SpringBoot内嵌的Tomcat启动过程以及请求-编程知识

1.springboot内嵌的tomcat的pom坐标

启动后可以看到tomcat版本为9.0.46

2.springboot 内嵌tomcat启动流程

点击进入SpringApplication.run()方法里面

看这次tomcat启动相关的核心代码refreshContext(context);刷新上下文方法

public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;configureHeadlessProperty();SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);context = createApplicationContext();prepareContext(context, environment, listeners, applicationArguments, printedBanner);// 此次的核心代码是刷新上下文方法refreshContext(context);afterRefresh(context, applicationArguments);stopWatch.stop();if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);}listeners.started(context);callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, listeners);throw new IllegalStateException(ex);}try {listeners.running(context);}catch (Throwable ex) {handleRunFailure(context, ex, null);throw new IllegalStateException(ex);}return context;
}

进入私有方法refreshContext

private void refreshContext(ConfigurableApplicationContext context) {if (this.registerShutdownHook) {try {context.registerShutdownHook();}catch (AccessControlException ex) {// Not allowed in some environments.}}// 核心方法，继续进入refresh((ApplicationContext) context);
}

@Deprecated
protected void refresh(ApplicationContext applicationContext) {Assert.isInstanceOf(ConfigurableApplicationContext.class, applicationContext);// 核心代码，继续进入方法refresh((ConfigurableApplicationContext) applicationContext);
}

进入refresh方法

protected void refresh(ConfigurableApplicationContext applicationContext) {// 核心方法，查看调用的方法refresh()applicationContext.refresh();
}

查看refresh方法，有三个实现类，进入默认的实现类ServletWebServerApplicationContext，定位到实现方法

@Override
public final void refresh() throws BeansException, IllegalStateException {try {// 进入父类refreshsuper.refresh();}catch (RuntimeException ex) {WebServer webServer = this.webServer;if (webServer != null) {webServer.stop();}throw ex;}
}

看onRefresh()方法

@Override
public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);// Initialize message source for this context.initMessageSource();// Initialize event multicaster for this context.initApplicationEventMulticaster();// 我们进入这个方法// Initialize other special beans in specific context subclasses.onRefresh();// Check for listener beans and register them.registerListeners();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.finishRefresh();}catch (BeansException ex) {if (logger.isWarnEnabled()) {logger.warn("Exception encountered during context initialization - " +"cancelling refresh attempt: " + ex);}// Destroy already created singletons to avoid dangling resources.destroyBeans();// Reset 'active' flag.cancelRefresh(ex);// Propagate exception to caller.throw ex;}finally {// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}
}

找到子类的实现方法：

@Override
protected void onRefresh() {super.onRefresh();try {// 核心代码，创建web服务器createWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}
}

进入createWebServer方法，ServletContext、WebServer在tomcat启动的时候一定为空，还没有初始化完成，会进入if代码块执行

private void createWebServer() {WebServer webServer = this.webServer;// tomcat启动servletContext才进行创建ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {ServletWebServerFactory factory = getWebServerFactory();// 核心代码，通过工厂获取一个WebServer实例this.webServer = factory.getWebServer(getSelfInitializer());getBeanFactory().registerSingleton("webServerGracefulShutdown",new WebServerGracefulShutdownLifecycle(this.webServer));getBeanFactory().registerSingleton("webServerStartStop",new WebServerStartStopLifecycle(this, this.webServer));}else if (servletContext != null) {try {getSelfInitializer().onStartup(servletContext);}catch (ServletException ex) {throw new ApplicationContextException("Cannot initialize servlet context", ex);}}initPropertySources();
}

getWebServer方法需要参数，该参数是一个方法getSelfInitializer()，找到

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {// 方法引用，ServletContextInitializer是一个函数是接口，通过方法引用返回一个实现        // ServletContextInitializer的一个实现子类,作用同匿名内部类。return this::selfInitialize;
}

函数式接口：

@FunctionalInterface
public interface ServletContextInitializer {/*** Configure the given {@link ServletContext} with any servlets, filters, listeners* context-params and attributes necessary for initialization.* @param servletContext the {@code ServletContext} to initialize* @throws ServletException if any call against the given {@code ServletContext}* throws a {@code ServletException}*/void onStartup(ServletContext servletContext) throws ServletException;}

this::selfInitialize就是实例::方法名调用方式，this代表当前对象，selfInitialize是this(ServletWebServerApplicationContext)对象的一个私有方法

private void selfInitialize(ServletContext servletContext) throws ServletException {prepareWebApplicationContext(servletContext);registerApplicationScope(servletContext);WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);for (ServletContextInitializer beans : getServletContextInitializerBeans()) {beans.onStartup(servletContext);}
}

继续进入getWebServer，发现是一个接口，有三个实现类Jetty、Tomcat、UnderTow，我们是Tomcat服务器，进入TomcatServletWebServerFactory中的实现方法

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {if (this.disableMBeanRegistry) {Registry.disableRegistry();}Tomcat tomcat = new Tomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");// 用于设置临时文件的目录，这些目录用于存放JSP生成的源代码及Class文件xtomcat.setBaseDir(baseDir.getAbsolutePath());Connector connector = new Connector(this.protocol);connector.setThrowOnFailure(true);tomcat.getService().addConnector(connector);customizeConnector(connector);// 用于设置链接器，包括协议、I/0、端口、压缩、加密等等tomcat.setConnector(connector);// 是否自动部署tomcat.getHost().setAutoDeploy(false);configureEngine(tomcat.getEngine());for (Connector additionalConnector : this.additionalTomcatConnectors) {tomcat.getService().addConnector(additionalConnector);}prepareContext(tomcat.getHost(), initializers);// 核心代码return getTomcatWebServer(tomcat);
}

protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
}

进入TomcatWebServer构造方法

public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {Assert.notNull(tomcat, "Tomcat Server must not be null");this.tomcat = tomcat;this.autoStart = autoStart;this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;// 核心代码initialize();
}

进入initialize()方法

private void initialize() throws WebServerException {logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));synchronized (this.monitor) {try {addInstanceIdToEngineName();Context context = findContext();context.addLifecycleListener((event) -> {if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {// Remove service connectors so that protocol binding doesn't// happen when the service is started.removeServiceConnectors();}});// 核心代码// Start the server to trigger initialization listenersthis.tomcat.start();// We can re-throw failure exception directly in the main threadrethrowDeferredStartupExceptions();try {ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());}catch (NamingException ex) {// Naming is not enabled. Continue}// Unlike Jetty, all Tomcat threads are daemon threads. We create a// blocking non-daemon to stop immediate shutdownstartDaemonAwaitThread();}catch (Exception ex) {stopSilently();destroySilently();throw new WebServerException("Unable to start embedded Tomcat", ex);}}
}

public void start() throws LifecycleException {getServer();// 核心代码server.start();
}

进入start()方法，找到LifecycleBase实现类方法

LifecycleBase类中init();startInternal();两个核心方法

 @Override
public final synchronized void start() throws LifecycleException {if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||LifecycleState.STARTED.equals(state)) {if (log.isDebugEnabled()) {Exception e = new LifecycleException();log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e);} else if (log.isInfoEnabled()) {log.info(sm.getString("lifecycleBase.alreadyStarted", toString()));}return;}if (state.equals(LifecycleState.NEW)) {// 核心代码1init();} else if (state.equals(LifecycleState.FAILED)) {stop();} else if (!state.equals(LifecycleState.INITIALIZED) &&!state.equals(LifecycleState.STOPPED)) {invalidTransition(Lifecycle.BEFORE_START_EVENT);}try {setStateInternal(LifecycleState.STARTING_PREP, null, false);// 核心代码2startInternal();if (state.equals(LifecycleState.FAILED)) {// This is a 'controlled' failure. The component put itself into the// FAILED state so call stop() to complete the clean-up.stop();} else if (!state.equals(LifecycleState.STARTING)) {// Shouldn't be necessary but acts as a check that sub-classes are// doing what they are supposed to.invalidTransition(Lifecycle.AFTER_START_EVENT);} else {setStateInternal(LifecycleState.STARTED, null, false);}} catch (Throwable t) {// This is an 'uncontrolled' failure so put the component into the// FAILED state and throw an exception.handleSubClassException(t, "lifecycleBase.startFail", toString());}
}

init()方法

 @Override
public final synchronized void init() throws LifecycleException {if (!state.equals(LifecycleState.NEW)) {invalidTransition(Lifecycle.BEFORE_INIT_EVENT);}try {setStateInternal(LifecycleState.INITIALIZING, null, false);// 核心代码initInternal();setStateInternal(LifecycleState.INITIALIZED, null, false);} catch (Throwable t) {handleSubClassException(t, "lifecycleBase.initFail", toString());}
}

initInternal有很多实现类，看组件Connector中的实现方法

组件Connector类型方法

@Override
protected void initInternal() throws LifecycleException {super.initInternal();if (protocolHandler == null) {throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerInstantiationFailed"));}// Initialize adapteradapter = new CoyoteAdapter(this);protocolHandler.setAdapter(adapter);if (service != null) {protocolHandler.setUtilityExecutor(service.getServer().getUtilityExecutor());}// Make sure parseBodyMethodsSet has a defaultif (null == parseBodyMethodsSet) {setParseBodyMethods(getParseBodyMethods());}if (protocolHandler.isAprRequired() && !AprStatus.isInstanceCreated()) {throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerNoAprListener",getProtocolHandlerClassName()));}if (protocolHandler.isAprRequired() && !AprStatus.isAprAvailable()) {throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerNoAprLibrary",getProtocolHandlerClassName()));}if (AprStatus.isAprAvailable() && AprStatus.getUseOpenSSL() &&protocolHandler instanceof AbstractHttp11JsseProtocol) {AbstractHttp11JsseProtocol<?> jsseProtocolHandler =(AbstractHttp11JsseProtocol<?>) protocolHandler;if (jsseProtocolHandler.isSSLEnabled() &&jsseProtocolHandler.getSslImplementationName() == null) {// OpenSSL is compatible with the JSSE configuration, so use it if APR is availablejsseProtocolHandler.setSslImplementationName(OpenSSLImplementation.class.getName());}}try {// 核心方法protocolHandler.init();} catch (Exception e) {throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerInitializationFailed"), e);}
}

进入实现类AbstractProtocol中init()方法

@Override
public void init() throws Exception {if (getLog().isInfoEnabled()) {getLog().info(sm.getString("abstractProtocolHandler.init", getName()));logPortOffset();}if (oname == null) {// Component not pre-registered so register itoname = createObjectName();if (oname != null) {Registry.getRegistry(null, null).registerComponent(this, oname, null);}}if (this.domain != null) {ObjectName rgOname = new ObjectName(domain + ":type=GlobalRequestProcessor,name=" + getName());this.rgOname = rgOname;Registry.getRegistry(null, null).registerComponent(getHandler().getGlobal(), rgOname, null);}String endpointName = getName();endpoint.setName(endpointName.substring(1, endpointName.length()-1));endpoint.setDomain(domain);// 核心代码endpoint.init();
}

进入endpoint.init()方法

public final void init() throws Exception {if (bindOnInit) {// 核心代码bindWithCleanup();bindState = BindState.BOUND_ON_INIT;}if (this.domain != null) {// Register endpoint (as ThreadPool - historical name)oname = new ObjectName(domain + ":type=ThreadPool,name=\"" + getName() + "\"");Registry.getRegistry(null, null).registerComponent(this, oname, null);ObjectName socketPropertiesOname = new ObjectName(domain +":type=SocketProperties,name=\"" + getName() + "\"");socketProperties.setObjectName(socketPropertiesOname);Registry.getRegistry(null, null).registerComponent(socketProperties, socketPropertiesOname, null);for (SSLHostConfig sslHostConfig : findSslHostConfigs()) {registerJmx(sslHostConfig);}}
}

进入bindWithCleanup方法里面

private void bindWithCleanup() throws Exception {try {// 核心代码bind();} catch (Throwable t) {// Ensure open sockets etc. are cleaned up if something goes// wrong during bindExceptionUtils.handleThrowable(t);unbind();throw t;}
}

@Override
public void bind() throws Exception {// 核心代码initServerSocket();setStopLatch(new CountDownLatch(1));// Initialize SSL if neededinitialiseSsl();selectorPool.open(getName());
}

初始化Socket

protected void initServerSocket() throws Exception {if (getUseInheritedChannel()) {// Retrieve the channel provided by the OSChannel ic = System.inheritedChannel();if (ic instanceof ServerSocketChannel) {serverSock = (ServerSocketChannel) ic;}if (serverSock == null) {throw new IllegalArgumentException(sm.getString("endpoint.init.bind.inherited"));}} else if (getUnixDomainSocketPath() != null) {SocketAddress sa = JreCompat.getInstance().getUnixDomainSocketAddress(getUnixDomainSocketPath());serverSock = JreCompat.getInstance().openUnixDomainServerSocketChannel();serverSock.bind(sa, getAcceptCount());if (getUnixDomainSocketPathPermissions() != null) {Path path = Paths.get(getUnixDomainSocketPath());Set<PosixFilePermission> permissions =PosixFilePermissions.fromString(getUnixDomainSocketPathPermissions());if (path.getFileSystem().supportedFileAttributeViews().contains("posix")) {FileAttribute<Set<PosixFilePermission>> attrs = PosixFilePermissions.asFileAttribute(permissions);Files.setAttribute(path, attrs.name(), attrs.value());} else {java.io.File file = path.toFile();if (permissions.contains(PosixFilePermission.OTHERS_READ) && !file.setReadable(true, false)) {log.warn(sm.getString("endpoint.nio.perms.readFail", file.getPath()));}if (permissions.contains(PosixFilePermission.OTHERS_WRITE) && !file.setWritable(true, false)) {log.warn(sm.getString("endpoint.nio.perms.writeFail", file.getPath()));}}}} else {serverSock = ServerSocketChannel.open();socketProperties.setProperties(serverSock.socket());InetSocketAddress addr = new InetSocketAddress(getAddress(), getPortWithOffset());serverSock.bind(addr, getAcceptCount());}serverSock.configureBlocking(true); //mimic APR behavior
}

startInternal();

看下这个方法，找到Connector里面的startInternal()方法实现

@Override
protected void startInternal() throws LifecycleException {// Validate settings before startingString id = (protocolHandler != null) ? protocolHandler.getId() : null;if (id == null && getPortWithOffset() < 0) {throw new LifecycleException(sm.getString("coyoteConnector.invalidPort", Integer.valueOf(getPortWithOffset())));}setState(LifecycleState.STARTING);try {// 核心代码protocolHandler.start();} catch (Exception e) {throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerStartFailed"), e);}
}

进入protocolHandler.start();方法里面。进入AbstractProtocol里面的实现方法

@Override
public void start() throws Exception {if (getLog().isInfoEnabled()) {getLog().info(sm.getString("abstractProtocolHandler.start", getName()));logPortOffset();}// 核心代码endpoint.start();monitorFuture = getUtilityExecutor().scheduleWithFixedDelay(() -> {if (!isPaused()) {startAsyncTimeout();}}, 0, 60, TimeUnit.SECONDS);
}

找到endpoint.start()方法

public final void start() throws Exception {if (bindState == BindState.UNBOUND) {bindWithCleanup();bindState = BindState.BOUND_ON_START;}// 核心代码startInternal();
}

进入这个实现类里，找到实现方法

@Overridepublic 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();// 核心代码 *******// Start poller threadpoller = new Poller();Thread pollerThread = new Thread(poller, getName() + "-ClientPoller");// 设置进程的优先级pollerThread.setPriority(threadPriority);// 守护线程:当非守护线程销毁的时候，守护线程跟着销毁。当运行的唯一线程是守护线程时，        // Java虚拟机将退出pollerThread.setDaemon(true);// 开启线程pollerThread.start();startAcceptorThread();}}

Poller类是什么？

Poller里面的run方法

@Override
public void run() {// Loop until destroy() is called// 核心代码1 轮询while (true) {boolean hasEvents = false;try {if (!close) {hasEvents = events();if (wakeupCounter.getAndSet(-1) > 0) {// If we are here, means we have other stuff to do// Do a non blocking selectkeyCount = 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;// Walk through the collection of ready keys and dispatch// any active event.while (iterator != null && iterator.hasNext()) {SelectionKey sk = iterator.next();iterator.remove();NioSocketWrapper socketWrapper = (NioSocketWrapper) sk.attachment();// Attachment may be null if another thread has called// cancelledKey()if (socketWrapper != null) {// 核心代码2processKey(sk, socketWrapper);}}// Process timeoutstimeout(keyCount,hasEvents);}getStopLatch().countDown();
}

启动一个Poller线程轮询监听NIO接收的请求，直到执行核心代码2processKey(sk, socketWrapper);Poller类会轮询监听Socket连接，到这里tomcat算启动成功啦。

springboot嵌入tomcat处理请求的过程

主要看下Poller类中的processKey方法，我们发起一个请求怎么处理的呢？

processKey方法

protected void processKey(SelectionKey sk, NioSocketWrapper socketWrapper) {try {if (close) {cancelledKey(sk, socketWrapper);} else if (sk.isValid() && socketWrapper != null) {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) {// 核心代码if (!socketWrapper.readOperation.process()) {closeSocket = true;}// 核心代码} 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 (!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);}
}

进入processSocket方法

public boolean processSocket(SocketWrapperBase<S> socketWrapper,SocketEvent event, boolean dispatch) {try {if (socketWrapper == null) {return false;}// 核心代码 线程SocketProcessorBase<S> sc = null;if (processorCache != null) {sc = processorCache.pop();}if (sc == null) {sc = createSocketProcessor(socketWrapper, event);} else {sc.reset(socketWrapper, event);}// 核心代码 获取线程池，通过线程池执行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;
}

看看SocketProcessBase是什么？实现了Runnable接口

看下SocketProcessBase的run()方法

 @Overridepublic final void run() {synchronized (socketWrapper) {// It is possible that processing may be triggered for read and// write at the same time. The sync above makes sure that processing// does not occur in parallel. The test below ensures that if the// first event to be processed results in the socket being closed,// the subsequent events are not processed.if (socketWrapper.isClosed()) {return;}// 核心代码doRun();}}

看下doRun()方法

找到过滤器链ApplicationFilterChain，找到doFilter()方法

@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((java.security.PrivilegedExceptionAction<Void>) () -> {// 核心代码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;elsethrow new ServletException(e.getMessage(), e);}} else {internalDoFilter(request,response);}
}

定位到internalDoFilter()方法

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 {// 核心代码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);}}
}

定位到service()方法的实现类HttpServlet中的service()方法中

@Overridepublic void service(ServletRequest req, ServletResponse res)throws ServletException, IOException {HttpServletRequest  request;HttpServletResponse response;try {request = (HttpServletRequest) req;response = (HttpServletResponse) res;} catch (ClassCastException e) {throw new ServletException(lStrings.getString("http.non_http"));}// 核心代码service(request, response);}

继续进入到service()方法中

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {long startTime = System.currentTimeMillis();Throwable failureCause = null;LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();LocaleContext localeContext = buildLocaleContext(request);RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());initContextHolders(request, localeContext, requestAttributes);try {// 核心代码doService(request, response);}catch (ServletException | IOException ex) {failureCause = ex;throw ex;}catch (Throwable ex) {failureCause = ex;throw new NestedServletException("Request processing failed", ex);}finally {resetContextHolders(request, previousLocaleContext, previousAttributes);if (requestAttributes != null) {requestAttributes.requestCompleted();}logResult(request, response, failureCause, asyncManager);publishRequestHandledEvent(request, response, startTime, failureCause);}
}

进入doService()方法