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/*
Copyright (c) 2007 Health Market Science, Inc.
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library 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. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
USA
You can contact Health Market Science at info@healthmarketscience.com
or at the following address:
Health Market Science
2700 Horizon Drive
Suite 200
King of Prussia, PA 19406
*/
package com.healthmarketscience.jackcess;
import java.io.IOException;
import java.lang.ref.SoftReference;
import java.nio.ByteBuffer;
/**
* Manages a reference to a page buffer.
*
* @author James Ahlborn
*/
public abstract class TempPageHolder {
private static final SoftReference<ByteBuffer> EMPTY_BUFFER_REF =
new SoftReference<ByteBuffer>(null);
protected int _pageNumber = PageChannel.INVALID_PAGE_NUMBER;
protected TempPageHolder() {
}
/**
* Creates a new TempPageHolder.
* @param hard iff true, the TempPageHolder will maintain a hard reference
* to the current page buffer, otherwise will maintain a
* SoftReference.
*/
public static TempPageHolder newHolder(boolean hard) {
if(hard) {
return new HardTempPageHolder();
}
return new SoftTempPageHolder();
}
/**
* @return the currently set page number
*/
public int getPageNumber() {
return _pageNumber;
}
/**
* @return the page for the current page number, reading as necessary,
* position and limit are unchanged
*/
public ByteBuffer getPage(PageChannel pageChannel)
throws IOException
{
return setPage(pageChannel, _pageNumber, false);
}
/**
* Sets the current page number and returns that page
* @return the page for the new page number, reading as necessary, resets
* position
*/
public ByteBuffer setPage(PageChannel pageChannel, int pageNumber)
throws IOException
{
return setPage(pageChannel, pageNumber, true);
}
private ByteBuffer setPage(PageChannel pageChannel, int pageNumber,
boolean rewind)
throws IOException
{
ByteBuffer buffer = getBuffer(pageChannel);
if(pageNumber != _pageNumber) {
_pageNumber = pageNumber;
pageChannel.readPage(buffer, _pageNumber);
} else if(rewind) {
buffer.rewind();
}
return buffer;
}
/**
* Allocates a new buffer in the database (with undefined data) and returns
* a new empty buffer.
*/
public ByteBuffer setNewPage(PageChannel pageChannel)
throws IOException
{
// ditch any current data
clear();
// allocate a new page in the database
_pageNumber = pageChannel.allocateNewPage();
// return a new buffer
return getBuffer(pageChannel);
}
/**
* Forces any current page data to be disregarded (any
* <code>getPage</code>/<code>setPage</code> call must reload page data).
* Does not necessarily release any memory.
*/
public void invalidate() {
possiblyInvalidate(_pageNumber, null);
}
/**
* Forces any current page data to be disregarded if it matches the given
* page number (any <code>getPage</code>/<code>setPage</code> call must
* reload page data) and is not the given buffer. Does not necessarily
* release any memory.
*/
public void possiblyInvalidate(int modifiedPageNumber,
ByteBuffer modifiedBuffer) {
if(modifiedBuffer == getExistingBuffer()) {
// no worries, our buffer was the one modified (or is null, either way
// we'll need to reload)
return;
}
if(modifiedPageNumber == _pageNumber) {
_pageNumber = PageChannel.INVALID_PAGE_NUMBER;
}
}
/**
* Forces any current page data to be disregarded (any
* <code>getPage</code>/<code>setPage</code> call must reload page data) and
* releases any referenced memory.
*/
public void clear() {
invalidate();
}
protected abstract ByteBuffer getExistingBuffer();
protected abstract ByteBuffer getBuffer(PageChannel pageChannel);
/**
* TempPageHolder which has a hard reference to the page buffer.
*/
private static class HardTempPageHolder extends TempPageHolder
{
private ByteBuffer _buffer;
@Override
protected ByteBuffer getExistingBuffer() {
return _buffer;
}
@Override
protected ByteBuffer getBuffer(PageChannel pageChannel) {
if(_buffer == null) {
_buffer = pageChannel.createPageBuffer();
}
return _buffer;
}
@Override
public void clear() {
super.clear();
_buffer = null;
}
}
/**
* TempPageHolder which has a soft reference to the page buffer.
*/
private static class SoftTempPageHolder extends TempPageHolder
{
private SoftReference<ByteBuffer> _buffer = EMPTY_BUFFER_REF;
@Override
protected ByteBuffer getExistingBuffer() {
return _buffer.get();
}
@Override
protected ByteBuffer getBuffer(PageChannel pageChannel) {
ByteBuffer buffer = _buffer.get();
if(buffer == null) {
buffer = pageChannel.createPageBuffer();
_buffer = new SoftReference<ByteBuffer>(buffer);
}
return buffer;
}
@Override
public void clear() {
super.clear();
_buffer.clear();
}
}
}
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