Apache Tomcat如何高并发处理请求

目录

• 介绍
• 接收Socket请求
• Socket请求轮询
• 请求具体处理
• 总结
• 参考：

介绍

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

接收Socket请求

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：

• 请求最大连接数限制： 最大为 8*1024；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
• 获取socketChannel

public 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nullsocket = endpoint.serverSocketAccept();    } catch (Exception ioe) {// We didn"t get a socketendpoint.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falseif (!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：

• 创建NioChannel
• 设置socket为非阻塞
• 将socket添加到Poller的队列中

 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 neededreturn false;    }

Socket请求轮询

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 timeoutstimeout(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socketWrappersc.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);    }}

请求具体处理

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 cacheif (running && !paused && processorCache != null) {    processorCache.push(this);}    }}

总结

• 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.NioSocketWrapper
    • org.apache.tomcat.util.net.NioEndpoint.SocketProcessor: 具体处理请求的线程类

参考：

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