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本文主要研究一下AsyncHttpClient的ChannelPool

ChannelPool

org/asynchttpclient/channel/ChannelPool.java

public interface ChannelPool {/*** Add a channel to the pool** @param channel      an I/O channel* @param partitionKey a key used to retrieve the cached channel* @return true if added.*/boolean offer(Channel channel, Object partitionKey);/*** Remove the channel associated with the uri.** @param partitionKey the partition used when invoking offer* @return the channel associated with the uri*/Channel poll(Object partitionKey);/*** Remove all channels from the cache. A channel might have been associated* with several uri.** @param channel a channel* @return the true if the channel has been removed*/boolean removeAll(Channel channel);/*** Return true if a channel can be cached. A implementation can decide based* on some rules to allow caching Calling this method is equivalent of* checking the returned value of {@link ChannelPool#offer(Channel, Object)}** @return true if a channel can be cached.*/boolean isOpen();/*** Destroy all channels that has been cached by this instance.*/void destroy();/*** Flush partitions based on a predicate** @param predicate the predicate*/void flushPartitions(Predicate<Object> predicate);/*** @return The number of idle channels per host.*/Map<String, Long> getIdleChannelCountPerHost();
}

ChannelPool定义了offer、poll、removeAll、isOpen、destroy、flushPartitions、getIdleChannelCountPerHost方法，它有两个实现类，分别是NoopChannelPool及DefaultChannelPool

NoopChannelPool

org/asynchttpclient/channel/NoopChannelPool.java

public enum NoopChannelPool implements ChannelPool {INSTANCE;@Overridepublic boolean offer(Channel channel, Object partitionKey) {return false;}@Overridepublic Channel poll(Object partitionKey) {return null;}@Overridepublic boolean removeAll(Channel channel) {return false;}@Overridepublic boolean isOpen() {return true;}@Overridepublic void destroy() {}@Overridepublic void flushPartitions(Predicate<Object> predicate) {}@Overridepublic Map<String, Long> getIdleChannelCountPerHost() {return Collections.emptyMap();}
}

NoopChannelPool是个枚举，用枚举实现了单例，其方法默认为空操作

DefaultChannelPool

/*** A simple implementation of {@link ChannelPool} based on a {@link java.util.concurrent.ConcurrentHashMap}*/
public final class DefaultChannelPool implements ChannelPool {private static final Logger LOGGER = LoggerFactory.getLogger(DefaultChannelPool.class);private final ConcurrentHashMap<Object, ConcurrentLinkedDeque<IdleChannel>> partitions = new ConcurrentHashMap<>();private final ConcurrentHashMap<ChannelId, ChannelCreation> channelId2Creation;private final AtomicBoolean isClosed = new AtomicBoolean(false);private final Timer nettyTimer;private final int connectionTtl;private final boolean connectionTtlEnabled;private final int maxIdleTime;private final boolean maxIdleTimeEnabled;private final long cleanerPeriod;private final PoolLeaseStrategy poolLeaseStrategy;public DefaultChannelPool(AsyncHttpClientConfig config, Timer hashedWheelTimer) {this(config.getPooledConnectionIdleTimeout(),config.getConnectionTtl(),hashedWheelTimer,config.getConnectionPoolCleanerPeriod());}public DefaultChannelPool(int maxIdleTime,int connectionTtl,Timer nettyTimer,int cleanerPeriod) {this(maxIdleTime,connectionTtl,PoolLeaseStrategy.LIFO,nettyTimer,cleanerPeriod);}public DefaultChannelPool(int maxIdleTime,int connectionTtl,PoolLeaseStrategy poolLeaseStrategy,Timer nettyTimer,int cleanerPeriod) {this.maxIdleTime = maxIdleTime;this.connectionTtl = connectionTtl;connectionTtlEnabled = connectionTtl > 0;channelId2Creation = connectionTtlEnabled ? new ConcurrentHashMap<>() : null;this.nettyTimer = nettyTimer;maxIdleTimeEnabled = maxIdleTime > 0;this.poolLeaseStrategy = poolLeaseStrategy;this.cleanerPeriod = Math.min(cleanerPeriod, Math.min(connectionTtlEnabled ? connectionTtl : Integer.MAX_VALUE, maxIdleTimeEnabled ? maxIdleTime : Integer.MAX_VALUE));if (connectionTtlEnabled || maxIdleTimeEnabled)scheduleNewIdleChannelDetector(new IdleChannelDetector());}//......
}  

DefaultChannelPool基于ConcurrentHashMap实现了ChannelPool接口，主要的参数为connectionTtl、maxIdleTime、cleanerPeriod、poolLeaseStrategy；cleanerPeriod会取connectionTtl、maxIdleTime、传入的cleanerPeriod的最小值；开启connectionTtl或者maxIdleTime的话，会往nettyTimer添加IdleChannelDetector，延后cleanerPeriod时间执行

offer

  public boolean offer(Channel channel, Object partitionKey) {if (isClosed.get())return false;long now = unpreciseMillisTime();if (isTtlExpired(channel, now))return false;boolean offered = offer0(channel, partitionKey, now);if (connectionTtlEnabled && offered) {registerChannelCreation(channel, partitionKey, now);}return offered;}private boolean isTtlExpired(Channel channel, long now) {if (!connectionTtlEnabled)return false;ChannelCreation creation = channelId2Creation.get(channel.id());return creation != null && now - creation.creationTime >= connectionTtl;}  private boolean offer0(Channel channel, Object partitionKey, long now) {ConcurrentLinkedDeque<IdleChannel> partition = partitions.get(partitionKey);if (partition == null) {partition = partitions.computeIfAbsent(partitionKey, pk -> new ConcurrentLinkedDeque<>());}return partition.offerFirst(new IdleChannel(channel, now));}  private void registerChannelCreation(Channel channel, Object partitionKey, long now) {ChannelId id = channel.id();if (!channelId2Creation.containsKey(id)) {channelId2Creation.putIfAbsent(id, new ChannelCreation(now, partitionKey));}}  

offer接口先判断isTtlExpired，如果channel的存活时间超过connectionTtl则返回false，否则执行offer0，往ConcurrentLinkedDeque添加，若添加成功且connectionTtlEnabled则执行registerChannelCreation，维护创建时间

poll

  /*** {@inheritDoc}*/public Channel poll(Object partitionKey) {IdleChannel idleChannel = null;ConcurrentLinkedDeque<IdleChannel> partition = partitions.get(partitionKey);if (partition != null) {while (idleChannel == null) {idleChannel = poolLeaseStrategy.lease(partition);if (idleChannel == null)// pool is emptybreak;else if (!Channels.isChannelActive(idleChannel.channel)) {idleChannel = null;LOGGER.trace("Channel is inactive, probably remotely closed!");} else if (!idleChannel.takeOwnership()) {idleChannel = null;LOGGER.trace("Couldn't take ownership of channel, probably in the process of being expired!");}}}return idleChannel != null ? idleChannel.channel : null;}

poll方法是根据partitionKey找到对应的ConcurrentLinkedDeque，然后循环执行poolLeaseStrategy.lease(partition)，若idleChannel为null直接break，若isChannelActive为false则重置为null继续循环，若idleChannel.takeOwnership()为false也重置为null继续循环

removeAll

  /*** {@inheritDoc}*/public boolean removeAll(Channel channel) {ChannelCreation creation = connectionTtlEnabled ? channelId2Creation.remove(channel.id()) : null;return !isClosed.get() && creation != null && partitions.get(creation.partitionKey).remove(new IdleChannel(channel, Long.MIN_VALUE));}

removeAll方法会将指定的channel从channelId2Creation及ConcurrentLinkedDeque中移除

isOpen

  /*** {@inheritDoc}*/public boolean isOpen() {return !isClosed.get();}

isOpen则取的isClosed变量

destroy

  /*** {@inheritDoc}*/public void destroy() {if (isClosed.getAndSet(true))return;partitions.clear();if (connectionTtlEnabled) {channelId2Creation.clear();}}

destroy会设置isClosed为true，然后清空partitions及channelId2Creation

flushPartitions

  public void flushPartitions(Predicate<Object> predicate) {for (Map.Entry<Object, ConcurrentLinkedDeque<IdleChannel>> partitionsEntry : partitions.entrySet()) {Object partitionKey = partitionsEntry.getKey();if (predicate.test(partitionKey))flushPartition(partitionKey, partitionsEntry.getValue());}}private void flushPartition(Object partitionKey, ConcurrentLinkedDeque<IdleChannel> partition) {if (partition != null) {partitions.remove(partitionKey);for (IdleChannel idleChannel : partition)close(idleChannel.channel);}}private void close(Channel channel) {// FIXME pity to have to do this hereChannels.setDiscard(channel);if (connectionTtlEnabled) {channelId2Creation.remove(channel.id());}Channels.silentlyCloseChannel(channel);}    

flushPartitions会遍历partitions，然后执行predicate.test，为true则执行flushPartition，它将从partitions移除指定的partitionKey，然后遍历idleChannels挨个执行close

getIdleChannelCountPerHost

  public Map<String, Long> getIdleChannelCountPerHost() {return partitions.values().stream().flatMap(ConcurrentLinkedDeque::stream).map(idle -> idle.getChannel().remoteAddress()).filter(a -> a.getClass() == InetSocketAddress.class).map(a -> (InetSocketAddress) a).map(InetSocketAddress::getHostName).collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));}

getIdleChannelCountPerHost则遍历partitions，然后map出remoteAddress获取hostName，然后进行groupBy

PoolLeaseStrategy

  public enum PoolLeaseStrategy {LIFO {public <E> E lease(Deque<E> d) {return d.pollFirst();}},FIFO {public <E> E lease(Deque<E> d) {return d.pollLast();}};abstract <E> E lease(Deque<E> d);}

PoolLeaseStrategy是个枚举，定义了LIFO及FIFO两个枚举，LIFO则是对Deque执行pollFirst，FIFO则是对Deque执行pollLast

IdleChannelDetector

  private final class IdleChannelDetector implements TimerTask {private boolean isIdleTimeoutExpired(IdleChannel idleChannel, long now) {return maxIdleTimeEnabled && now - idleChannel.start >= maxIdleTime;}private List<IdleChannel> expiredChannels(ConcurrentLinkedDeque<IdleChannel> partition, long now) {// lazy createList<IdleChannel> idleTimeoutChannels = null;for (IdleChannel idleChannel : partition) {boolean isIdleTimeoutExpired = isIdleTimeoutExpired(idleChannel, now);boolean isRemotelyClosed = !Channels.isChannelActive(idleChannel.channel);boolean isTtlExpired = isTtlExpired(idleChannel.channel, now);if (isIdleTimeoutExpired || isRemotelyClosed || isTtlExpired) {LOGGER.debug("Adding Candidate expired Channel {} isIdleTimeoutExpired={} isRemotelyClosed={} isTtlExpired={}", idleChannel.channel, isIdleTimeoutExpired, isRemotelyClosed, isTtlExpired);if (idleTimeoutChannels == null)idleTimeoutChannels = new ArrayList<>(1);idleTimeoutChannels.add(idleChannel);}}return idleTimeoutChannels != null ? idleTimeoutChannels : Collections.emptyList();}private List<IdleChannel> closeChannels(List<IdleChannel> candidates) {// lazy create, only if we hit a non-closeable channelList<IdleChannel> closedChannels = null;for (int i = 0; i < candidates.size(); i++) {// We call takeOwnership here to avoid closing a channel that has just been taken out// of the pool, otherwise we risk closing an active connection.IdleChannel idleChannel = candidates.get(i);if (idleChannel.takeOwnership()) {LOGGER.debug("Closing Idle Channel {}", idleChannel.channel);close(idleChannel.channel);if (closedChannels != null) {closedChannels.add(idleChannel);}} else if (closedChannels == null) {// first non closeable to be skipped, copy all// previously skipped closeable channelsclosedChannels = new ArrayList<>(candidates.size());for (int j = 0; j < i; j++)closedChannels.add(candidates.get(j));}}return closedChannels != null ? closedChannels : candidates;}public void run(Timeout timeout) {if (isClosed.get())return;if (LOGGER.isDebugEnabled())for (Object key : partitions.keySet()) {int size = partitions.get(key).size();if (size > 0) {LOGGER.debug("Entry count for : {} : {}", key, size);}}long start = unpreciseMillisTime();int closedCount = 0;int totalCount = 0;for (ConcurrentLinkedDeque<IdleChannel> partition : partitions.values()) {// store in intermediate unsynchronized lists to minimize// the impact on the ConcurrentLinkedDequeif (LOGGER.isDebugEnabled())totalCount += partition.size();List<IdleChannel> closedChannels = closeChannels(expiredChannels(partition, start));if (!closedChannels.isEmpty()) {if (connectionTtlEnabled) {for (IdleChannel closedChannel : closedChannels)channelId2Creation.remove(closedChannel.channel.id());}partition.removeAll(closedChannels);closedCount += closedChannels.size();}}if (LOGGER.isDebugEnabled()) {long duration = unpreciseMillisTime() - start;if (closedCount > 0) {LOGGER.debug("Closed {} connections out of {} in {} ms", closedCount, totalCount, duration);}}scheduleNewIdleChannelDetector(timeout.task());}}

IdleChannelDetector实现了netty的TimerTask接口，其run方法主要是遍历partitions，通过expiredChannels取出过期的IdleChannel，这里isIdleTimeoutExpired、isRemotelyClosed、isTtlExpired都算在内，然后挨个执行takeOwnership及close，再从channelId2Creation及partition中移除，最后再次调度一下IdleChannelDetector

小结

AsyncHttpClient的ChannelPool定义了offer、poll、removeAll、isOpen、destroy、flushPartitions、getIdleChannelCountPerHost方法，它有两个实现类，分别是NoopChannelPool及DefaultChannelPool；DefaultChannelPool基于ConcurrentHashMap实现了ChannelPool接口，主要的参数为connectionTtl、maxIdleTime、cleanerPeriod、poolLeaseStrategy；cleanerPeriod会取connectionTtl、maxIdleTime、传入的cleanerPeriod的最小值；开启connectionTtl或者maxIdleTime的话，会往nettyTimer添加IdleChannelDetector，延后cleanerPeriod时间执行。

poll方法会判断是active，不是的话继续循环lease，而IdleChannelDetector则会定期检查，isIdleTimeoutExpired、isRemotelyClosed、isTtlExpired都会被close，offer的时候还会判断isTtlExpired，这样子来保证连接的活性。