}

// FIXME what happens if an object's finalizer creates a Reference to
// the object, and the object has never before been added to the hash?
// Madness!

// Add an item to the hash table.  If the item is new, we also add a
// finalizer item.  We keep items in the hash table until they are
// completely collected; this lets us know when an item is new, even
// if it has been resurrected after its finalizer has been run.
static void
add_to_hash (java::lang::ref::Reference *the_reference)
{
  JvSynchronize sync (java::lang::ref::Reference::lock);

  if (3 * hash_count >= 2 * hash_size)
    rehash ();

  jobject referent = the_reference->referent;
  object_list *item = find_slot (referent);
  if (item->reference == NULL)
    {
      // New item, so make an entry for the finalizer.
      item->reference = referent;
      item->weight = HEAD;

      item->next = (object_list *) _Jv_Malloc (sizeof (object_list));
      item->next->reference = NULL;
      item->next->weight = FINALIZE;
      item->next->next = NULL;
      ++hash_count;
    }

  object_list *n = (object_list *) _Jv_Malloc (sizeof (object_list));
  n->reference = the_reference;

  enum weight w = PHANTOM;
  if (java::lang::ref::SoftReference::class$.isInstance (the_reference))
    w = SOFT;
  else if (java::lang::ref::WeakReference::class$.isInstance (the_reference))
    w = WEAK;
  n->weight = w;

  object_list **link = &item->next;
  object_list *iter = *link;
  while (iter && iter->weight < n->weight)
    {
      link = &iter->next;
      iter = *link;
    }
  n->next = (*link) ? (*link)->next : NULL;
  *link = n;
}

// This is called when an object is ready to be finalized.  This
// actually implements the appropriate Reference semantics.
static void
finalize_referred_to_object (jobject obj)
{
  JvSynchronize sync (java::lang::ref::Reference::lock);

  object_list *list = find_slot (obj);
  object_list *head = list->next;
  if (head == NULL)
    {
      // We have a truly dead object: the object's finalizer has been
      // run, all the object's references have been processed, and the
      // object is unreachable.  There is, at long last, no way to
      // resurrect it.
      list->weight = DELETED;
      --hash_count;
      return;
    }

  enum weight w = head->weight;
  if (w == FINALIZE)
    {
      // If we have a Reference A to a Reference B, and B is
      // finalized, then we have to take special care to make sure
      // that B is properly deregistered.  This is super gross.  FIXME
      // will it fail if B's finalizer resurrects B?
      if (java::lang::ref::Reference::class$.isInstance (obj))
	finalize_reference (obj);
      else
	_Jv_FinalizeObject (obj);
      list->next = head->next;
      _Jv_Free (head);
    }
  else if (w != SOFT || _Jv_GCCanReclaimSoftReference (obj))
    {
      // If we just decided to reclaim a soft reference, we might as
      // well do all the weak references at the same time.
      if (w == SOFT)
	w = WEAK;

      while (head && head->weight <= w)
	{
	  java::lang::ref::Reference *ref
	    = reinterpret_cast<java::lang::ref::Reference *> (head->reference);
	  // If the copy is already NULL then the user must have
	  // called Reference.clear().
	  if (ref->copy != NULL)
	    {
	      if (w == PHANTOM)
		ref->referent = ref->copy;
	      else
		ref->copy = NULL;
	      ref->enqueue ();
	    }

	  object_list *next = head->next;
	  _Jv_Free (head);
	  head = next;
	}
      list->next = head;
    }

  // Re-register this finalizer.  We always re-register because we
  // can't know until the next collection cycle whether or not the
  // object is truly unreachable.
  _Jv_RegisterFinalizer (obj, finalize_referred_to_object);
}

// This is called when a Reference object is finalized.  If there is a
// Reference pointing to this Reference then that case is handled by
// finalize_referred_to_object.
static void
finalize_reference (jobject ref)
{
  JvSynchronize sync (java::lang::ref::Reference::lock);
  remove_from_hash (ref);
  // The user might have a subclass of Reference with a finalizer.
  _Jv_FinalizeObject (ref);
}

void
::java::lang::ref::Reference::create (jobject ref)
{
  // Nothing says you can't make a Reference with a NULL referent.
  // But there's nothing to do in such a case.
  referent = reinterpret_cast<gnu::gcj::RawData *> (ref);
  copy = referent;
  if (referent != NULL)
    {
      JvSynchronize sync (java::lang::ref::Reference::lock);
      // `this' is a new Reference object.  We register a new
      // finalizer for pointed-to object and we arrange a special
      // finalizer for ourselves as well.
      _Jv_RegisterFinalizer (this, finalize_reference);
      _Jv_RegisterFinalizer (referent, finalize_referred_to_object);
      jobject *objp = reinterpret_cast<jobject *> (&referent);
      _Jv_GCRegisterDisappearingLink (objp);
      add_to_hash (this);
    }
}