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Carrot2 search results clustering framework. Minimal functional subset (core algorithms and infrastructure, no document sources).
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/*
 * Carrot2 project.
 *
 * Copyright (C) 2002-2013, Dawid Weiss, Stanisław Osiński.
 * All rights reserved.
 *
 * Refer to the full license file "carrot2.LICENSE"
 * in the root folder of the repository checkout or at:
 * http://www.carrot2.org/carrot2.LICENSE
 */

package org.carrot2.util;

import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;

import com.google.common.collect.Lists;
import com.google.common.util.concurrent.ForwardingExecutorService;

/**
 * A number of utility methods for working with the {@link Executor}s framework.
 */
public class ExecutorServiceUtils
{
    final static class AccountingThreadFactory implements ThreadFactory
    {
        private final ClassLoader classLoader;
        private final String baseName;
        private final AtomicInteger counter = new AtomicInteger();
        private final List> threads = 
            Collections.synchronizedList(Lists.> newArrayList());

        public AccountingThreadFactory(ClassLoader classLoader, String baseName)
        {
            this.classLoader = classLoader;
            this.baseName = baseName;
        }

        @Override
        public Thread newThread(Runnable r)
        {
            final Thread t = new Thread(r, "SharedExecutor-" + baseName + "-" + counter.getAndIncrement());
            t.setDaemon(true);
            t.setContextClassLoader(classLoader);
            threads.add(new WeakReference(t));
            return t;
        }

        void join()
        {
            for (WeakReference r : threads) {
                Thread t = r.get();
                if (t != null) {
                    try {
                        t.join();
                    } catch (InterruptedException e) {
                        break;
                    }
                }
            }
        }
    }

    final static class AccountingExecutorService extends ForwardingExecutorService {
        private ExecutorService delegate;
        private AccountingThreadFactory threadFactory;

        public AccountingExecutorService(int maxConcurrentThreads, AccountingThreadFactory threadFactory)
        {
            this.delegate = Executors.newFixedThreadPool(maxConcurrentThreads, threadFactory);
            this.threadFactory = threadFactory;
        }

        public void shutdown()
        {
            super.shutdown();
            threadFactory.join();
        }

        public List shutdownNow()
        {
            final List result = super.shutdownNow();
            threadFactory.join();
            return result;
        }
        
        @Override
        protected ExecutorService delegate()
        {
            return delegate;
        }
    }
    
    /**
     * @return Return an executor service with a fixed thread pool of
     *         maxConcurrentThreads threads and context class loader
     *         initialized to clazz's context class loader.
     *         
     *         A weak reference to the returned object is saved internally to make the
     *         necessary cleanups in Web applications and other dynamic environments
     *         possible. 
     *         See CARROT-388.
     */
    public static ExecutorService createExecutorService(int maxConcurrentThreads,
        Class clazz)
    {
        final String baseName = clazz.getSimpleName();
        final ClassLoader classLoader = clazz.getClassLoader();
        final AccountingThreadFactory threadFactory = 
            new AccountingThreadFactory(classLoader, baseName);
        return new AccountingExecutorService(maxConcurrentThreads, threadFactory);
    }
}