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//  Copyright (c) 1995-2019 Mort Bay Consulting Pty. Ltd.
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package org.eclipse.jetty.util.thread;

import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;

import org.eclipse.jetty.util.component.AbstractLifeCycle;
import org.eclipse.jetty.util.log.Log;
import org.eclipse.jetty.util.log.Logger;

A utility class to perform periodic sweeping of resources.
Sweepable resources may be added to or removed from a Sweeper and the resource implementation decides whether it should be swept or not.
If a Sweepable resources is itself a container of other sweepable resources, it will forward the sweep operation to children resources, and so on recursively.
Typical usage is to add Sweeper as a bean to an existing container:
Server server = new Server();
server.addBean(new Sweeper(), true);
server.start();
 Code that knows it has sweepable resources can then lookup the Sweeper and offer the sweepable resources to it: 
class MyComponent implements Sweeper.Sweepable
{
    private final long creation;
    private volatile destroyed;
    MyComponent(Server server)
    {
        this.creation = System.nanoTime();
        Sweeper sweeper = server.getBean(Sweeper.class);
        sweeper.offer(this);
    }
    void destroy()
    {
        destroyed = true;
    }
    @Override
    public boolean sweep()
    {
        return destroyed;
    }
}

/**
 * <p>A utility class to perform periodic sweeping of resources.</p>
 * <p>{@link Sweepable} resources may be added to or removed from a
 * {@link Sweeper} and the resource implementation decides whether
 * it should be swept or not.</p>
 * <p>If a {@link Sweepable} resources is itself a container of
 * other sweepable resources, it will forward the sweep operation
 * to children resources, and so on recursively.</p>
 * <p>Typical usage is to add {@link Sweeper} as a bean to an existing
 * container:</p>
 * <pre>
 * Server server = new Server();
 * server.addBean(new Sweeper(), true);
 * server.start();
 * </pre>
 * Code that knows it has sweepable resources can then lookup the
 * {@link Sweeper} and offer the sweepable resources to it:
 * <pre>
 * class MyComponent implements Sweeper.Sweepable
 * {
 *     private final long creation;
 *     private volatile destroyed;
 *
 *     MyComponent(Server server)
 *     {
 *         this.creation = System.nanoTime();
 *         Sweeper sweeper = server.getBean(Sweeper.class);
 *         sweeper.offer(this);
 *     }
 *
 *     void destroy()
 *     {
 *         destroyed = true;
 *     }
 *
 *     &#64;Override
 *     public boolean sweep()
 *     {
 *         return destroyed;
 *     }
 * }
 * </pre>
 */
public class Sweeper extends AbstractLifeCycle implements Runnable
{
    private static final Logger LOG = Log.getLogger(Sweeper.class);

    private final AtomicReference<List<Sweepable>> items = new AtomicReference<>();
    private final AtomicReference<Scheduler.Task> task = new AtomicReference<>();
    private final Scheduler scheduler;
    private final long period;

    public Sweeper(Scheduler scheduler, long period)
    {
        this.scheduler = scheduler;
        this.period = period;
    }

    @Override
    protected void doStart() throws Exception
    {
        super.doStart();
        items.set(new CopyOnWriteArrayList<Sweepable>());
        activate();
    }

    @Override
    protected void doStop() throws Exception
    {
        deactivate();
        items.set(null);
        super.doStop();
    }

    public int getSize()
    {
        List<Sweepable> refs = items.get();
        return refs == null ? 0 : refs.size();
    }

    public boolean offer(Sweepable sweepable)
    {
        List<Sweepable> refs = items.get();
        if (refs == null)
            return false;
        refs.add(sweepable);
        if (LOG.isDebugEnabled())
            LOG.debug("Resource offered {}", sweepable);
        return true;
    }

    public boolean remove(Sweepable sweepable)
    {
        List<Sweepable> refs = items.get();
        return refs != null && refs.remove(sweepable);
    }

    @Override
    public void run()
    {
        List<Sweepable> refs = items.get();
        if (refs == null)
            return;
        for (Sweepable sweepable : refs)
        {
            try
            {
                if (sweepable.sweep())
                {
                    refs.remove(sweepable);
                    if (LOG.isDebugEnabled())
                        LOG.debug("Resource swept {}", sweepable);
                }
            }
            catch (Throwable x)
            {
                LOG.info("Exception while sweeping " + sweepable, x);
            }
        }
        activate();
    }

    private void activate()
    {
        if (isRunning())
        {
            Scheduler.Task t = scheduler.schedule(this, period, TimeUnit.MILLISECONDS);
            if (LOG.isDebugEnabled())
                LOG.debug("Scheduled in {} ms sweep task {}", period, t);
            task.set(t);
        }
        else
        {
            if (LOG.isDebugEnabled())
                LOG.debug("Skipping sweep task scheduling");
        }
    }

    private void deactivate()
    {
        Scheduler.Task t = task.getAndSet(null);
        if (t != null)
        {
            boolean cancelled = t.cancel();
            if (LOG.isDebugEnabled())
                LOG.debug("Cancelled ({}) sweep task {}", cancelled, t);
        }
    }

    
A Sweepable resource implements this interface to signal to a Sweeper or to a parent container if it needs to be swept or not.
Typical implementations will check their own internal state and return true or false from sweep() to indicate whether they should be swept.
/**
     * <p>A {@link Sweepable} resource implements this interface to
     * signal to a {@link Sweeper} or to a parent container if it
     * needs to be swept or not.</p>
     * <p>Typical implementations will check their own internal state
     * and return true or false from {@link #sweep()} to indicate
     * whether they should be swept.</p>
     */
    public interface Sweepable
    {
        
Returns:
whether this resource should be swept/**
         * @return whether this resource should be swept
         */
        public boolean sweep();
    }
}