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Utility classes for Jetty
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//
//  ========================================================================
//  Copyright (c) 1995-2016 Mort Bay Consulting Pty. Ltd.
//  ------------------------------------------------------------------------
//  All rights reserved. This program and the accompanying materials
//  are made available under the terms of the Eclipse Public License v1.0
//  and Apache License v2.0 which accompanies this distribution.
//
//      The Eclipse Public License is available at
//      http://www.eclipse.org/legal/epl-v10.html
//
//      The Apache License v2.0 is available at
//      http://www.opensource.org/licenses/apache2.0.php
//
//  You may elect to redistribute this code under either of these licenses.
//  ========================================================================
//

package org.eclipse.jetty.util;

/**
 * A callback abstraction that handles completed/failed events of asynchronous operations.
 *
 * Semantically this is equivalent to an optimise Promise<Void>, but callback is a more meaningful
 * name than EmptyPromise
 */
public interface Callback
{
    /**
     * Instance of Adapter that can be used when the callback methods need an empty
     * implementation without incurring in the cost of allocating a new Adapter object.
     */
    Callback NOOP = new Callback()
    {
    };

    /**
     * Callback invoked when the operation completes.
     *
     * @see #failed(Throwable)
     */
    default void succeeded()
    {
    }

    /**
     * Callback invoked when the operation fails.
     * @param x the reason for the operation failure
     */
    default void failed(Throwable x)
    {
    }

    /**
     * @return True if the callback is known to never block the caller
     */
    default boolean isNonBlocking()
    {
        return false;
    }

    /**
     * Callback interface that declares itself as non-blocking
     */
    interface NonBlocking extends Callback
    {
        @Override
        default boolean isNonBlocking()
        {
            return true;
        }
    }

    class Nested implements Callback
    {
        private final Callback callback;

        public Nested(Callback callback)
        {
            this.callback = callback;
        }

        public Nested(Nested nested)
        {
            this.callback = nested.callback;
        }

        @Override
        public void succeeded()
        {
            callback.succeeded();
        }

        @Override
        public void failed(Throwable x)
        {
            callback.failed(x);
        }

        @Override
        public boolean isNonBlocking()
        {
            return callback.isNonBlocking();
        }
    }

    /**
     * Empty implementation of {@link Callback}
     */
    @Deprecated
    class Adapter implements Callback
    {
    }
}