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Disruptor - Concurrent Programming Framework
There is a newer version: 2.10.4
/*
 * Copyright 2011 LMAX Ltd.
 *
 * 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.
 */
package com.lmax.disruptor;

import java.util.concurrent.atomic.AtomicBoolean;

/**
 * Convenience class for handling the batching semantics of consuming entries from a {@link RingBuffer}
 * and delegating the available events to a {@link EventHandler}.
 *
 * If the {@link EventHandler} also implements {@link LifecycleAware} it will be notified just after the thread
 * is started and just before the thread is shutdown.
 *
 * @param  event implementation storing the data for sharing during exchange or parallel coordination of an event.
 */
public final class BatchEventProcessor
    implements EventProcessor
{
    private final AtomicBoolean running = new AtomicBoolean(false);
    private ExceptionHandler exceptionHandler = new FatalExceptionHandler();
    private final RingBuffer ringBuffer;
    private final SequenceBarrier sequenceBarrier;
    private final EventHandler eventHandler;
    private final Sequence sequence = new Sequence(Sequencer.INITIAL_CURSOR_VALUE);

    /**
     * Construct a {@link EventProcessor} that will automatically track the progress by updating its sequence when
     * the {@link EventHandler#onEvent(Object, long, boolean)} method returns.
     *
     * @param ringBuffer to which events are published.
     * @param sequenceBarrier on which it is waiting.
     * @param eventHandler is the delegate to which events are dispatched.
     */
    public BatchEventProcessor(final RingBuffer ringBuffer,
                               final SequenceBarrier sequenceBarrier,
                               final EventHandler eventHandler)
    {
        this.ringBuffer = ringBuffer;
        this.sequenceBarrier = sequenceBarrier;
        this.eventHandler = eventHandler;

        if (eventHandler instanceof SequenceReportingEventHandler)
        {
            ((SequenceReportingEventHandler)eventHandler).setSequenceCallback(sequence);
        }
    }

    @Override
    public Sequence getSequence()
    {
        return sequence;
    }

    @Override
    public void halt()
    {
        running.set(false);
        sequenceBarrier.alert();
    }

    /**
     * Set a new {@link ExceptionHandler} for handling exceptions propagated out of the {@link BatchEventProcessor}
     *
     * @param exceptionHandler to replace the existing exceptionHandler.
     */
    public void setExceptionHandler(final ExceptionHandler exceptionHandler)
    {
        if (null == exceptionHandler)
        {
            throw new NullPointerException();
        }

        this.exceptionHandler = exceptionHandler;
    }

    /**
     * It is ok to have another thread rerun this method after a halt().
     */
    @Override
    public void run()
    {
        if (!running.compareAndSet(false, true))
        {
            throw new IllegalStateException("Thread is already running");
        }
        sequenceBarrier.clearAlert();

        notifyStart();

        T event = null;
        long nextSequence = sequence.get() + 1L;
        while (true)
        {
            try
            {
                final long availableSequence = sequenceBarrier.waitFor(nextSequence);
                while (nextSequence <= availableSequence)
                {
                    event = ringBuffer.get(nextSequence);
                    eventHandler.onEvent(event, nextSequence, nextSequence == availableSequence);
                    nextSequence++;
                }

                sequence.set(nextSequence - 1L);
            }
            catch (final AlertException ex)
            {
               if (!running.get())
               {
                   break;
               }
            }
            catch (final Throwable ex)
            {
                exceptionHandler.handleEventException(ex, nextSequence, event);
                sequence.set(nextSequence);
                nextSequence++;
            }
        }

        notifyShutdown();

        running.set(false);
    }

    private void notifyStart()
    {
        if (eventHandler instanceof LifecycleAware)
        {
            try
            {
                ((LifecycleAware)eventHandler).onStart();
            }
            catch (final Throwable ex)
            {
                exceptionHandler.handleOnStartException(ex);
            }
        }
    }

    private void notifyShutdown()
    {
        if (eventHandler instanceof LifecycleAware)
        {
            try
            {
                ((LifecycleAware)eventHandler).onShutdown();
            }
            catch (final Throwable ex)
            {
                exceptionHandler.handleOnShutdownException(ex);
            }
        }
    }
}