https://eclipse.org/jetty/jetty-util: Utility classes for Jetty (Webtide) 
//
// ========================================================================
// Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under
// the terms of the Eclipse Public License 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0
//
// This Source Code may also be made available under the following
// Secondary Licenses when the conditions for such availability set
// forth in the Eclipse Public License, v. 2.0 are satisfied:
// the Apache License v2.0 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;

import java.io.Closeable;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Function;
import java.util.stream.Collectors;

import org.eclipse.jetty.util.component.Dumpable;
import org.eclipse.jetty.util.component.DumpableCollection;
import org.eclipse.jetty.util.thread.AutoLock;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


A fast pool of objects, with optional support for multiplexing, max usage count and several optimized strategies plus an optional ThreadLocal cache of the last release entry. 
 When the method close() is called, all Closeables in the pool are also closed. 


Type parameters:
  • <T> – 
/**
 * A fast pool of objects, with optional support for
 * multiplexing, max usage count and several optimized strategies plus
 * an optional {@link ThreadLocal} cache of the last release entry.
 * <p>
 * When the method {@link #close()} is called, all {@link Closeable}s in the pool
 * are also closed.
 * </p>
 * @param <T>
 */
public class Pool<T> implements AutoCloseable, Dumpable
{
    private static final Logger LOGGER = LoggerFactory.getLogger(Pool.class);

    private final List<Entry> entries = new CopyOnWriteArrayList<>();

    private final int maxEntries;
    private final AtomicInteger pending = new AtomicInteger();
    private final StrategyType strategyType;

    /*
     * The cache is used to avoid hammering on the first index of the entry list.
     * Caches can become poisoned (i.e.: containing entries that are in use) when
     * the release isn't done by the acquiring thread or when the entry pool is
     * undersized compared to the load applied on it.
     * When an entry can't be found in the cache, the global list is iterated
     * with the configured strategy so the cache has no visible effect besides performance.
     */
    private final AutoLock lock = new AutoLock();
    private final ThreadLocal<Entry> cache;
    private final AtomicInteger nextIndex;
    private volatile boolean closed;
    private volatile int maxMultiplex = 1;
    private volatile int maxUsageCount = -1;

    
The type of the strategy to use for the pool.
The strategy primarily determines where iteration over the pool entries begins.

/**
     * The type of the strategy to use for the pool.
     * The strategy primarily determines where iteration over the pool entries begins.
     */
    public enum StrategyType
    {
        
A strategy that looks for an entry always starting from the first entry.
It will favour the early entries in the pool, but may contend on them more.

/**
         * A strategy that looks for an entry always starting from the first entry.
         * It will favour the early entries in the pool, but may contend on them more.
         */
        FIRST,

        
A strategy that looks for an entry by iterating from a random starting
index.  No entries are favoured and contention is reduced.

/**
         * A strategy that looks for an entry by iterating from a random starting
         * index.  No entries are favoured and contention is reduced.
         */
        RANDOM,

        
A strategy that uses the Thread.getId() of the current thread to select a starting point for an entry search. Whilst not as performant as using the ThreadLocal cache, it may be suitable when the pool is substantially smaller than the number of available threads. No entries are favoured and contention is reduced. 
/**
         * A strategy that uses the {@link Thread#getId()} of the current thread
         * to select a starting point for an entry search.  Whilst not as performant as
         * using the {@link ThreadLocal} cache, it may be suitable when the pool is substantially smaller
         * than the number of available threads.
         * No entries are favoured and contention is reduced.
         */
        THREAD_ID,

        
A strategy that looks for an entry by iterating from a starting point
that is incremented on every search. This gives similar results to the
random strategy but with more predictable behaviour.
No entries are favoured and contention is reduced.

/**
         * A strategy that looks for an entry by iterating from a starting point
         * that is incremented on every search. This gives similar results to the
         * random strategy but with more predictable behaviour.
         * No entries are favoured and contention is reduced.
         */
        ROUND_ROBIN,
    }

    
Construct a Pool with a specified lookup strategy and no ThreadLocal cache. 
Params:
  • strategyType – The strategy to used for looking up entries.
  • maxEntries – the maximum amount of entries that the pool will accept.
/**
     * Construct a Pool with a specified lookup strategy and no
     * {@link ThreadLocal} cache.
     *
     * @param strategyType The strategy to used for looking up entries.
     * @param maxEntries the maximum amount of entries that the pool will accept.
     */
    public Pool(StrategyType strategyType, int maxEntries)
    {
        this(strategyType, maxEntries, false);
    }

    
Construct a Pool with the specified thread-local cache size and an optional ThreadLocal cache. 
Params:
  • strategyType – The strategy to used for looking up entries.
  • maxEntries – the maximum amount of entries that the pool will accept.
  • cache – True if a ThreadLocal cache should be used to try the most recently released entry.
/**
     * Construct a Pool with the specified thread-local cache size and
     * an optional {@link ThreadLocal} cache.
     * @param strategyType The strategy to used for looking up entries.
     * @param maxEntries the maximum amount of entries that the pool will accept.
     * @param cache True if a {@link ThreadLocal} cache should be used to try the most recently released entry.
     */
    public Pool(StrategyType strategyType, int maxEntries, boolean cache)
    {
        this.maxEntries = maxEntries;
        this.strategyType = strategyType;
        this.cache = cache ? new ThreadLocal<>() : null;
        nextIndex = strategyType == StrategyType.ROUND_ROBIN ? new AtomicInteger() : null;
    }

    public int getReservedCount()
    {
        return pending.get();
    }

    public int getIdleCount()
    {
        return (int)entries.stream().filter(Entry::isIdle).count();
    }

    public int getInUseCount()
    {
        return (int)entries.stream().filter(Entry::isInUse).count();
    }

    public int getMaxEntries()
    {
        return maxEntries;
    }

    public int getMaxMultiplex()
    {
        return maxMultiplex;
    }

    public final void setMaxMultiplex(int maxMultiplex)
    {
        if (maxMultiplex < 1)
            throw new IllegalArgumentException("Max multiplex must be >= 1");
        this.maxMultiplex = maxMultiplex;
    }

    
Get the maximum number of times the entries of the pool
can be acquired.

Returns:the max usage count.
/**
     * Get the maximum number of times the entries of the pool
     * can be acquired.
     * @return the max usage count.
     */
    public int getMaxUsageCount()
    {
        return maxUsageCount;
    }

    
Change the max usage count of the pool's entries. All existing
idle entries over this new max usage are removed and closed.

Params:
  • maxUsageCount – the max usage count.
/**
     * Change the max usage count of the pool's entries. All existing
     * idle entries over this new max usage are removed and closed.
     * @param maxUsageCount the max usage count.
     */
    public final void setMaxUsageCount(int maxUsageCount)
    {
        if (maxUsageCount == 0)
            throw new IllegalArgumentException("Max usage count must be != 0");
        this.maxUsageCount = maxUsageCount;

        // Iterate the entries, remove overused ones and collect a list of the closeable removed ones.
        List<Closeable> copy;
        try (AutoLock l = lock.lock())
        {
            if (closed)
                return;

            copy = entries.stream()
                .filter(entry -> entry.isIdleAndOverUsed() && remove(entry) && entry.pooled instanceof Closeable)
                .map(entry -> (Closeable)entry.pooled)
                .collect(Collectors.toList());
        }

        // Iterate the copy and close the collected entries.
        copy.forEach(IO::close);
    }

    
Create a new disabled slot into the pool. The returned entry must ultimately have the Entry.enable(Object, boolean) method called or be removed via Entry.remove() or remove(Entry). 
Params:
  • allotment – the desired allotment, where each entry handles an allotment of maxMultiplex,
or a negative number to always trigger the reservation of a new entry.
Returns:a disabled entry that is contained in the pool, or null if the pool is closed or if the pool already contains getMaxEntries() entries, or the allotment has already been reserved
/**
     * Create a new disabled slot into the pool.
     * The returned entry must ultimately have the {@link Entry#enable(Object, boolean)}
     * method called or be removed via {@link Pool.Entry#remove()} or
     * {@link Pool#remove(Pool.Entry)}.
     *
     * @param allotment the desired allotment, where each entry handles an allotment of maxMultiplex,
     * or a negative number to always trigger the reservation of a new entry.
     * @return a disabled entry that is contained in the pool,
     * or null if the pool is closed or if the pool already contains
     * {@link #getMaxEntries()} entries, or the allotment has already been reserved
     */
    public Entry reserve(int allotment)
    {
        try (AutoLock l = lock.lock())
        {
            if (closed)
                return null;

            int space = maxEntries - entries.size();
            if (space <= 0)
                return null;

            // The pending count is an AtomicInteger that is only ever incremented here with
            // the lock held.  Thus the pending count can be reduced immediately after the
            // test below, but never incremented.  Thus the allotment limit can be enforced.
            if (allotment >= 0 && (pending.get() * getMaxMultiplex()) >= allotment)
                return null;
            pending.incrementAndGet();

            Entry entry = new Entry();
            entries.add(entry);
            return entry;
        }
    }

    
Acquire an entry from the pool.
Only enabled entries will be returned from this method and their enable method must not be called.

Returns:an entry from the pool or null if none is available.
/**
     * Acquire an entry from the pool.
     * Only enabled entries will be returned from this method and their enable method must not be called.
     * @return an entry from the pool or null if none is available.
     */
    public Entry acquire()
    {
        if (closed)
            return null;

        int size = entries.size();
        if (size == 0)
            return null;

        if (cache != null)
        {
            Pool<T>.Entry entry = cache.get();
            if (entry != null && entry.tryAcquire())
                return entry;
        }

        int index = startIndex(size);

        for (int tries = size; tries-- > 0;)
        {
            try
            {
                Pool<T>.Entry entry = entries.get(index);
                if (entry != null && entry.tryAcquire())
                    return entry;
            }
            catch (IndexOutOfBoundsException e)
            {
                LOGGER.trace("IGNORED", e);
                size = entries.size();
                // Size can be 0 when the pool is in the middle of
                // acquiring a connection while another thread
                // removes the last one from the pool.
                if (size == 0)
                    break;
            }
            index = (index + 1) % size;
        }
        return null;
    }

    private int startIndex(int size)
    {
        switch (strategyType)
        {
            case FIRST:
                return 0;
            case RANDOM:
                return ThreadLocalRandom.current().nextInt(size);
            case ROUND_ROBIN:
                return nextIndex.getAndUpdate(c -> Math.max(0, c + 1)) % size;
            case THREAD_ID:
                return (int)(Thread.currentThread().getId() % size);
            default:
                throw new IllegalArgumentException("Unknown strategy type: " + strategyType);
        }
    }

    
Utility method to acquire an entry from the pool,
reserving and creating a new entry if necessary.

Params:
  • creator – a function to create the pooled value for a reserved entry.
Returns:an entry from the pool or null if none is available.
/**
     * Utility method to acquire an entry from the pool,
     * reserving and creating a new entry if necessary.
     *
     * @param creator a function to create the pooled value for a reserved entry.
     * @return an entry from the pool or null if none is available.
     */
    public Entry acquire(Function<Pool<T>.Entry, T> creator)
    {
        Entry entry = acquire();
        if (entry != null)
            return entry;

        entry = reserve(-1);
        if (entry == null)
            return null;

        T value;
        try
        {
            value = creator.apply(entry);
        }
        catch (Throwable th)
        {
            remove(entry);
            throw th;
        }

        if (value == null)
        {
            remove(entry);
            return null;
        }

        return entry.enable(value, true) ? entry : null;
    }

    
This method will return an acquired object to the pool. Objects
that are acquired from the pool but never released will result
in a memory leak.

Params:
  • entry – the value to return to the pool
Throws:
Returns:true if the entry was released and could be acquired again, false if the entry should be removed by calling remove(Entry) and the object contained by the entry should be disposed.
/**
     * This method will return an acquired object to the pool. Objects
     * that are acquired from the pool but never released will result
     * in a memory leak.
     *
     * @param entry the value to return to the pool
     * @return true if the entry was released and could be acquired again,
     * false if the entry should be removed by calling {@link #remove(Pool.Entry)}
     * and the object contained by the entry should be disposed.
     * @throws NullPointerException if value is null
     */
    public boolean release(Entry entry)
    {
        if (closed)
            return false;

        boolean released = entry.tryRelease();
        if (released && cache != null)
            cache.set(entry);
        return released;
    }

    
Remove a value from the pool.

Params:
  • entry – the value to remove
Returns:true if the entry was removed, false otherwise
/**
     * Remove a value from the pool.
     *
     * @param entry the value to remove
     * @return true if the entry was removed, false otherwise
     */
    public boolean remove(Entry entry)
    {
        if (closed)
            return false;

        if (!entry.tryRemove())
        {
            if (LOGGER.isDebugEnabled())
                LOGGER.debug("Attempt to remove an object from the pool that is still in use: {}", entry);
            return false;
        }

        boolean removed = entries.remove(entry);
        if (!removed && LOGGER.isDebugEnabled())
            LOGGER.debug("Attempt to remove an object from the pool that does not exist: {}", entry);

        return removed;
    }

    public boolean isClosed()
    {
        return closed;
    }

    @Override
    public void close()
    {
        List<Entry> copy;
        try (AutoLock l = lock.lock())
        {
            closed = true;
            copy = new ArrayList<>(entries);
            entries.clear();
        }

        // iterate the copy and close its entries
        for (Entry entry : copy)
        {
            if (entry.tryRemove() && entry.pooled instanceof Closeable)
                IO.close((Closeable)entry.pooled);
        }
    }

    public int size()
    {
        return entries.size();
    }

    public Collection<Entry> values()
    {
        return Collections.unmodifiableCollection(entries);
    }

    @Override
    public void dump(Appendable out, String indent) throws IOException
    {
        Dumpable.dumpObjects(out, indent, this,
            new DumpableCollection("entries", entries));
    }

    @Override
    public String toString()
    {
        return String.format("%s@%x[size=%d closed=%s pending=%d]",
            getClass().getSimpleName(),
            hashCode(),
            entries.size(),
            closed,
            pending.get());
    }

    public class Entry
    {
        // hi: positive=open/maxUsage counter; negative=closed; MIN_VALUE pending
        // lo: multiplexing counter
        private final AtomicBiInteger state;

        // The pooled item.  This is not volatile as it is set once and then never changed.
        // Other threads accessing must check the state field above first, so a good before/after
        // relationship exists to make a memory barrier.
        private T pooled;

        Entry()
        {
            this.state = new AtomicBiInteger(Integer.MIN_VALUE, 0);
        }

        // for testing only
        void setUsageCount(int usageCount)
        {
            this.state.getAndSetHi(usageCount);
        }

        
Enable a reserved entry Entry. An entry returned from the Pool.reserve(int) method must be enabled with this method, once and only once, before it is usable by the pool. The entry may be enabled and not acquired, in which case it is immediately available to be acquired, potentially by another thread; or it can be enabled and acquired atomically so that no other thread can acquire it, although the acquire may still fail if the pool has been closed. 
Params:
  • pooled – The pooled item for the entry
  • acquire – If true the entry is atomically enabled and acquired.
Throws:
Returns:true If the entry was enabled.
/** Enable a reserved entry {@link Entry}.
         * An entry returned from the {@link #reserve(int)} method must be enabled with this method,
         * once and only once, before it is usable by the pool.
         * The entry may be enabled and not acquired, in which case it is immediately available to be
         * acquired, potentially by another thread; or it can be enabled and acquired atomically so that
         * no other thread can acquire it, although the acquire may still fail if the pool has been closed.
         * @param pooled The pooled item for the entry
         * @param acquire If true the entry is atomically enabled and acquired.
         * @return true If the entry was enabled.
         * @throws IllegalStateException if the entry was already enabled
         */
        public boolean enable(T pooled, boolean acquire)
        {
            Objects.requireNonNull(pooled);

            if (state.getHi() != Integer.MIN_VALUE)
            {
                if (state.getHi() == -1)
                    return false; // Pool has been closed
                throw new IllegalStateException("Entry already enabled: " + this);
            }
            this.pooled = pooled;
            int usage = acquire ? 1 : 0;
            if (!state.compareAndSet(Integer.MIN_VALUE, usage, 0, usage))
            {
                this.pooled = null;
                if (state.getHi() == -1)
                    return false; // Pool has been closed
                throw new IllegalStateException("Entry already enabled: " + this);
            }
            pending.decrementAndGet();
            return true;
        }

        public T getPooled()
        {
            return pooled;
        }

        
Release the entry. This is equivalent to calling Pool.release(Entry) passing this entry. 
Returns:true if released.
/**
         * Release the entry.
         * This is equivalent to calling {@link Pool#release(Pool.Entry)} passing this entry.
         * @return true if released.
         */
        public boolean release()
        {
            return Pool.this.release(this);
        }

        
Remove the entry. This is equivalent to calling Pool.remove(Entry) passing this entry. 
Returns:true if remove.
/**
         * Remove the entry.
         * This is equivalent to calling {@link Pool#remove(Pool.Entry)} passing this entry.
         * @return true if remove.
         */
        public boolean remove()
        {
            return Pool.this.remove(this);
        }

        
Try to acquire the entry if possible by incrementing both the usage
count and the multiplex count.

Returns:true if the usage count is <= maxUsageCount and
the multiplex count is maxMultiplex and the entry is not closed,
false otherwise.
/**
         * Try to acquire the entry if possible by incrementing both the usage
         * count and the multiplex count.
         * @return true if the usage count is &lt;= maxUsageCount and
         * the multiplex count is maxMultiplex and the entry is not closed,
         * false otherwise.
         */
        boolean tryAcquire()
        {
            while (true)
            {
                long encoded = state.get();
                int usageCount = AtomicBiInteger.getHi(encoded);
                boolean closed = usageCount < 0;
                int multiplexingCount = AtomicBiInteger.getLo(encoded);
                int currentMaxUsageCount = maxUsageCount;
                if (closed || multiplexingCount >= maxMultiplex || (currentMaxUsageCount > 0 && usageCount >= currentMaxUsageCount))
                    return false;

                // Prevent overflowing the usage counter by capping it at Integer.MAX_VALUE.
                int newUsageCount = usageCount == Integer.MAX_VALUE ? Integer.MAX_VALUE : usageCount + 1;
                if (state.compareAndSet(encoded, newUsageCount, multiplexingCount + 1))
                    return true;
            }
        }

        
Try to release the entry if possible by decrementing the multiplexing
count unless the entity is closed.

Returns:true if the entry was released, false if tryRemove() should be called.
/**
         * Try to release the entry if possible by decrementing the multiplexing
         * count unless the entity is closed.
         * @return true if the entry was released,
         * false if {@link #tryRemove()} should be called.
         */
        boolean tryRelease()
        {
            int newMultiplexingCount;
            int usageCount;
            while (true)
            {
                long encoded = state.get();
                usageCount = AtomicBiInteger.getHi(encoded);
                boolean closed = usageCount < 0;
                if (closed)
                    return false;

                newMultiplexingCount = AtomicBiInteger.getLo(encoded) - 1;
                if (newMultiplexingCount < 0)
                    throw new IllegalStateException("Cannot release an already released entry");

                if (state.compareAndSet(encoded, usageCount, newMultiplexingCount))
                    break;
            }

            int currentMaxUsageCount = maxUsageCount;
            boolean overUsed = currentMaxUsageCount > 0 && usageCount >= currentMaxUsageCount;
            return !(overUsed && newMultiplexingCount == 0);
        }

        
Try to mark the entry as removed.

Returns:true if the entry has to be removed from the containing pool, false otherwise.
/**
         * Try to mark the entry as removed.
         * @return true if the entry has to be removed from the containing pool, false otherwise.
         */
        boolean tryRemove()
        {
            while (true)
            {
                long encoded = state.get();
                int usageCount = AtomicBiInteger.getHi(encoded);
                int multiplexCount = AtomicBiInteger.getLo(encoded);
                int newMultiplexCount = Math.max(multiplexCount - 1, 0);

                boolean removed = state.compareAndSet(usageCount, -1, multiplexCount, newMultiplexCount);
                if (removed)
                {
                    if (usageCount == Integer.MIN_VALUE)
                        pending.decrementAndGet();
                    return newMultiplexCount == 0;
                }
            }
        }

        public boolean isClosed()
        {
            return state.getHi() < 0;
        }

        public boolean isIdle()
        {
            long encoded = state.get();
            return AtomicBiInteger.getHi(encoded) >= 0 && AtomicBiInteger.getLo(encoded) == 0;
        }

        public boolean isInUse()
        {
            long encoded = state.get();
            return AtomicBiInteger.getHi(encoded) >= 0 && AtomicBiInteger.getLo(encoded) > 0;
        }

        public boolean isOverUsed()
        {
            int currentMaxUsageCount = maxUsageCount;
            int usageCount = state.getHi();
            return currentMaxUsageCount > 0 && usageCount >= currentMaxUsageCount;
        }

        boolean isIdleAndOverUsed()
        {
            int currentMaxUsageCount = maxUsageCount;
            long encoded = state.get();
            int usageCount = AtomicBiInteger.getHi(encoded);
            int multiplexCount = AtomicBiInteger.getLo(encoded);
            return currentMaxUsageCount > 0 && usageCount >= currentMaxUsageCount && multiplexCount == 0;
        }

        public int getUsageCount()
        {
            return Math.max(state.getHi(), 0);
        }

        @Override
        public String toString()
        {
            long encoded = state.get();
            int usageCount = AtomicBiInteger.getHi(encoded);
            int multiplexCount = AtomicBiInteger.getLo(encoded);

            String state = usageCount < 0 ? "CLOSED" : multiplexCount == 0 ? "IDLE" : "INUSE";

            return String.format("%s@%x{%s, usage=%d, multiplex=%d/%d, pooled=%s}",
                getClass().getSimpleName(),
                hashCode(),
                state,
                Math.max(usageCount, 0),
                Math.max(multiplexCount, 0),
                getMaxMultiplex(),
                pooled);
        }
    }
}