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Utility classes for Jetty
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//
// ========================================================================
// Copyright (c) 1995 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under the
// terms of the Eclipse Public License v. 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
// which is available at https://www.apache.org/licenses/LICENSE-2.0.
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//

package org.eclipse.jetty.util.thread;

import java.util.concurrent.Executor;

import org.eclipse.jetty.util.VirtualThreads;
import org.eclipse.jetty.util.annotation.ManagedAttribute;
import org.eclipse.jetty.util.annotation.ManagedObject;
import org.eclipse.jetty.util.component.LifeCycle;

/**
 * A pool for threads.
 * A specialization of Executor interface that provides reporting methods (eg {@link #getThreads()})
 * and the option of configuration methods (e.g. @link {@link SizedThreadPool#setMaxThreads(int)}).
 */
@ManagedObject("Pool of Threads")
public interface ThreadPool extends Executor
{
    /**
     * Blocks until the thread pool is {@link LifeCycle#stop stopped}.
     *
     * @throws InterruptedException if thread was interrupted
     */
    void join() throws InterruptedException;

    /**
     * @return The total number of threads currently in the pool
     */
    @ManagedAttribute("number of threads in pool")
    int getThreads();

    /**
     * @return The number of idle threads in the pool
     */
    @ManagedAttribute("number of idle threads in pool")
    int getIdleThreads();

    /**
     * @return True if the pool is low on threads
     */
    @ManagedAttribute("indicates the pool is low on available threads")
    boolean isLowOnThreads();

    /**
     * Specialized sub-interface of ThreadPool that allows to get/set
     * the minimum and maximum number of threads of the pool.
     */
    interface SizedThreadPool extends ThreadPool
    {
        /**
         * @return the minimum number of threads
         */
        int getMinThreads();

        /**
         * @return the maximum number of threads
         */
        int getMaxThreads();

        /**
         * @param threads the minimum number of threads
         */
        void setMinThreads(int threads);

        /**
         * @param threads the maximum number of threads
         */
        void setMaxThreads(int threads);

        /**
         * @return a ThreadPoolBudget for this sized thread pool,
         * or null of no ThreadPoolBudget can be returned
         */
        default ThreadPoolBudget getThreadPoolBudget()
        {
            return null;
        }
    }

    /**
     * Execute a task immediately without queueing.   This may use a
     * {@code ReservedThread}, a {@code Virtual Thread}, a call to {@link Invocable#invokeNonBlocking(Runnable)},
     * a newly spawned thread, or direct execution.
     *
     * @param executor An executor that may be used
     * @param task The task that must be executed.
     */
    static void executeImmediately(Executor executor, Runnable task)
    {
        if (task == null)
            return;

        if (executor instanceof TryExecutor tryExecutor && tryExecutor.tryExecute(task))
            return;

        Executor virtual = VirtualThreads.getVirtualThreadsExecutor(executor);
        if (virtual != null)
        {
            virtual.execute(task);
            return;
        }

        switch (Invocable.getInvocationType(task))
        {
            case NON_BLOCKING -> task.run();
            case EITHER -> Invocable.invokeNonBlocking(task);
            default ->
            {
                try
                {
                    new Thread(task).start();
                }
                catch (Throwable ignored)
                {
                    task.run();
                }
            }
        }
    }
}