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/*
* Copyright (c) 1998-2018 Caucho Technology -- all rights reserved
*
* This file is part of Resin(R) Open Source
*
* Each copy or derived work must preserve the copyright notice and this
* notice unmodified.
*
* Resin Open Source is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* Resin Open Source is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, or any warranty
* of NON-INFRINGEMENT. See the GNU General Public License for more
* details.
*
* You should have received a copy of the GNU General Public License
* along with Resin Open Source; if not, write to the
*
* Free Software Foundation, Inc.
* 59 Temple Place, Suite 330
* Boston, MA 02111-1307 USA
*
* @author Scott Ferguson
*/
package com.caucho.util;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReferenceArray;
/**
* FreeList provides a simple class to manage free objects. This is useful
* for large data structures that otherwise would gobble up huge GC time.
*
* The free list is bounded. Freeing an object when the list is full will
* do nothing.
*/
public final class FreeList {
private final int _size;
private final AtomicReferenceArray _freeStack;
private final AtomicInteger _top = new AtomicInteger();
/**
* Create a new free list.
*
* @param initialSize maximum number of free objects to store.
*/
public FreeList(int size)
{
_size = size;
_freeStack = new AtomicReferenceArray(size);
}
/**
* Try to get an object from the free list. Returns null if the free list
* is empty.
*
* @return the new object or null.
*/
public T allocate()
{
int top = _top.get();
if (top > 0 && _top.compareAndSet(top, top - 1))
return _freeStack.getAndSet(top - 1, null);
else
return null;
}
/**
* Frees the object. If the free list is full, the object will be garbage
* collected.
*
* @param obj the object to be freed.
*/
public boolean free(T obj)
{
final int top = _top.get();
if (top < _size) {
boolean isFree = _freeStack.compareAndSet(top, null, obj);
_top.compareAndSet(top, top + 1);
return isFree;
}
else
return false;
}
/**
* Frees the object. If the free list is full, the object will be garbage
* collected.
*
* @param obj the object to be freed.
*/
public boolean freeCareful(T obj)
{
if (checkDuplicate(obj))
throw new IllegalStateException("tried to free object twice: " + obj);
return free(obj);
}
/**
* Debugging to see if the object has already been freed.
*/
public boolean checkDuplicate(T obj)
{
final int top = _top.get();
for (int i = top - 1; i >= 0; i--) {
if (_freeStack.get(i) == obj)
return true;
}
return false;
}
}