/* 
 * Copyright 2001-2009 James House 
 * 
 * Licensed under the Apache 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.apache.org/licenses/LICENSE-2.0 
 *   
 * Unless required by applicable law or agreed to in writing, software 
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 
 * License for the specific language governing permissions and limitations 
 * under the License.
 * 
 */

/*
 * Previously Copyright (c) 2001-2004 James House
 */
package org.quartz.spi;

import org.quartz.SchedulerConfigException;

/**
 * <p>
 * The interface to be implemented by classes that want to provide a thread
 * pool for the <code>{@link org.quartz.core.QuartzScheduler}</code>'s use.
 * </p>
 *
 * <p>
 * <code>ThreadPool</code> implementation instances should ideally be made
 * for the sole use of Quartz.  Most importantly, when the method
 * <code>blockForAvailableThreads()</code> returns a value of 1 or greater,
 * there must still be at least one available thread in the pool when the
 * method <code>runInThread(Runnable)</code> is called a few moments (or
 * many moments) later.  If this assumption does not hold true, it may
 * result in extra JobStore queries and updates, and if clustering features
 * are being used, it may result in greater imballance of load.
 * </p>
 *
 * @see org.quartz.core.QuartzScheduler
 * 
 * @author James House
 */
public interface ThreadPool {

    /*
     * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     * 
     * Interface.
     * 
     * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     */

    /**
     * <p>
     * Execute the given <code>{@link java.lang.Runnable}</code> in the next
     * available <code>Thread</code>.
     * </p>
     * 
     * <p>
     * The implementation of this interface should not throw exceptions unless
     * there is a serious problem (i.e. a serious misconfiguration). If there
     * are no immediately available threads <code>false</code> should be returned.
     * </p>
     *
     * @return true, if the runnable was assigned to run on a Thread.
     */
    boolean runInThread(Runnable runnable);

    /**
     * <p>
     * Determines the number of threads that are currently available in in
     * the pool.  Useful for determining the number of times
     * <code>runInThread(Runnable)</code> can be called before returning
     * false.
     * </p>
     *
     * <p>The implementation of this method should block until there is at
     * least one available thread.</p>
     *
     * @return the number of currently available threads
     */
    int blockForAvailableThreads();

    /**
     * <p>
     * Called by the QuartzScheduler before the <code>ThreadPool</code> is
     * used, in order to give the it a chance to initialize.
     * </p>
     */
    void initialize() throws SchedulerConfigException;

    /**
     * <p>
     * Called by the QuartzScheduler to inform the <code>ThreadPool</code>
     * that it should free up all of it's resources because the scheduler is
     * shutting down.
     * </p>
     */
    void shutdown(boolean waitForJobsToComplete);

    int getPoolSize();
}