在使用 Tomcat 的时候，我们只需要在 Servlet 实现类中写我们的业务逻辑代码即可，不需要管 Socket 连接、协议处理要怎么实现，因为这部分作为不经常变动的部分，被封装到了 Tomcat 中，程序员只需要引入 Tomcat 中即可，这也是面向对象编程的经典实践。

那么 Tomcat 中的一次请求都要经过哪些类的处理呢？以及 Tomcat 在处理的时候做了哪些方面的考量和设计呢？

Connector

我们知道，Tomcat 中的 Connector 组件负责 Socket 连接的创建和管理，以及网络字节流的传输。在 Connector 组件中，有三个组件 Endpoint、Processor、Adapt。

• Endpoint：负责 ServerSocket 的创建和循环获取 Socket 连接
• Processor：根据具体的协议，解析字节流中的数据
• Adapt：将 Tomcat Request 转换成 ServletRequest，将 ServletResponse 转换成 Tomcat Response

Tomcat 的设计者将 Endpoint 和 Processor 又做了一次封装，将它们封装在 ProtocolHandler 中，表示协议处理器。

从上面看，请求会先在 Endpoint 对象中被接受，然后在 Processor 中被解析，最后通过 Adapt 转换后发送给 Servlet 容器。

Endpoint

NioEndpoint#startInternal

生命周期方法，在启动 Tomcat 的时候会调用 Server#start -> Service.start -> Connector.start -> ProtocolHandler.start() -> EndPoint.start() -> Endpoint.startInternal()

@Override
public void startInternal() throws Exception {if (!running) {running = true;paused = false;if (socketProperties.getProcessorCache() != 0) {processorCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,socketProperties.getProcessorCache());}if (socketProperties.getEventCache() != 0) {eventCache = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,socketProperties.getEventCache());}if (socketProperties.getBufferPool() != 0) {nioChannels = new SynchronizedStack<>(SynchronizedStack.DEFAULT_SIZE,socketProperties.getBufferPool());}// Create worker collectionif (getExecutor() == null) {createExecutor();}initializeConnectionLatch();// Start poller thread// 启动一个 Poller 线程并赋值给 NioEndpoint 的 poller 成员变量// 这个变量在后续处理 PollerEvent 的时候会用到poller = new Poller();Thread pollerThread = new Thread(poller, getName() + "-Poller");pollerThread.setPriority(threadPriority);pollerThread.setDaemon(true);pollerThread.start();// 启动 Acceptor 线程startAcceptorThread();}
}

AbstractEndpoint#startAcceptorThread

创建并启动 Acceptor 线程

protected void startAcceptorThread() {acceptor = new Acceptor<>(this);String threadName = getName() + "-Acceptor";acceptor.setThreadName(threadName);Thread t = new Thread(acceptor, threadName);t.setPriority(getAcceptorThreadPriority());t.setDaemon(getDaemon());t.start();
}

Acceptor#run

Acceptor 的 run 方法中循环获取 socket 连接，为了减少篇幅和方便阅读，我简化了代码：

public void run() {int errorDelay = 0;long pauseStart = 0;try {// 循环，直到我们接收到一个 shutdown 命令while (!stopCalled) {if (stopCalled) {break;}state = AcceptorState.RUNNING;try {// 如果我们到达了最大连接数，则阻塞该线程endpoint.countUpOrAwaitConnection();if (endpoint.isPaused()) {continue;}U socket = null;try {// 获取 socket 连接socket = endpoint.serverSocketAccept();} catch (Exception ioe) {...}// Successful accept, reset the error delayerrorDelay = 0;if (!stopCalled && !endpoint.isPaused()) {// setSocketOptions() 方法会把 socket 交给合适的 processor 处理if (!endpoint.setSocketOptions(socket)) {endpoint.closeSocket(socket);}} else {endpoint.destroySocket(socket);}} catch (Throwable t) {...}}} finally {stopLatch.countDown();}state = AcceptorState.ENDED;
}

NioEndpoint#setSocketOptions

这里主要做了三件事：

1. 设置 socket 为非阻塞模式
2. 将 socket 封装到 NioSocketWrapper 中
3. 将 NioSocketWrapper 注册到 Poller 中

其中 NioSocketWrapper 是 NioEndpoint 的静态内部类，Poller 是 NioEndpoint 的内部类，它实现了 Runnable 接口。

@Override
protected boolean setSocketOptions(SocketChannel socket) {NioSocketWrapper socketWrapper = null;try {// Allocate channel and wrapperNioChannel 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);}}// NioSocketWrapper 是 NioEndpoint 的静态内部类// 将 socket 封装到 NioSocketWrapper 中NioSocketWrapper newWrapper = new NioSocketWrapper(channel, this);channel.reset(socket, newWrapper);connections.put(socket, newWrapper);socketWrapper = newWrapper;// 设置 socket 为非阻塞模式socket.configureBlocking(false);socketProperties.setProperties(socket.socket());socketWrapper.setReadTimeout(getConnectionTimeout());socketWrapper.setWriteTimeout(getConnectionTimeout());socketWrapper.setKeepAliveLeft(NioEndpoint.this.getMaxKeepAliveRequests());// Poller 是 NioEndpoint 的内部类，它实现了 Runnable 接口poller.register(socketWrapper);return true;} catch (Throwable t) {...}// Tell to close the socket if neededreturn false;
}

Poller#register -> Poller#addEvent

• 在 NioSocketWrapper 中设置感兴趣的事件是 SelectionKey.OP_READ，也就是读事件，此时还没有注册到 Selector 上
• 将 NioSocketWrapper 封装成 PollerEvent，并添加到 Poller 持有的同步队列中

/*** Registers a newly created socket with the poller.** @param socketWrapper The socket wrapper*/
public void register(final NioSocketWrapper socketWrapper) {// 设置感兴趣的事件是读事件socketWrapper.interestOps(SelectionKey.OP_READ);//this is what OP_REGISTER turns into.// 将 NioSocketWrapper 封装成 PollerEventPollerEvent pollerEvent = createPollerEvent(socketWrapper, OP_REGISTER);addEvent(pollerEvent);
}private void addEvent(PollerEvent event) {// 将 PollerEvent 添加到 Poller 中 Tomcat 自定义的同步队列中events.offer(event);if (wakeupCounter.incrementAndGet() == 0) {selector.wakeup();}
}

Poller#run

Poller 线程在一个死循环中，首先通过 events() 方法，将 PollerEvent 同步队列中的 socket 用持有的 Selector 注册感兴趣的事件。

然后获取准备好的通道， 每一个通道都传入 processKey 方法中

@Override
public void run() {// Loop until destroy() is calledwhile (true) {boolean hasEvents = false;try {if (!close) {hasEvents = events();if (wakeupCounter.getAndSet(-1) > 0) {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 firstif (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;// 如果 Selector 存在已经准备好的通道，则遍历就绪的通道集while (iterator != null && iterator.hasNext()) {SelectionKey sk = iterator.next();iterator.remove();NioSocketWrapper socketWrapper = (NioSocketWrapper) sk.attachment();if (socketWrapper != null) {processKey(sk, socketWrapper);}}// Process timeoutstimeout(keyCount,hasEvents);}getStopLatch().countDown();
}

Poller#events

public boolean events() {boolean result = false;PollerEvent pe = null;for (int i = 0, size = events.size(); i < size && (pe = events.poll()) != null; i++ ) {// 遍历存储 PollerEvent 的同步队列，如果队列大于零，则设置返回值 result 为 trueresult = 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) {try {sc.register(getSelector(), SelectionKey.OP_READ, socketWrapper);} catch (Exception x) {log.error(sm.getString("endpoint.nio.registerFail"), x);}} else {// 当前 socketChannel 是否在 selector 中注册final SelectionKey key = sc.keyFor(getSelector());if (key == null) {socketWrapper.close();} else {final NioSocketWrapper attachment = (NioSocketWrapper) key.attachment();if (attachment != null) {try {int ops = key.interestOps() | interestOps;attachment.interestOps(ops);// 在 selector 上注册 READ 事件，每个 Poller 都会创建一个单独的 Selectorkey.interestOps(ops);} catch (CancelledKeyException ckx) {cancelledKey(key, socketWrapper);}} else {cancelledKey(key, socketWrapper);}}}if (running && eventCache != null) {// 重置 PollerEvent 并放回缓存 PollerEvent 的同步栈中pe.reset();eventCache.push(pe);}}return result;
}

Poller#processKey

该方法的核心逻辑是做条件判断，通过判断 socket 准备好的事件的类型，调用不同的方法来处理。如果是读事件，最终会走到 processSocket(socketWrapper, SocketEvent.OPEN_READ, true) 。

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 writeif (sk.isReadable()) {if (socketWrapper.readOperation != null) {if (!socketWrapper.readOperation.process()) {closeSocket = true;}} else if (socketWrapper.readBlocking) {synchronized (socketWrapper.readLock) {socketWrapper.readBlocking = false;socketWrapper.readLock.notify();}} else if (!processSocket(socketWrapper, SocketEvent.OPEN_READ, true)) {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 keycancelledKey(sk, socketWrapper);}} catch (CancelledKeyException ckx) {cancelledKey(sk, socketWrapper);} catch (Throwable t) {ExceptionUtils.handleThrowable(t);log.error(sm.getString("endpoint.nio.keyProcessingError"), t);}
}

Poller#processSocket

该方法用来将 NioSocketWrapper 封装到 SocketProcessor 中，SocketProcessor 是 NioEndpoint 的内部类，它实现了 Runnable 接口，创建好 SocketProcessor 之后将它放到 executor 线程池中执行。

public boolean processSocket(SocketWrapperBase<S> socketWrapper,SocketEvent event, boolean dispatch) {try {if (socketWrapper == null) {return false;}SocketProcessorBase<S> sc = null;if (processorCache != null) {sc = processorCache.pop();}if (sc == null) {sc = createSocketProcessor(socketWrapper, event);} else {sc.reset(socketWrapper, event);}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 fullgetLog().error(sm.getString("endpoint.process.fail"), t);return false;}return true;
}

SocketProcessor#doRun

前面我们说过 Connector 组件中包含了 Endpoint、Processor 和 Adapt 三个组件，三个组件各司其职完成了 Socket 客户端的连接、网络字节流的解析和 Request 和 Response 对象的转换。

在 SocketProcessor 线程会调用 Processor 组件的 process 方法来根据不同的协议解析字节流。

@Override
protected void doRun() {boolean launch = false;try {SocketState state = SocketState.OPEN;// Process the request from this socketif (event == null) {state = getHandler().process(socketWrapper, SocketEvent.OPEN_READ);} else {state = getHandler().process(socketWrapper, event);}if (state == SocketState.CLOSED) {// Close socket and poolsocketWrapper.close();} else if (state == SocketState.UPGRADING) {launch = true;}} catch (VirtualMachineError vme) {ExceptionUtils.handleThrowable(vme);} catch (Throwable t) {log.error(sm.getString("endpoint.processing.fail"), t);if (socketWrapper != null) {((Nio2SocketWrapper) socketWrapper).close();}} finally {...}
}

AbstractProtocol#process

调用 AbstractProcessLight 类的 process 方法

@Override
public SocketState process(SocketWrapperBase<S> wrapper, SocketEvent status) {if (wrapper == null) {// Nothing to do. Socket has been closed.return SocketState.CLOSED;}S socket = wrapper.getSocket();Processor processor = (Processor) wrapper.takeCurrentProcessor();try {...processor.setSslSupport(wrapper.getSslSupport(getProtocol().getClientCertProvider()));SocketState state = SocketState.CLOSED;do {// 调用 AbstractProcessLight 类的 process 方法state = processor.process(wrapper, status);...} while ( state == SocketState.UPGRADING);...if (processor != null) {wrapper.setCurrentProcessor(processor);}return state;} catch(java.net.SocketException e) {...}// Make sure socket/processor is removed from the list of current// connectionsrelease(processor);return SocketState.CLOSED;
}

AbstractProcessLight#process

该方法主要做一些条件判断，如果我们的请求是一个简单的 GET 请求，则会执行到 state = service(socketWrapper); 这行代码。

@Override
public SocketState process(SocketWrapperBase<?> socketWrapper, SocketEvent status)throws IOException {SocketState state = SocketState.CLOSED;Iterator<DispatchType> dispatches = null;do {if (dispatches != null) {DispatchType nextDispatch = dispatches.next();if (getLog().isDebugEnabled()) {getLog().debug("Processing dispatch type: [" + nextDispatch + "]");}state = dispatch(nextDispatch.getSocketStatus());if (!dispatches.hasNext()) {state = checkForPipelinedData(state, socketWrapper);}} else if (status == SocketEvent.DISCONNECT) {// Do nothing here, just wait for it to get recycled} else if (isAsync() || isUpgrade() || state == SocketState.ASYNC_END) {state = dispatch(status);state = checkForPipelinedData(state, socketWrapper);} else if (status == SocketEvent.OPEN_WRITE) {// Extra write event likely after async, ignorestate = SocketState.LONG;} else if (status == SocketEvent.OPEN_READ) {// 读事件state = service(socketWrapper);} else if (status == SocketEvent.CONNECT_FAIL) {logAccess(socketWrapper);} else {state = SocketState.CLOSED;}...} while (state == SocketState.ASYNC_END ||dispatches != null && state != SocketState.CLOSED);return state;
}

Http11Processor#service

Http11Processor 类是 HTTP/1.1 协议的实现，这里会按照请求行、请求头、请求体的顺序解析字节流。

因为代码太多，这里不做展示，请求最终会调用 CoyoteAdapter#service，CoyoteAdapter 是 Adapter 的实现类，它属于 Connector 组件中的 Adapter 组件，用于完成 Request 和 Response 对象的适配工作，并调用 Container 的 Pipeline，至此请求进入到 Servlet 容器中。

@Override
public void service(org.apache.coyote.Request req, org.apache.coyote.Response res)throws Exception {Request request = (Request) req.getNote(ADAPTER_NOTES);Response response = (Response) res.getNote(ADAPTER_NOTES);...try {// Parse and set Catalina and configuration specific// request parameterspostParseSuccess = postParseRequest(req, request, res, response);if (postParseSuccess) {//check valves if we support asyncrequest.setAsyncSupported(connector.getService().getContainer().getPipeline().isAsyncSupported());// Calling the containerconnector.getService().getContainer().getPipeline().getFirst().invoke(request, response);}...} catch (IOException e) {// Ignore} finally {...}
}

总结

简单的梳理了 HTTP 请求进入 Tomcat 的代码调用栈之后，我们可以通过上面的流程画出请求的流程图：

Http11Processor 中主要是对 HTTP 协议的实现，相比于这部分，我对 Endpoint 中的处理更感兴趣，因为这部分更接近操作系统，这里我们只讨论了 Endpoint 在 NIO 模式下的处理流程，在使用 NIO 模式的时候，Tomcat 做了哪些设计和努力来让 Tomcat 能够支持高并发呢？

随着 Tomcat 版本的不断更新，每个版本都会对这部分做一些小优化，该篇文章我用的 Tomcat 是 8.5.84 的版本，其中 Endpoint 中的 Acceptor 和 Poller 都只开启了一个线程，在之后的版本中改成了可以开启多个线程，增加线程可以提高吞吐量。

Container

在上节讲解 Connector 组件的时候我们了解到，Connector 最终会调用到 CoyoteAdapter#service 方法，该方法会通过 Engine 的 Pipeline 将请求发送给 Servlet 容器。

CoyoteAdapter#service

@Override
public void service(org.apache.coyote.Request req, org.apache.coyote.Response res)throws Exception {...try {// Parse and set Catalina and configuration specific// request parameterspostParseSuccess = postParseRequest(req, request, res, response);if (postParseSuccess) {//check valves if we support asyncrequest.setAsyncSupported(connector.getService().getContainer().getPipeline().isAsyncSupported());// Calling the containerconnector.getService().getContainer().getPipeline().getFirst().invoke(request, response);}...} catch (IOException e) {// Ignore} finally {...}
}

在 Servlet 容器中，组件被组织成具有层次结构，大容器包小容器，最小的容器就是 Servlet，Engine -> Host -> Context -> Wrapper。每个容器组件都实现了 Container 接口，且都持有 Pipeline 对象，Pipeline 和 Valve 组合形成了责任链模式。

Pipeline 持有链路中第一个 First Valve 和最后一个 Basic Valve，每个 Valve 持有下一个 Valve 的应用，每个容器在初始化的时候会给 Pipeline 配置好 Basic Valve，每个 Pipeline 的 Basic Valve 会调用子容器 Pipeline 的 First Valve，于是请求就可以在容器中流转。

我们启动本地的 Tomcat，并发起一个 GET 请求，在 IDEA 中 debug 看在默认配置下各容器都持有哪些 Valve。

首先请求会到 StandardEngine，可以看出持有的是 StandardPipeline，它没有 First Valve，只有 Basic Valve，所以它只有一个 Valve，在该 Valve 中完成请求的转发。

之后请求到 StandardHost，它的 StandardPipeline 中既有 First Valve 也有 Basic Valve，Pipeline 中整个责任链如下：

• NonLoginAuthenticator

• StandardContextValve

之后请求到 StandardHost，它的 StandardPipeline 也只有一个 Basic Valve。

最后是 StandardWrapper，它的 StandardPipeline 也只有一个 Basic Valve。

StandardWrapperValve

有上可知，StandardWrapperValve 应该是最后一个 Valve 了，再之后就是 Servlet 了。这里单独拿出来细究是怎么到 Servlet 的 service 方法的。

@Override
public final void invoke(Request request, Response response)throws IOException, ServletException {...// Allocate a servlet instance to process this requesttry {if (!unavailable) {// 实例化 Servlet，Servlet 是延迟实例化的，只有用到了才会实例化，在实例化的时候会调用 Servlet 的 init 方法servlet = wrapper.allocate();}} catch (UnavailableException e) {...}...// Create the filter chain for this request// 为该请求创建 filterChainApplicationFilterChain filterChain =ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);// Call the filter chain for this request// NOTE: This also calls the servlet's service() methodContainer container = this.container;try {if ((servlet != null) && (filterChain != null)) {// Swallow output if neededif (context.getSwallowOutput()) {try {SystemLogHandler.startCapture();if (request.isAsyncDispatching()) {request.getAsyncContextInternal().doInternalDispatch();} else {// 触发过滤器链filterChain.doFilter(request.getRequest(),response.getResponse());}} finally {String log = SystemLogHandler.stopCapture();if (log != null && log.length() > 0) {context.getLogger().info(log);}}} else {if (request.isAsyncDispatching()) {request.getAsyncContextInternal().doInternalDispatch();} else {// 触发过滤器链filterChain.doFilter(request.getRequest(), response.getResponse());}}}} catch (ClientAbortException | CloseNowException e) {...} finally {// 释放资源...}
}

由代码可知会先创建一个 filterChain，servlet 就是在这里面被调用的，我们继续看创建 filterChain 的方法。

public static ApplicationFilterChain createFilterChain(ServletRequest request,Wrapper wrapper, Servlet servlet) {// If there is no servlet to execute, return nullif (servlet == null) {return null;}// Create and initialize a filter chain objectApplicationFilterChain filterChain = null;if (request instanceof Request) {Request req = (Request) request;if (Globals.IS_SECURITY_ENABLED) {// Security: Do not recyclefilterChain = new ApplicationFilterChain();} else {filterChain = (ApplicationFilterChain) req.getFilterChain();if (filterChain == null) {filterChain = new ApplicationFilterChain();req.setFilterChain(filterChain);}}} else {// Request dispatcher in usefilterChain = new ApplicationFilterChain();}filterChain.setServlet(servlet);filterChain.setServletSupportsAsync(wrapper.isAsyncSupported());// Acquire the filter mappings for this ContextStandardContext context = (StandardContext) wrapper.getParent();FilterMap filterMaps[] = context.findFilterMaps();// If there are no filter mappings, we are doneif ((filterMaps == null) || (filterMaps.length == 0)) {return filterChain;}// Acquire the information we will need to match filter mappingsDispatcherType dispatcher =(DispatcherType) request.getAttribute(Globals.DISPATCHER_TYPE_ATTR);String requestPath = null;Object attribute = request.getAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR);if (attribute != null){requestPath = attribute.toString();}String servletName = wrapper.getName();// Add the relevant path-mapped filters to this filter chainfor (FilterMap filterMap : filterMaps) {if (!matchDispatcher(filterMap, dispatcher)) {continue;}if (!matchFiltersURL(filterMap, requestPath)) {continue;}ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)context.findFilterConfig(filterMap.getFilterName());if (filterConfig == null) {// FIXME - log configuration problemcontinue;}filterChain.addFilter(filterConfig);}// Add filters that match on servlet name secondfor (FilterMap filterMap : filterMaps) {if (!matchDispatcher(filterMap, dispatcher)) {continue;}if (!matchFiltersServlet(filterMap, servletName)) {continue;}ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)context.findFilterConfig(filterMap.getFilterName());if (filterConfig == null) {// FIXME - log configuration problemcontinue;}filterChain.addFilter(filterConfig);}// Return the completed filter chainreturn filterChain;
}

1. 通过 new 关键字创建 ApplicationFilterChain
2. 获取 Context 中所有的 Filter 实例，并通过路径和 servlet name 筛选出匹配当前 Servlet 的 Filter 实例添加到 ApplicationFilterChain 中

创建成功后调用 filterChain.doFilter() 方法触发过滤器链。

@Override
public void doFilter(ServletRequest request, ServletResponse response)throws IOException, ServletException {if( Globals.IS_SECURITY_ENABLED ) {final ServletRequest req = request;final ServletResponse res = response;try {java.security.AccessController.doPrivileged(new java.security.PrivilegedExceptionAction<Void>() {@Overridepublic Void run()throws ServletException, IOException {internalDoFilter(req,res);return null;}});} catch( PrivilegedActionException pe) {Exception e = pe.getException();if (e instanceof ServletException) {throw (ServletException) e;} else if (e instanceof IOException) {throw (IOException) e;} else if (e instanceof RuntimeException) {throw (RuntimeException) e;} else {throw new ServletException(e.getMessage(), e);}}} else {internalDoFilter(request,response);}
}private void internalDoFilter(ServletRequest request,ServletResponse response)throws IOException, ServletException {// Call the next filter if there is oneif (pos < n) {ApplicationFilterConfig filterConfig = filters[pos++];try {Filter filter = filterConfig.getFilter();if (request.isAsyncSupported() && "false".equalsIgnoreCase(filterConfig.getFilterDef().getAsyncSupported())) {request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);}if( Globals.IS_SECURITY_ENABLED ) {final ServletRequest req = request;final ServletResponse res = response;Principal principal =((HttpServletRequest) req).getUserPrincipal();Object[] args = new Object[]{req, res, this};SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal);} else {filter.doFilter(request, response, this);}} catch (IOException | ServletException | RuntimeException e) {throw e;} catch (Throwable e) {e = ExceptionUtils.unwrapInvocationTargetException(e);ExceptionUtils.handleThrowable(e);throw new ServletException(sm.getString("filterChain.filter"), e);}return;}// We fell off the end of the chain -- call the servlet instancetry {if (ApplicationDispatcher.WRAP_SAME_OBJECT) {lastServicedRequest.set(request);lastServicedResponse.set(response);}if (request.isAsyncSupported() && !servletSupportsAsync) {request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,Boolean.FALSE);}// Use potentially wrapped request from this pointif ((request instanceof HttpServletRequest) &&(response instanceof HttpServletResponse) &&Globals.IS_SECURITY_ENABLED ) {final ServletRequest req = request;final ServletResponse res = response;Principal principal =((HttpServletRequest) req).getUserPrincipal();Object[] args = new Object[]{req, res};SecurityUtil.doAsPrivilege("service",servlet,classTypeUsedInService,args,principal);} else {servlet.service(request, response);}} catch (IOException | ServletException | RuntimeException e) {throw e;} catch (Throwable e) {e = ExceptionUtils.unwrapInvocationTargetException(e);ExceptionUtils.handleThrowable(e);throw new ServletException(sm.getString("filterChain.servlet"), e);} finally {if (ApplicationDispatcher.WRAP_SAME_OBJECT) {lastServicedRequest.set(null);lastServicedResponse.set(null);}}
}

ApplicationFilterChain 中有一个 pos 变量来记录过滤器链执行位置，执行每次从 Filter 数组 filters 中获取 Filter 后 pos 就加一。等到 Filter 被调用完之后会执行 servlet.service(request, response);。由此可见 Filter 是在 Servlet init 方法执行之后，service 方法执行之前执行的。