natReference.cc   [plain text]

// natReference.cc - Native code for References

/* Copyright (C) 2001, 2002  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 ();

  // Use `copy' here because the `referent' field has been cleared.
  jobject referent = the_reference->copy;
  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 = 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)
	    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);
    }
}