作为常用的http协议服务器，tomcat应用非常广泛。tomcat也是遵循Servelt协议的，Servelt协议可以让服务器与真实服务逻辑代码进行解耦。各自只需要关注Servlet协议即可。
对于tomcat是如何作为一个高性能的服务器的呢？你是不是也会有这样的疑问？
tomcat是如何接收网络请求？
如何做到高性能的http协议服务器？
tomcat从8.0往后开始使用了NIO非阻塞io模型，提高了吞吐量，本文的源码是tomcat 9.0.48版本
org.apache.tomcat.util.net.Acceptor实现了Runnable接口，在一个单独的线程中以死循环的方式一直进行socket的监听
线程的初始化及启动是在方法org.apache.tomcat.util.net.AbstractEndpoint#startAcceptorThread
有个很重要的属性org.apache.tomcat.util.net.AbstractEndpoint；同时实现了run方法，方法中主要有以下功能：
socketChannelpublic void run() { int errorDelay = 0; try { // Loop until we receive a shutdown command while (!stopCalled) { ... if (stopCalled) { break; } state = AcceptorState.RUNNING; try { //if we have reached max connections, wait // 如果连接超过了 8*1024，则线程阻塞等待; 是使用org.apache.tomcat.util.threads.LimitLatch类实现了分享锁(内部实现了AbstractQueuedSynchronizer) // 请你注意到达最大连接数后操作系统底层还是会接收客户端连接，但用户层已经不再接收。 endpoint.countUpOrAwaitConnection(); // Endpoint might have been paused while waiting for latch // If that is the case, don't accept new connections if (endpoint.isPaused()) { continue; } U socket = null; try { // Accept the next incoming connection from the server // socket // 抽象方法，不同的endPoint有不同的实现方法。NioEndPoint为例，实现方法为serverSock.accept()，这个方法主要看serverSock实例化时如果为阻塞，accept方法为阻塞；反之为立即返回，如果没有socket链接，则为null socket = endpoint.serverSocketAccept(); } catch (Exception ioe) { // We didn't get a socket endpoint.countDownConnection(); if (endpoint.isRunning()) { // Introduce delay if necessary errorDelay = handleExceptionWithDelay(errorDelay); // re-throw throw ioe; } else { break; } } // Successful accept, reset the error delay errorDelay = 0; // Configure the socket if (!stopCalled && !endpoint.isPaused()) { // setSocketOptions() will hand the socket off to // an appropriate processor if successful // endPoint类的抽象方法，不同的endPoint有不同的实现。处理获取到的socketChannel链接，如果该socket链接能正常处理，那么该方法会返回true，否则为false if (!endpoint.setSocketOptions(socket)) { endpoint.closeSocket(socket); } } else { endpoint.destroySocket(socket); } } catch (Throwable t) { ... } } } finally { stopLatch.countDown(); } state = AcceptorState.ENDED; }再来看下org.apache.tomcat.util.net.NioEndpoint#setSocketOptions方法的具体实现(NioEndpoint为例)
这个方法中主要做的事：
protected boolean setSocketOptions(SocketChannel socket) { NioSocketWrapper socketWrapper = null; try { // Allocate channel and wrapper // 优先使用已有的缓存nioChannel NioChannel channel = null; if (nioChannels != null) { channel = nioChannels.pop(); } if (channel == null) { SocketBufferHandler bufhandler = new SocketBufferHandler( socketProperties.getAppReadBufSize(), socketProperties.getAppWriteBufSize(), socketProperties.getDirectBuffer()); if (isSSLEnabled()) { channel = new SecureNioChannel(bufhandler, this); } else { channel = new NioChannel(bufhandler); } } // 将nioEndpoint与NioChannel进行包装 NioSocketWrapper newWrapper = new NioSocketWrapper(channel, this); channel.reset(socket, newWrapper); connections.put(socket, newWrapper); socketWrapper = newWrapper; // Set socket properties // Disable blocking, polling will be used // 设置当前链接的socket为非阻塞 socket.configureBlocking(false); if (getUnixDomainSocketPath() == null) { socketProperties.setProperties(socket.socket()); } socketWrapper.setReadTimeout(getConnectionTimeout()); socketWrapper.setWriteTimeout(getConnectionTimeout()); socketWrapper.setKeepAliveLeft(NioEndpoint.this.getMaxKeepAliveRequests()); // 将包装后的nioChannel与nioEndpoint进行注册，注册到Poller，将对应的socket包装类添加到Poller的队列中，同时唤醒selector poller.register(socketWrapper); return true; } catch (Throwable t) { ExceptionUtils.handleThrowable(t); try { log.error(sm.getString("endpoint.socketOptionsError"), t); } catch (Throwable tt) { ExceptionUtils.handleThrowable(tt); } if (socketWrapper == null) { destroySocket(socket); } } // Tell to close the socket if needed return false; }上一小节是接收到了socket请求，进行包装之后，将socket添加到了Poller的队列上，并可能唤醒了Selector，本小节就来看看，Poller是如何进行socket的轮询的。
首先org.apache.tomcat.util.net.NioEndpoint.Poller也是实现了Runnable接口，是一个可以单独启动的线程
初始化及启动是在org.apache.tomcat.util.net.NioEndpoint#startInternal
重要的属性：
java.nio.channels.Selector：在Poller对象初始化的时候，就会启动轮询器SynchronizedQueue<PollerEvent>：同步的事件队列再来看下具体处理逻辑，run方法的源码
public void run() { // Loop until destroy() is called while (true) { boolean hasEvents = false; try { if (!close) { // 去SynchronizedQueue事件队列中拉去，看是否已经有了事件，如果有，则返回true // 如果从队列中拉取到了event(即上一步将NioSocketWrapper封装为PollerEvent添加到次队列中)，将socketChannel注册到Selector上，标记为SelectionKey.OP_READ，添加处理函数attachment(为Accetpor添加到Poller时的 // NioSocketWrapper) hasEvents = events(); if (wakeupCounter.getAndSet(-1) > 0) { // If we are here, means we have other stuff to do // Do a non blocking select keyCount = selector.selectNow(); } else { keyCount = selector.select(selectorTimeout); } wakeupCounter.set(0); } if (close) { events(); timeout(0, false); try { selector.close(); } catch (IOException ioe) { log.error(sm.getString("endpoint.nio.selectorCloseFail"), ioe); } break; } // Either we timed out or we woke up, process events first if (keyCount == 0) { hasEvents = (hasEvents | events()); } } catch (Throwable x) { ExceptionUtils.handleThrowable(x); log.error(sm.getString("endpoint.nio.selectorLoopError"), x); continue; } Iterator<SelectionKey> iterator = keyCount > 0 ? selector.selectedKeys().iterator() : null; // Walk through the collection of ready keys and dispatch // any active event. // selector轮询获取已经注册的事件，如果有事件准备好，此时通过selectKeys方法就能拿到对应的事件 while (iterator != null && iterator.hasNext()) { SelectionKey sk = iterator.next(); // 获取到事件后，从迭代器删除事件，防止事件重复轮询 iterator.remove(); // 获取事件的处理器，这个attachment是在event()方法中注册的，后续这个事件的处理，就交给这个wrapper去处理 NioSocketWrapper socketWrapper = (NioSocketWrapper) sk.attachment(); // Attachment may be null if another thread has called // cancelledKey() if (socketWrapper != null) { processKey(sk, socketWrapper); } } // Process timeouts timeout(keyCount,hasEvents); } getStopLatch().countDown(); }在这里，有一个很重要的方法，org.apache.tomcat.util.net.NioEndpoint.Poller#events()，他是从Poller的事件队列中获取Acceptor接收到的可用socket，并将其注册到Selector上
/** * Processes events in the event queue of the Poller. * * @return <code>true</code> if some events were processed, * <code>false</code> if queue was empty */ public boolean events() { boolean result = false; PollerEvent pe = null; // 如果Acceptor将socket添加到队列中，那么events.poll()方法就能拿到对应的事件，否则拿不到就返回false for (int i = 0, size = events.size(); i < size && (pe = events.poll()) != null; i++ ) { result = true; NioSocketWrapper socketWrapper = pe.getSocketWrapper(); SocketChannel sc = socketWrapper.getSocket().getIOChannel(); int interestOps = pe.getInterestOps(); if (sc == null) { log.warn(sm.getString("endpoint.nio.nullSocketChannel")); socketWrapper.close(); } else if (interestOps == OP_REGISTER) { // 如果是Acceptor刚添加到队列中的事件，那么此时的ops就是OP_REGISTER try {， // 将次socket注册到selector上，标记为OP_READ事件，添加事件触发时处理函数socketWrapper sc.register(getSelector(), SelectionKey.OP_READ, socketWrapper); } catch (Exception x) { log.error(sm.getString("endpoint.nio.registerFail"), x); } } else { // ？？这里的逻辑，不清楚什么情况下会进入到这个分支里面 final SelectionKey key = sc.keyFor(getSelector()); if (key == null) { // The key was cancelled (e.g. due to socket closure) // and removed from the selector while it was being // processed. Count down the connections at this point // since it won't have been counted down when the socket // closed. socketWrapper.close(); } else { final NioSocketWrapper attachment = (NioSocketWrapper) key.attachment(); if (attachment != null) { // We are registering the key to start with, reset the fairness counter. try { int ops = key.interestOps() | interestOps; attachment.interestOps(ops); key.interestOps(ops); } catch (CancelledKeyException ckx) { cancelledKey(key, socketWrapper); } } else { cancelledKey(key, socketWrapper); } } } if (running && !paused && eventCache != null) { pe.reset(); eventCache.push(pe); } } return result; }还有一个重要方法就是org.apache.tomcat.util.net.NioEndpoint.Poller#processKey，上一个方法是获取event，并注册到selector，那这个方法就是通过Selector获取到的数据准备好的event，并开始封装成对应的业务处理线程SocketProcessorBase，扔到线程池里开始处理
protected void processKey(SelectionKey sk, NioSocketWrapper socketWrapper) { try { if (close) { cancelledKey(sk, socketWrapper); } else if (sk.isValid()) { if (sk.isReadable() || sk.isWritable()) { if (socketWrapper.getSendfileData() != null) { processSendfile(sk, socketWrapper, false); } else { unreg(sk, socketWrapper, sk.readyOps()); boolean closeSocket = false; // Read goes before write if (sk.isReadable()) { //这里如果是异步的操作，就会走这里 if (socketWrapper.readOperation != null) { if (!socketWrapper.readOperation.process()) { closeSocket = true; } } else if (socketWrapper.readBlocking) { // readBlocking默认为false synchronized (socketWrapper.readLock) { socketWrapper.readBlocking = false; socketWrapper.readLock.notify(); } } else if (!processSocket(socketWrapper, SocketEvent.OPEN_READ, true)) { // 处理正常的事件，这里的processSocket就要正式开始处理请求了。 // 将对应的事件封装成对应的线程，然后交给线程池去处理正式的请求业务 closeSocket = true; } } if (!closeSocket && sk.isWritable()) { if (socketWrapper.writeOperation != null) { if (!socketWrapper.writeOperation.process()) { closeSocket = true; } } else if (socketWrapper.writeBlocking) { synchronized (socketWrapper.writeLock) { socketWrapper.writeBlocking = false; socketWrapper.writeLock.notify(); } } else if (!processSocket(socketWrapper, SocketEvent.OPEN_WRITE, true)) { closeSocket = true; } } if (closeSocket) { cancelledKey(sk, socketWrapper); } } } } else { // Invalid key cancelledKey(sk, socketWrapper); } } catch (CancelledKeyException ckx) { cancelledKey(sk, socketWrapper); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); log.error(sm.getString("endpoint.nio.keyProcessingError"), t); } }上一步，Selector获取到了就绪的请求socket，然后根据socket注册的触发处理函数等，将这些数据进行封装，扔到了线程池里，开始具体的业务逻辑处理。本节就是从工作线程封装开始，org.apache.tomcat.util.net.SocketProcessorBase为工作线程类的抽象类，实现了Runnable接口，不同的Endpoint实现具体的处理逻辑，本节以NioEndpoint为例
以下为org.apache.tomcat.util.net.AbstractEndpoint#processSocket方法源码
/** * Process the given SocketWrapper with the given status. Used to trigger * processing as if the Poller (for those endpoints that have one) * selected the socket. * * @param socketWrapper The socket wrapper to process * @param event The socket event to be processed * @param dispatch Should the processing be performed on a new * container thread * * @return if processing was triggered successfully */ public boolean processSocket(SocketWrapperBase<S> socketWrapper, SocketEvent event, boolean dispatch) { try { if (socketWrapper == null) { return false; } // 优先使用已经存在的线程 SocketProcessorBase<S> sc = null; if (processorCache != null) { sc = processorCache.pop(); } if (sc == null) { sc = createSocketProcessor(socketWrapper, event); } else { sc.reset(socketWrapper, event); } // 获取线程池。线程池的初始化，是在Acceptor、Poller这两个单独线程启动之前创建 // tomcat使用了自定义的org.apache.tomcat.util.threads.TaskQueue，这块tomcat也进行了小的适配开发 // 核心线程为10个，最大200线程 Executor executor = getExecutor(); if (dispatch && executor != null) { executor.execute(sc); } else { sc.run(); } } catch (RejectedExecutionException ree) { getLog().warn(sm.getString("endpoint.executor.fail", socketWrapper) , ree); return false; } catch (Throwable t) { ExceptionUtils.handleThrowable(t); // This means we got an OOM or similar creating a thread, or that // the pool and its queue are full getLog().error(sm.getString("endpoint.process.fail"), t); return false; } return true; }上面的方法是得到了处理业务逻辑的线程SocketProcessorBase，NioEndpoint内部类org.apache.tomcat.util.net.NioEndpoint.SocketProcessor继承了这个抽象类，也就是具体的业务处理逻辑在org.apache.tomcat.util.net.NioEndpoint.SocketProcessor#doRun方法中，最终调用到我们的Servlet
protected void doRun() { /* * Do not cache and re-use the value of socketWrapper.getSocket() in * this method. If the socket closes the value will be updated to * CLOSED_NIO_CHANNEL and the previous value potentially re-used for * a new connection. That can result in a stale cached value which * in turn can result in unintentionally closing currently active * connections. */ Poller poller = NioEndpoint.this.poller; if (poller == null) { socketWrapper.close(); return; } try { int handshake = -1; try { // 握手相关判断逻辑 ... } catch (IOException x) { ... } // 三次握手成功了 if (handshake == 0) { SocketState state = SocketState.OPEN; // Process the request from this socket // event为SocketEvent.OPEN_READ，这个变量是org.apache.tomcat.util.net.NioEndpoint.Poller#processKey方法赋值 if (event == null) { state = getHandler().process(socketWrapper, SocketEvent.OPEN_READ); } else { // 这里就开始正式处理请求了 state = getHandler().process(socketWrapper, event); } if (state == SocketState.CLOSED) { poller.cancelledKey(getSelectionKey(), socketWrapper); } } else if (handshake == -1 ) { getHandler().process(socketWrapper, SocketEvent.CONNECT_FAIL); poller.cancelledKey(getSelectionKey(), socketWrapper); } else if (handshake == SelectionKey.OP_READ){ socketWrapper.registerReadInterest(); } else if (handshake == SelectionKey.OP_WRITE){ socketWrapper.registerWriteInterest(); } } catch (CancelledKeyException cx) { poller.cancelledKey(getSelectionKey(), socketWrapper); } catch (VirtualMachineError vme) { ExceptionUtils.handleThrowable(vme); } catch (Throwable t) { log.error(sm.getString("endpoint.processing.fail"), t); poller.cancelledKey(getSelectionKey(), socketWrapper); } finally { socketWrapper = null; event = null; //return to cache if (running && !paused && processorCache != null) { processorCache.push(this); } } }Tomcat是如何接收网络请求？
使用java nio的同步非阻塞去进行网络监听。
org.apache.tomcat.util.net.AbstractEndpoint#bindWithCleanup中初始化网络监听、SSL
{ .... serverSock = ServerSocketChannel.open(); socketProperties.setProperties(serverSock.socket()); InetSocketAddress addr = new InetSocketAddress(getAddress(), getPortWithOffset()); // 当应用层面的连接数到达最大值时，操作系统可以继续接收连接，那么操作系统能继续接收的最大连接数就是这个队列长度，可以通过acceptCount 参数配置，默认是 100 serverSock.bind(addr, getAcceptCount()); } serverSock.configureBlocking(true); //mimic APR behavior
org.apache.tomcat.util.net.NioEndpoint#startInternal中初始化业务处理的线程池、连接限制器、Poller线程、Acceptor线程
如何做到高性能的http协议服务器？
Tomcat把接收连接、检测 I/O 事件以及处理请求进行了拆分，用不同规模的线程去做对应的事情，这也是tomcat能高并发处理请求的原因。不让线程阻塞，尽量让CPU忙起来
是怎么设计的呢？
通过接口、抽象类等，将不同的处理逻辑拆分，各司其职
org.apache.tomcat.util.net.AbstractEndpoint：I/O事件的检测、处理逻辑都在这个类的实现类里面。使用模板方法，不同的协议有不同的实现方法。NioEndpoint/Nio2Endpoint/AprEndpoint org.apache.tomcat.util.net.NioEndpoint.Poller：引用了java.nio.channels.Selector，内部有个事件队列，监听I/O事件具体就是在这里做的org.apache.tomcat.util.net.NioEndpoint.NioSocketWrapperorg.apache.tomcat.util.net.NioEndpoint.SocketProcessor: 具体处理请求的线程类