原文地址：https://www.iteye.com/blog/gearever-1844203

ller線程中維護(hù)的這個Selector標(biāo)為主Selector。?
Poller是NIO實(shí)現(xiàn)的主要線程。首先作為events queue的消費(fèi)者，從queue中取出PollerEvent對象，然后將此對象中的channel以O(shè)P_READ事件注冊到主Selector中，然后主Selector執(zhí)行select操作，遍歷出可以讀數(shù)據(jù)的socket，并從Worker線程池中拿到可用的Worker線程，然后將socket傳遞給Worker。整個過程是典型的NIO實(shí)現(xiàn)。?

Worker?
Worker線程拿到Poller傳過來的socket后，將socket封裝在SocketProcessor對象中。然后從Http11ConnectionHandler中取出Http11NioProcessor對象，從Http11NioProcessor中調(diào)用CoyoteAdapter的邏輯，跟BIO實(shí)現(xiàn)一樣。在Worker線程中，會完成從socket中讀取http request，解析成HttpServletRequest對象，分派到相應(yīng)的servlet并完成邏輯，然后將response通過socket發(fā)回client。在從socket中讀數(shù)據(jù)和往socket中寫數(shù)據(jù)的過程，并沒有像典型的非阻塞的NIO的那樣，注冊O(shè)P_READ或OP_WRITE事件到主Selector，而是直接通過socket完成讀寫，這時是阻塞完成的，但是在timeout控制上，使用了NIO的Selector機(jī)制，但是這個Selector并不是Poller線程維護(hù)的主Selector，而是BlockPoller線程中維護(hù)的Selector，稱之為輔Selector。?

NioSelectorPool?
NioEndpoint對象中維護(hù)了一個NioSelecPool對象，這個NioSelectorPool中又維護(hù)了一個BlockPoller線程，這個線程就是基于輔Selector進(jìn)行NIO的邏輯。以執(zhí)行servlet后，得到response，往socket中寫數(shù)據(jù)為例，最終寫的過程調(diào)用NioBlockingSelector的write方法。?

Java代碼??

public?int?write(ByteBuffer?buf,?NioChannel?socket,?long?writeTimeout,MutableInteger?lastWrite)?throws?IOException?{??

????????SelectionKey?key?=?socket.getIOChannel().keyFor(socket.getPoller().getSelector());??

????????if?(?key?==?null?)?throw?new?IOException("Key?no?longer?registered");??

????????KeyAttachment?att?=?(KeyAttachment)?key.attachment();??

????????int?written?=?0;??

????????boolean?timedout?=?false;??

????????int?keycount?=?1;?//assume?we?can?write??

????????long?time?=?System.currentTimeMillis();?//start?the?timeout?timer??

????????try?{??

????????????while?(?(!timedout)?&&?buf.hasRemaining())?{??

????????????????if?(keycount?>?0)?{?//only?write?if?we?were?registered?for?a?write??

????????????????????//直接往socket中寫數(shù)據(jù)??

????????????????????int?cnt?=?socket.write(buf);?//write?the?data??

????????????????????lastWrite.set(cnt);??

????????????????????if?(cnt?==?-1)??

????????????????????????throw?new?EOFException();??

????????????????????written?+=?cnt;??

????????????????????//寫數(shù)據(jù)成功，直接進(jìn)入下一次循環(huán)，繼續(xù)寫??

????????????????????if?(cnt?>?0)?{??

????????????????????????time?=?System.currentTimeMillis();?//reset?our?timeout?timer??

????????????????????????continue;?//we?successfully?wrote,?try?again?without?a?selector??

????????????????????}??

????????????????}??

????????????????//如果寫數(shù)據(jù)返回值cnt等于0，通常是網(wǎng)絡(luò)不穩(wěn)定造成的寫數(shù)據(jù)失敗??

????????????????try?{??

????????????????????//開始一個倒數(shù)計(jì)數(shù)器???

????????????????????if?(?att.getWriteLatch()==null?||?att.getWriteLatch().getCount()==0)?att.startWriteLatch(1);??

????????????????????//將socket注冊到輔Selector，這里poller就是BlockSelector線程??

????????????????????poller.add(att,SelectionKey.OP_WRITE);??

????????????????????//阻塞，直至超時時間喚醒，或者在還沒有達(dá)到超時時間，在BlockSelector中喚醒??

????????????????????att.awaitWriteLatch(writeTimeout,TimeUnit.MILLISECONDS);??

????????????????}catch?(InterruptedException?ignore)?{??

????????????????????Thread.interrupted();??

????????????????}??

????????????????if?(?att.getWriteLatch()!=null?&&?att.getWriteLatch().getCount()>?0)?{??

????????????????????keycount?=?0;??

????????????????}else?{??

????????????????????//還沒超時就喚醒，說明網(wǎng)絡(luò)狀態(tài)恢復(fù)，繼續(xù)下一次循環(huán)，完成寫socket??

????????????????????keycount?=?1;??

????????????????????att.resetWriteLatch();??

????????????????}??

??

????????????????if?(writeTimeout?>?0?&&?(keycount?==?0))??

????????????????????timedout?=?(System.currentTimeMillis()?-?time)?>=?writeTimeout;??

????????????}?//while??

????????????if?(timedout)???

????????????????throw?new?SocketTimeoutException();??

????????}?finally?{??

????????????poller.remove(att,SelectionKey.OP_WRITE);??

????????????if?(timedout?&&?key?!=?null)?{??

????????????????poller.cancelKey(socket,?key);??

????????????}??

????????}??

????????return?written;??

????}??

也就是說當(dāng)socket.write()返回0時，說明網(wǎng)絡(luò)狀態(tài)不穩(wěn)定，這時將socket注冊O(shè)P_WRITE事件到輔Selector，由BlockPoller線程不斷輪詢這個輔Selector，直到發(fā)現(xiàn)這個socket的寫狀態(tài)恢復(fù)了，通過那個倒數(shù)計(jì)數(shù)器，通知Worker線程繼續(xù)寫socket動作。看一下BlockSelector線程的邏輯；?

Java代碼??

public?void?run()?{??

????????????while?(run)?{??

????????????????try?{??

????????????????????......??

??

????????????????????Iterator?iterator?=?keyCount?>?0???selector.selectedKeys().iterator()?:?null;??

????????????????????while?(run?&&?iterator?!=?null?&&?iterator.hasNext())?{??

????????????????????????SelectionKey?sk?=?(SelectionKey)?iterator.next();??

????????????????????????KeyAttachment?attachment?=?(KeyAttachment)sk.attachment();??

????????????????????????try?{??

????????????????????????????attachment.access();??

????????????????????????????iterator.remove();?;??

????????????????????????????sk.interestOps(sk.interestOps()?&?(~sk.readyOps()));??

????????????????????????????if?(?sk.isReadable()?)?{??

????????????????????????????????countDown(attachment.getReadLatch());??

????????????????????????????}??

????????????????????????????//發(fā)現(xiàn)socket可寫狀態(tài)恢復(fù)，將倒數(shù)計(jì)數(shù)器置位，通知Worker線程繼續(xù)??

????????????????????????????if?(sk.isWritable())?{??

????????????????????????????????countDown(attachment.getWriteLatch());??

????????????????????????????}??

????????????????????????}catch?(CancelledKeyException?ckx)?{??

????????????????????????????if?(sk!=null)?sk.cancel();??

????????????????????????????countDown(attachment.getReadLatch());??

????????????????????????????countDown(attachment.getWriteLatch());??

????????????????????????}??

????????????????????}//while??

????????????????}catch?(?Throwable?t?)?{??

????????????????????log.error("",t);??

????????????????}??

????????????}??

????????????events.clear();??

????????????try?{??

????????????????selector.selectNow();//cancel?all?remaining?keys??

????????????}catch(?Exception?ignore?)?{??

????????????????if?(log.isDebugEnabled())log.debug("",ignore);??

????????????}??

????????}??

使用這個輔Selector主要是減少線程間的切換，同時還可減輕主Selector的負(fù)擔(dān)。以上描述了NIO connector工作的主要邏輯，可以看到在設(shè)計(jì)上還是比較精巧的。NIO connector還有一塊就是Comet，有時間再說吧。需要注意的是，上面從Acceptor開始，有很多對象的封裝，NioChannel及其KeyAttachment，PollerEvent和SocketProcessor對象，這些不是每次都重新生成一個新的，都是NioEndpoint分別維護(hù)了它們的對象池；?

Java代碼??

ConcurrentLinkedQueue<SocketProcessor>?processorCache?=?new?ConcurrentLinkedQueue<SocketProcessor>()??

ConcurrentLinkedQueue<KeyAttachment>?keyCache?=?new?ConcurrentLinkedQueue<KeyAttachment>()??

ConcurrentLinkedQueue<PollerEvent>?eventCache?=?new?ConcurrentLinkedQueue<PollerEvent>()??

ConcurrentLinkedQueue<NioChannel>?nioChannels?=?new?ConcurrentLinkedQueue<NioChannel>()??

當(dāng)需要這些對象時，分別從它們的對象池獲取，當(dāng)用完后返回給相應(yīng)的對象池，這樣可以減少因?yàn)閯?chuàng)建及GC對象時的性能消耗。

轉(zhuǎn)載于:https://www.cnblogs.com/davidwang456/articles/11452705.html

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