/*
 ***** BEGIN LICENSE BLOCK *****
 * Version: EPL 2.0/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Eclipse Public
 * License Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of
 * the License at http://www.eclipse.org/legal/epl-v20.html
 *
 * Software distributed under the License is distributed on an "AS
 * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
 * implied. See the License for the specific language governing
 * rights and limitations under the License.
 *
 * Copyright (C) 2010 Charles O Nutter <headius@headius.com>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either of the GNU General Public License Version 2 or later (the "GPL"),
 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
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package org.jruby.util.io;

import java.io.IOException;
import java.nio.channels.ClosedSelectorException;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.util.List;

import java.nio.channels.spi.SelectorProvider;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Iterator;

This is a simple implementation of a hard-referenced java.nio.channels.Selector pool. It is intended to allow us to reuse a small pool of selectors rather than creating them new for each use (which causes problem for Windows and its socket-per-selector impl) or saving them per-thread (which causes problems when there are many not-quite-dead threads in flight. The selectors are kept open in the pool and you should call cleanup() for releasing selectors. 
Author:
headius/**
 * This is a simple implementation of a hard-referenced java.nio.channels.Selector
 * pool. It is intended to allow us to reuse a small pool of selectors rather
 * than creating them new for each use (which causes problem for Windows and
 * its socket-per-selector impl) or saving them per-thread (which causes
 * problems when there are many not-quite-dead threads in flight.
 *
 * The selectors are kept open in the pool and you should call {@link #cleanup()}
 * for releasing selectors.
 *
 * @author headius
 */
public class SelectorPool {
    private final Map<SelectorProvider, List<Selector>> pool = new HashMap<SelectorProvider, List<Selector>>();

    
Get a selector from the pool (or create a new one). Selectors come from
the default selector provider on the current JVM.
Throws:
IOException – if there's a problem opening a new selector
Returns:
a java.nio.channels.Selector/**
     * Get a selector from the pool (or create a new one). Selectors come from
     * the default selector provider on the current JVM.
     *
     * @return a java.nio.channels.Selector
     * @throws IOException if there's a problem opening a new selector
     */
    public synchronized Selector get() throws IOException{
        return retrieveFromPool(SelectorProvider.provider());
    }

    
Get a selector from the pool (or create a new one). Selectors come from
the given provider.
Throws:
IOException – if there's a problem opening a new selector
Returns:
a java.nio.channels.Selector/**
     * Get a selector from the pool (or create a new one). Selectors come from
     * the given provider.
     *
     * @return a java.nio.channels.Selector
     * @throws IOException if there's a problem opening a new selector
     */
    public synchronized Selector get(SelectorProvider provider) throws IOException{
        return retrieveFromPool(provider);
    }

    
Put a selector back into the pool.
Params:
selector – the selector to put back/**
     * Put a selector back into the pool.
     *
     * @param selector the selector to put back
     */
    public void put(Selector selector) {
        try {
            for (SelectionKey key : selector.keys()) {
                if (key != null) {
                    key.cancel();
                }
            }

            try {
                selector.selectNow();
            } catch (Exception e) {
                //ignore
            }

            returnToPool(selector);
        } catch (ClosedSelectorException cse) {
            // ignore; this one is not usable and we'll just create a new one the next time
        }
    }
    
    
Clean up a pool.
All selectors in a pool are closed and the pool gets empty.
/**
     * Clean up a pool.
     * 
     * All selectors in a pool are closed and the pool gets empty.
     * 
     */
    public synchronized void cleanup() {
        for (Map.Entry<SelectorProvider, List<Selector>> entry : pool.entrySet()) {
            List<Selector> providerPool = entry.getValue();
            while (!providerPool.isEmpty()) {
                Selector selector = providerPool.remove(providerPool.size() - 1);
                try {
                    selector.close();
                } catch (IOException ioe) {
                    // ignore IOException at termination.
                }
            }
        }
        pool.clear();
    }

    private Selector retrieveFromPool(SelectorProvider provider) throws IOException {
        List<Selector> providerPool = pool.get(provider);
        if (providerPool != null && !providerPool.isEmpty()) {
            return providerPool.remove(providerPool.size() - 1);
        }

        return SelectorFactory.openWithRetryFrom(null, provider);
    }

    private synchronized void returnToPool(Selector selector) {
        if (selector.isOpen()) {
            SelectorProvider provider = selector.provider();
            List<Selector> providerPool = pool.get(provider);
            if (providerPool == null) {
                providerPool = new LinkedList<Selector>();
                pool.put(provider, providerPool);
            }
            providerPool.add(selector);
        }
    }
}