ConcurrentBag.java

/*
 * Copyright (C) 2013, 2014 Brett Wooldridge
 *
 * 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.zaxxer.hikari.util;

import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_IN_USE;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_NOT_IN_USE;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_REMOVED;
import static com.zaxxer.hikari.util.IConcurrentBagEntry.STATE_RESERVED;

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * This is a specialized concurrent bag that achieves superior performance
 * to LinkedBlockingQueue and LinkedTransferQueue for the purposes of a
 * connection pool.  It uses ThreadLocal storage when possible to avoid
 * locks, but resorts to scanning a common collection if there are no
 * available items in the ThreadLocal list.  Not-in-use items in the
 * ThreadLocal lists can be "stolen" when the borrowing thread has none
 * of its own.  It is a "lock-less" implementation using a specialized
 * AbstractQueuedLongSynchronizer to manage cross-thread signaling.
 *
 * Note that items that are "borrowed" from the bag are not actually
 * removed from any collection, so garbage collection will not occur
 * even if the reference is abandoned.  Thus care must be taken to
 * "requite" borrowed objects otherwise a memory leak will result.  Only
 * the "remove" method can completely remove an object from the bag.
 *
 * @author Brett Wooldridge
 *
 * @param <T> the templated type to store in the bag
 */
@SuppressWarnings("rawtypes")
public class ConcurrentBag<T extends IConcurrentBagEntry> implements AutoCloseable
{
   private static final Logger LOGGER = LoggerFactory.getLogger(ConcurrentBag.class);

   private final QueuedSequenceSynchronizer synchronizer;
   private final CopyOnWriteArrayList<T> sharedList;
   private final boolean weakThreadLocals;

   private final ThreadLocal<List> threadList;
   private final IBagStateListener listener;
   private volatile boolean closed;


   /**
    * Construct a ConcurrentBag with the specified listener.
    *
    * @param listener the IBagStateListener to attach to this bag
    */
   public ConcurrentBag(IBagStateListener listener)
   {
      this.listener = listener;
      this.weakThreadLocals = useWeakThreadLocals();

      this.sharedList = new CopyOnWriteArrayList<>();
      this.synchronizer = new QueuedSequenceSynchronizer();
      if (weakThreadLocals) {
         this.threadList = new ThreadLocal<>(); 
      }
      else {
         this.threadList = new ThreadLocal<List>() {
            @Override
            protected List initialValue()
            {
               return new FastList<>(IConcurrentBagEntry.class, 16);
            }
         };
      }
   }

   /**
    * The method will borrow a BagEntry from the bag, blocking for the
    * specified timeout if none are available.
    * 
    * @param timeout how long to wait before giving up, in units of unit
    * @param timeUnit a <code>TimeUnit</code> determining how to interpret the timeout parameter
    * @return a borrowed instance from the bag or null if a timeout occurs
    * @throws InterruptedException if interrupted while waiting
    */
   @SuppressWarnings("unchecked")
   public T borrow(long timeout, final TimeUnit timeUnit) throws InterruptedException
   {
      // Try the thread-local list first, if there are no blocked threads waiting already
      if (!synchronizer.hasQueuedThreads()) {
         List<?> list = threadList.get();
         if (weakThreadLocals && list == null) {
            list = new ArrayList<>(16);
            threadList.set(list);
         }

         for (int i = list.size() - 1; i >= 0; i--) {
            final T bagEntry = (T) (weakThreadLocals ? ((WeakReference) list.remove(i)).get() : list.remove(i));
            if (bagEntry != null && bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
               return bagEntry;
            }
         }
      }

      // Otherwise, scan the shared list ... for maximum of timeout
      timeout = timeUnit.toNanos(timeout);
      Future<Boolean> addItemFuture = null;
      final long startScan = System.nanoTime();
      final long originTimeout = timeout;
      long startSeq;
      try {
         do {
            do {
               startSeq = synchronizer.currentSequence();
               for (final T bagEntry : sharedList) {
                  if (bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
                     return bagEntry;
                  }
               }
            } while (startSeq < synchronizer.currentSequence());

            if (addItemFuture == null || addItemFuture.isDone()) {
               addItemFuture = listener.addBagItem();
            }
   
            timeout = originTimeout - (System.nanoTime() - startScan);
         } while (timeout > 1000L && synchronizer.waitUntilSequenceExceeded(startSeq, timeout));
      }
      finally {
         synchronizer.signal();
      }

      return null;
   }

   /**
    * This method will return a borrowed object to the bag.  Objects
    * that are borrowed from the bag but never "requited" will result
    * in a memory leak.
    *
    * @param bagEntry the value to return to the bag
    * @throws NullPointerException if value is null
    * @throws IllegalStateException if the requited value was not borrowed from the bag
    */
   @SuppressWarnings("unchecked")
   public void requite(final T bagEntry)
   {
      if (bagEntry.state().compareAndSet(STATE_IN_USE, STATE_NOT_IN_USE)) {
         final List threadLocalList = threadList.get();
         if (threadLocalList != null) {
            threadLocalList.add((weakThreadLocals ? new WeakReference<>(bagEntry) : bagEntry));
         }

         synchronizer.signal();
      }
      else {
         LOGGER.warn("Attempt to remove an object from the bag that does not exist: {}", bagEntry);
      }
   }

   /**
    * Add a new object to the bag for others to borrow.
    *
    * @param bagEntry an object to add to the bag
    */
   public void add(final T bagEntry)
   {
      if (closed) {
         LOGGER.info("ConcurrentBag has been closed, ignoring add()");
         throw new IllegalStateException("ConcurrentBag has been closed, ignoring add()");
      }

      sharedList.add(bagEntry);
      synchronizer.signal();
   }

   /**
    * Remove a value from the bag.  This method should only be called
    * with objects obtained by <code>borrow(long, TimeUnit)</code> or <code>reserve(T)</code>
    *
    * @param bagEntry the value to remove
    * @return true if the entry was removed, false otherwise
    * @throws IllegalStateException if an attempt is made to remove an object
    *         from the bag that was not borrowed or reserved first
    */
   public boolean remove(final T bagEntry)
   {
      if (!bagEntry.state().compareAndSet(STATE_IN_USE, STATE_REMOVED) && !bagEntry.state().compareAndSet(STATE_RESERVED, STATE_REMOVED) && !closed) {
         LOGGER.warn("Attempt to remove an object from the bag that was not borrowed or reserved: {}", bagEntry);
         return false;
      }

      final boolean removed = sharedList.remove(bagEntry);
      if (!removed && !closed) {
         LOGGER.warn("Attempt to remove an object from the bag that does not exist: {}", bagEntry);
      }
      return removed;
   }

   /**
    * Close the bag to further adds.
    */
   @Override
   public void close()
   {
      closed = true;
   }

   /**
    * This method provides a "snapshot" in time of the BagEntry
    * items in the bag in the specified state.  It does not "lock"
    * or reserve items in any way.  Call <code>reserve(T)</code>
    * on items in list before performing any action on them.
    *
    * @param state one of the {@link IConcurrentBagEntry} states
    * @return a possibly empty list of objects having the state specified
    */
   public List<T> values(final int state)
   {
      final ArrayList<T> list = new ArrayList<>(sharedList.size());
      for (final T reference : sharedList) {
         if (reference.state().get() == state) {
            list.add(reference);
         }
      }

      return list;
   }

   /**
    * This method provides a "snapshot" in time of the bag items.  It
    * does not "lock" or reserve items in any way.  Call <code>reserve(T)</code>
    * on items in the list, or understand the concurrency implications of
    * modifying items, before performing any action on them.
    *
    * @return a possibly empty list of (all) bag items
    */
   @SuppressWarnings("unchecked")
   public List<T> values()
   {
      return (List<T>) sharedList.clone();
   }

   /**
    * The method is used to make an item in the bag "unavailable" for
    * borrowing.  It is primarily used when wanting to operate on items
    * returned by the <code>values(int)</code> method.  Items that are
    * reserved can be removed from the bag via <code>remove(T)</code>
    * without the need to unreserve them.  Items that are not removed
    * from the bag can be make available for borrowing again by calling
    * the <code>unreserve(T)</code> method.
    *
    * @param bagEntry the item to reserve
    * @return true if the item was able to be reserved, false otherwise
    */
   public boolean reserve(final T bagEntry)
   {
      return bagEntry.state().compareAndSet(STATE_NOT_IN_USE, STATE_RESERVED);
   }

   /**
    * This method is used to make an item reserved via <code>reserve(T)</code>
    * available again for borrowing.
    *
    * @param bagEntry the item to unreserve
    */
   public void unreserve(final T bagEntry)
   {
      if (bagEntry.state().compareAndSet(STATE_RESERVED, STATE_NOT_IN_USE)) {
         synchronizer.signal();
      }
      else {
         LOGGER.warn("Attempt to relinquish an object to the bag that was not reserved: {}", bagEntry);
      }
   }

   /**
    * Get the number of threads pending (waiting) for an item from the
    * bag to become available.
    *
    * @return the number of threads waiting for items from the bag
    */
   public int getPendingQueue()
   {
      return synchronizer.getQueueLength();
   }

   /**
    * Get a count of the number of items in the specified state at the time of this call.
    *
    * @param state the state of the items to count
    * @return a count of how many items in the bag are in the specified state
    */
   public int getCount(final int state)
   {
      int count = 0;
      for (final T reference : sharedList) {
         if (reference.state().get() == state) {
            count++;
         }
      }
      return count;
   }

   /**
    * Get the total number of items in the bag.
    *
    * @return the number of items in the bag 
    */
   public int size()
   {
      return sharedList.size();
   }

   public void dumpState()
   {
      for (T bagEntry : sharedList) {
         LOGGER.info(bagEntry.toString());
      }
   }

   /**
    * Determine whether to use WeakReferences based on whether there is a
    * custom ClassLoader implementation sitting between this class and the
    * System ClassLoader.
    *
    * @return true if we should use WeakReferences in our ThreadLocals, false otherwise
    */
   private boolean useWeakThreadLocals()
   {
      try {
         if (System.getProperty("com.zaxxer.hikari.useWeakReferences") != null) {   // undocumented manual override of WeakReference behavior
            return Boolean.getBoolean("com.zaxxer.hikari.useWeakReferences");
         }

         return getClass().getClassLoader() != ClassLoader.getSystemClassLoader();
      }
      catch (SecurityException se) {
         return true;
      }
   }
}