Tomcat 请求处理流程

请求流程

设计了这么多层次的容器，Tomcat是怎么确定每一个请求应该由哪个Wrapper容器里的 Servlet来处理的呢？

答案是，Tomcat是用Mapper组件来完成这个任务的。

Mapper组件的功能就是将用户请求的URL定位到一个Servlet，它的工作原理是：

Mapper组件里保存了Web应用的配置信息，其实就是容器组件与访问路径的映射关系， 比如Host容器里配置的域名、Context容器里的Web应用路径，以及Wrapper容器里 Servlet映射的路径，你可以想象这些配置信息就是一个多层次的Map。

当一个请求到来时，Mapper组件通过解析请求URL里的域名和路径，再到自己保存的 Map里去查找，就能定位到一个Servlet。请你注意，一个请求URL最后只会定位到一个 Wrapper容器，也就是一个Servlet。

下面的示意图中 ， 就描述了 当用户请求链接 http://www.itcast.cn/bbs/findAll 之 后, 是如何找到最终处理业务逻辑的servlet 。

那上面这幅图只是描述了根据请求的URL如何查找到需要执行的Servlet ， 那么下面我们 再来解析一下 ， 从Tomcat的设计架构层面来分析Tomcat的请求处理

步骤如下:

1. Connector组件Endpoint中的Acceptor监听客户端套接字连接并接收Socket。
2. 将连接交给线程池Executor处理，开始执行请求响应任务。
3. Processor组件读取消息报文，解析请求行、请求体、请求头，封装成Request对象。
4. Mapper组件根据请求行的URL值和请求头的Host值匹配由哪个Host容器、Context容器、Wrapper容器处理请求。
5. CoyoteAdaptor组件负责将Connector组件和Engine容器关联起来，把生成的 Request对象和响应对象Response传递到Engine容器中，调用 Pipeline。
6. Engine容器的管道开始处理，管道中包含若干个Valve、每个Valve负责部分处理逻 辑。执行完Valve后会执行基础的 Valve–StandardEngineValve，负责调用Host容器的 Pipeline。
7. Host容器的管道开始处理，流程类似，最后执行 Context容器的Pipeline。
8. Context容器的管道开始处理，流程类似，最后执行 Wrapper容器的Pipeline。
9. Wrapper容器的管道开始处理，流程类似，最后执行 Wrapper容器对应的Servlet对象 的 处理方法。

请求流程源码解析

在前面所讲解的Tomcat的整体架构中，我们发现Tomcat中的各个组件各司其职，组件 之间松耦合，确保了整体架构的可伸缩性和可拓展性，那么在组件内部，如何增强组件 的灵活性和拓展性呢？ 在Tomcat中，每个Container组件采用责任链模式来完成具体的 请求处理。

在Tomcat中定义了Pipeline 和 Valve 两个接口，Pipeline 用于构建责任链， 后者代表责 任链上的每个处理器。Pipeline 中维护了一个基础的Valve，它始终位于Pipeline的末端 （最后执行），封装了具体的请求处理和输出响应的过程。当然，我们也可以调用 addValve()方法， 为Pipeline 添加其他的Valve， 后添加的Valve 位于基础的Valve之 前，并按照添加顺序执行。Pipiline通过获得首个Valve来启动整合链条的执行 。

源码研究

建议看源码流程前先去回顾一下责任链模式，因为tomcat的请求流程中主要使用了责任链模式

责任链模式

我们把请求过程的源码分为两部分来进行分析:
第一部分: 请求由Endpoint捕获,并转交给Processor处理.

1.Acceptor.run()

请求的流程由NioEndpoint中的Accepter类的run方法开始.

首先一个浏览器的请求会由tomcat中的Endpoint中的Accepter所捕获并开启会话.

protected class Acceptor extends AbstractEndpoint.Acceptor {
 
        @Override
        public void run() {
            int errorDelay = 0;
            while (running) {
                while (paused && running) {
                    state = AcceptorState.PAUSED;
                    try {
                        Thread.sleep(50);
                    } catch (InterruptedException e) {
                        // Ignore
                    }
                }
                if (!running) {
                    break;
                }
                state = AcceptorState.RUNNING;
                try {
                    //if we have reached max connections, wait
                    countUpOrAwaitConnection();
                    SocketChannel socket = null;
                    try {
                        //接受客户端请求
                        socket = serverSock.accept();
                    } catch (IOException ioe) {
                        // We didn't get a socket
                        countDownConnection();
                        if (running) {
                            // 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 (running && !paused) {
                        // setSocketOptions() will hand the socket off to
                        // an appropriate processor if successful
                        if (!setSocketOptions(socket)) {
                            closeSocket(socket);
                        }
                    } else {
                        closeSocket(socket);
                    }
                } catch (Throwable t) {
                    ExceptionUtils.handleThrowable(t);
                    log.error(sm.getString("endpoint.accept.fail"), t);
                }
            }
            state = AcceptorState.ENDED;
        }

2.Poller.run()

当接收客户端请求时,NioEndpoint.Poller会获取事件并开始迭代执行.(一下省略部分代码)

public void run() {
            
            while (true) {
                .......
                while (iterator != null && iterator.hasNext()) {
                    SelectionKey sk = iterator.next();
                    NioSocketWrapper attachment = (NioSocketWrapper)sk.attachment();
                    // Attachment may be null if another thread has called
                    // cancelledKey()
                    if (attachment == null) {
                        iterator.remove();
                    } else {
                        iterator.remove();
                        //开始正式执行请求流程
                        processKey(sk, attachment);
                    }
                }//while
 
                //process timeouts
                timeout(keyCount,hasEvents);
            }//while
 
            getStopLatch().countDown();
        }

2.1 Poller.processKey()

processKey方法继续调用其中的processSocket方法,并开始处理会话.

protected void processKey(SelectionKey sk, NioSocketWrapper attachment) {
            try {
                if ( close ) {
                    cancelledKey(sk);
                } else if ( sk.isValid() && attachment != null ) {
                    if (sk.isReadable() || sk.isWritable() ) {
                        if ( attachment.getSendfileData() != null ) {
                            processSendfile(sk,attachment, false);
                        } else {
                            unreg(sk, attachment, sk.readyOps());
                            boolean closeSocket = false;
                            // Read goes before write
                            if (sk.isReadable()) {
                                //开始处理会话
                                if (!processSocket(attachment, SocketEvent.OPEN_READ, true)) {
                                    closeSocket = true;
                                }
                            }
                            if (!closeSocket && sk.isWritable()) {
                                if (!processSocket(attachment, SocketEvent.OPEN_WRITE, true)) {
                                    closeSocket = true;
                                }
                            }
                            if (closeSocket) {
                                cancelledKey(sk);
                            }
                        }
                    }
                } else {
                    //invalid key
                    cancelledKey(sk);
                }
            } catch ( CancelledKeyException ckx ) {
                cancelledKey(sk);
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
                log.error("",t);
            }
        }

2.2 AbstractEndpoint.processSocket()

当跟踪进processSocket时会发现调用的是AbstractEndpoint的processSocket方法.

他会首先获取Socket的处理器,并获取线程池为处理Socket单独开启一个线程.

public boolean processSocket(SocketWrapperBase<S> socketWrapper,
            SocketEvent event, boolean dispatch) {
        try {
            if (socketWrapper == null) {
                return false;
            }
            //获取socket的处理器
            SocketProcessorBase<S> sc = processorCache.pop();
            if (sc == null) {
                sc = createSocketProcessor(socketWrapper, event);
            } else {
                sc.reset(socketWrapper, event);
            }
            //获取到线程池
            Executor executor = getExecutor();
            if (dispatch && executor != null) {
                //由线程池调用一个线程来执行Socket处理器
                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;
    }

3. NioEndpoint.SocketProcessor.doRun()

经过跟踪会最终发现,sc.run()方法实际上是在调用NioEndpoint.SocketProcessor.doRun();

protected void doRun() {
            NioChannel socket = socketWrapper.getSocket();
            SelectionKey key = socket.getIOChannel().keyFor(socket.getPoller().getSelector());
 
            ....
                if (handshake == 0) {
                    SocketState state = SocketState.OPEN;
                    // Process the request from this socket
                    if (event == null) {
                        //获取Handler处理器:Servlet(处理器),调度处理器的处理方法
                        state = getHandler().process(socketWrapper, SocketEvent.OPEN_READ);
                    } else {
                        state = getHandler().process(socketWrapper, event);
                    }
                    if (state == SocketState.CLOSED) {
                        close(socket, key);
                    }
                } 
              ....
        }
    }

4. AbstractProtocol.ConnectionHandler.process

在doRun的方法中最后会交给AbstractProtocol.ConnectionHandler.process方法执行.

@Override
        public SocketState process(SocketWrapperBase<S> wrapper, SocketEvent status) {
            if (getLog().isDebugEnabled()) {
                getLog().debug(sm.getString("abstractConnectionHandler.process",
                        wrapper.getSocket(), status));
            }
            if (wrapper == null) {
                // Nothing to do. Socket has been closed.
                return SocketState.CLOSED;
            }
 
            //获取Socket
            S socket = wrapper.getSocket();
            //交给Processor处理 -> 真正开始处理请求
            Processor processor = connections.get(socket);
            ....
             do {
                    //开始解析请求
                    state = processor.process(wrapper, status);
 
                    .....
                    }
                } while ( state == SocketState.UPGRADING);
}

至此,请求由Endpoint组件正式转交给Processor进行处理.

主要内容为在请求转交给Coyote适配器后的流程分析,紧接上文中请求交由Processor处理.

5.AbstractProcessorLight.process()

当请求由Endpoint交由Processor处理时,首先经过的就是AbstractProcessorLight.process()

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) {
            } 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, ignore
                state = SocketState.LONG;
            } else if (status == SocketEvent.OPEN_READ) {
                //调度Http11Processor.service方法
                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;
    }

6.Http11Processor的service方法

@Override
    public SocketState service(SocketWrapperBase<?> socketWrapper)
        
          ....    
        while (!getErrorState().isError() && keepAlive && !isAsync() && upgradeToken == null &&
                sendfileState == SendfileState.DONE && !endpoint.isPaused()) {

            // Parsing the request header
            try {
            //解析socket请求数据中每一行,按照http协议解析请求头----只负责解析请求头
                if (!inputBuffer.parseRequestLine(keptAlive)) {
                    if (inputBuffer.getParsingRequestLinePhase() == -1) {
                        return SocketState.UPGRADING;
                    } else if (handleIncompleteRequestLineRead()) {
                        break;
                    }
                }

                if (endpoint.isPaused()) {
                    // 503 - Service unavailable
                    response.setStatus(503);
                    setErrorState(ErrorState.CLOSE_CLEAN, null);
                } else {
                    keptAlive = true;
                    // Set this every time in case limit has been changed via JMX
                    request.getMimeHeaders().setLimit(endpoint.getMaxHeaderCount());
                    if (!inputBuffer.parseHeaders()) {
                        // We've read part of the request, don't recycle it
                        // instead associate it with the socket
                        openSocket = true;
                        readComplete = false;
                        break;
                    }
                    if (!disableUploadTimeout) {
                        socketWrapper.setReadTimeout(connectionUploadTimeout);
                    }
                }
            } catch (IOException e) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("http11processor.header.parse"), e);
                }
                setErrorState(ErrorState.CLOSE_CONNECTION_NOW, e);
                break;
            } catch (Throwable t) {
                   ....
                }
                // 400 - Bad Request
                response.setStatus(400);
                setErrorState(ErrorState.CLOSE_CLEAN, t);
            }

            // Has an upgrade been requested?
            Enumeration<String> connectionValues = request.getMimeHeaders().values("Connection");
            boolean foundUpgrade = false;
            while (connectionValues.hasMoreElements() && !foundUpgrade) {
                foundUpgrade = connectionValues.nextElement().toLowerCase(
                        Locale.ENGLISH).contains("upgrade");
            }

            if (foundUpgrade) {
                // Check the protocol
                String requestedProtocol = request.getHeader("Upgrade");

                UpgradeProtocol upgradeProtocol = httpUpgradeProtocols.get(requestedProtocol);
                if (upgradeProtocol != null) {
                    if (upgradeProtocol.accept(request)) {
                        // TODO Figure out how to handle request bodies at this
                        // point.
                        response.setStatus(HttpServletResponse.SC_SWITCHING_PROTOCOLS);
                        response.setHeader("Connection", "Upgrade");
                        response.setHeader("Upgrade", requestedProtocol);
                        action(ActionCode.CLOSE,  null);
                        getAdapter().log(request, response, 0);

                        InternalHttpUpgradeHandler upgradeHandler =
                                upgradeProtocol.getInternalUpgradeHandler(
                                        getAdapter(), cloneRequest(request));
                        UpgradeToken upgradeToken = new UpgradeToken(upgradeHandler, null, null);
                        action(ActionCode.UPGRADE, upgradeToken);
                        return SocketState.UPGRADING;
                    }
                }
            }

            if (getErrorState().isIoAllowed()) {
                // Setting up filters, and parse some request headers
                rp.setStage(org.apache.coyote.Constants.STAGE_PREPARE);
                try {
                //After reading the request headers, we have to setup the request filters.
                    prepareRequest();
                } catch (Throwable t) {
                    ExceptionUtils.handleThrowable(t);
                    if (log.isDebugEnabled()) {
                        log.debug(sm.getString("http11processor.request.prepare"), t);
                    }
                    // 500 - Internal Server Error
                    response.setStatus(500);
                    setErrorState(ErrorState.CLOSE_CLEAN, t);
                }
            }
              ...
            // Process the request in the adapter
            if (getErrorState().isIoAllowed()) {
                try {
                    rp.setStage(org.apache.coyote.Constants.STAGE_SERVICE);
                    //将上面socket请求封装而成的request和response对象，再交给适配器进行处理
                  //获取Coyote适配器,并调用其service方法
                    getAdapter().service(request, response);
                    ...

7.CoyoteAdapter.service()

至此请求就即将被发送给Engine引擎.

在CoyoteAdapter中将Request和Response装换成Servlet容器中处理的Request和Response，然后从service中获取容器，再调用管道Pipeline的阀门Valve的invoke方法

在Coyote适配器中,会获取Engine容器的第一个Valve,即StandardEngineValve,并执行.

@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);
         
         .......   
         
        if (connector.getXpoweredBy()) {
            response.addHeader("X-Powered-By", POWERED_BY);
        }

        boolean async = false;
        boolean postParseSuccess = false;

        req.getRequestProcessor().setWorkerThreadName(THREAD_NAME.get());

        try {
            // Parse and set Catalina and configuration specific
            // request parameters
            //解析和设置 Catalina 和特定配置,包括session的查找
            //在这里将Request和Response装换成Servlet容器中处理的Request和Response
            postParseSuccess = postParseRequest(req, request, res, response);
            if (postParseSuccess) {
                //check valves if we support async
                request.setAsyncSupported(
                        connector.getService().getContainer().getPipeline().isAsyncSupported());
                // Calling the container
                //获取Engine容器的第一个Valve,即StandardEngineValve,并执行.
                connector.getService().getContainer().getPipeline().getFirst().invoke(
                        request, response);
            }
            .......

整个容器中执行的链路如图：

8.StandardEngineValve#invoke

因为这里默认只有一个标准的Engine value实现:

@Override
    public final void invoke(Request request, Response response)
        throws IOException, ServletException {

        // Select the Host to be used for this Request
        Host host = request.getHost();
        if (host == null) {
            response.sendError
                (HttpServletResponse.SC_BAD_REQUEST,
                 sm.getString("standardEngine.noHost",
                              request.getServerName()));
            return;
        }
        if (request.isAsyncSupported()) {
            request.setAsyncSupported(host.getPipeline().isAsyncSupported());
        }

        // Ask this Host to process this request
        //这里调用Host管道中的每个pipeline
        host.getPipeline().getFirst().invoke(request, response);

    }

9.StandardHostValve#invoke

说明一下:在Host管道中,默认还会加入两个Value，一个是accessLogValue，一个是errorLogValue后者用来处理请求处理过程中发生的错误

接下来依次调用context,wrapper的valve并执行.

10.StandardContextValve#invoke

获取当前请求对应的wrapper

// Select the Wrapper to be used for this Request
        Wrapper wrapper = request.getWrapper();

调用wrapper管道中所有value

wrapper.getPipeline().getFirst().invoke(request, response);

10.StandardWrapperValve#invoke

重点代码一:分配一个servlet实例对象，来处理当前请求，这里的servlet其实就是我们自己定义的

// Allocate a servlet instance to process this request
        try {
            if (!unavailable) {
                servlet = wrapper.allocate();
            }

这里的allocate就是从mapped中获取到当前请求映射的servlet，联系初始化时,扫描web.xml和相关注解

// Create the filter chain for this request
        ApplicationFilterChain filterChain =
                ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

给当前请求创建一个过滤器链

/**
     * Construct a FilterChain implementation that will wrap the execution of
     * the specified servlet instance.
     *
     * @param request The servlet request we are processing
     * @param wrapper The wrapper managing the servlet instance
     * @param servlet The servlet instance to be wrapped
     *
     * @return The configured FilterChain instance or null if none is to be
     *         executed.
     */
    public static ApplicationFilterChain createFilterChain(ServletRequest request,
            Wrapper wrapper, Servlet servlet) {

        // If there is no servlet to execute, return null
        if (servlet == null)
            return null;

        // Create and initialize a filter chain object
        ApplicationFilterChain filterChain = null;
        if (request instanceof Request) {
            Request req = (Request) request;
            if (Globals.IS_SECURITY_ENABLED) {
                // Security: Do not recycle
                filterChain = new ApplicationFilterChain();
            } else {
                filterChain = (ApplicationFilterChain) req.getFilterChain();
                if (filterChain == null) {
                    filterChain = new ApplicationFilterChain();
                    req.setFilterChain(filterChain);
                }
            }
        } else {
            // Request dispatcher in use
            filterChain = new ApplicationFilterChain();
        }

        filterChain.setServlet(servlet);
        filterChain.setServletSupportsAsync(wrapper.isAsyncSupported());

        // Acquire the filter mappings for this Context
        StandardContext context = (StandardContext) wrapper.getParent();
        //wrapper的父容器context中的过滤器集合
        FilterMap filterMaps[] = context.findFilterMaps();

        // If there are no filter mappings, we are done
        if ((filterMaps == null) || (filterMaps.length == 0))
            return (filterChain);

        // Acquire the information we will need to match filter mappings
        DispatcherType 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 chain
        for (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersURL(filterMaps[i], requestPath))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                // FIXME - log configuration problem
                continue;
            }
            //将集合中符合要求当前请求的过滤器都加入过滤器链中
            filterChain.addFilter(filterConfig);
        }

        // Add filters that match on servlet name second
        for (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersServlet(filterMaps[i], servletName))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                // FIXME - log configuration problem
                continue;
            }
            //将集合中符合要求当前请求的过滤器都加入过滤器链中
            filterChain.addFilter(filterConfig);
        }

        // Return the completed filter chain
        //返回构建好的过滤器链
        return filterChain;
    }

调用完创建过滤器的方法后,回到invoke方法中

正式对当前请求调用过滤器链

filterChain.doFilter
                            (request.getRequest(), response.getResponse());

11.ApplicationFilterChain#doFilter

重点代码:

internalDoFilter(request,response);

开始真正执行过滤流程

12.ApplicationFilterChain#internalDoFilter

private void internalDoFilter(ServletRequest request,
                                  ServletResponse response)
        throws IOException, ServletException {

        // Call the next filter if there is one
        if (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 {
                    //重点代码---第三个参数是当前过滤器链对象--ApplicationFilterChain
                    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 instance
        try {
            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 point
            if ((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 {
            //过滤器链调用完毕,真正调用servlet的方法
                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);
            }
        }
    }

这是过滤器的原理

下面看sevlet的调用,下面演示的是正常流程:

13.HttpServlet#service

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);

        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);

        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);

        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);

        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);

        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }

这里的doGet和doPost等方法的调用就是在我们自己的servlet中调用了，因为我们自己的servlet没有重写service方法，所以这里service是调用父类的，而重写的doGet等方法，是调用对应子类的实现

13.DefaultServlet#service

如果没有找到对应的请求映射servlet，会返回DefaultServlet

@Override
    protected void service(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        if (req.getDispatcherType() == DispatcherType.ERROR) {
            doGet(req, resp);
        } else {
            super.service(req, resp);
        }
    }

最终走的是HttpSerlvet的最后一个else分支,因为DefaultServlet是HttpServlet的子类

else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }

到此请求流程代码分析完毕