Tomcat 一次请求的生命周期

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 collection
        if (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 delay
                errorDelay = 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 wrapper
        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);
            }
        }
        // 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 needed
    return 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 PollerEvent
    PollerEvent 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 called
    while (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 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;
        // 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 timeouts
        timeout(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 true
        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) {
            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 Selector
                        key.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 write
                    if (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 key
            cancelledKey(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 full
        getLog().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 socket
        if (event == null) {
            state = getHandler().process(socketWrapper, SocketEvent.OPEN_READ);
        } else {
            state = getHandler().process(socketWrapper, event);
        }
        if (state == SocketState.CLOSED) {
            // Close socket and pool
            socketWrapper.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
    // connections
    release(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, ignore
            state = 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 parameters
        postParseSuccess = postParseRequest(req, request, res, response);
        if (postParseSuccess) {
            //check valves if we support async
            request.setAsyncSupported(
                connector.getService().getContainer().getPipeline().isAsyncSupported());
            // Calling the container
            connector.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 parameters
        postParseSuccess = postParseRequest(req, request, res, response);
        if (postParseSuccess) {
            //check valves if we support async
            request.setAsyncSupported(
                connector.getService().getContainer().getPipeline().isAsyncSupported());
            // Calling the container
            connector.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 request
    try {
        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 filterChain
    ApplicationFilterChain filterChain =
        ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

    // Call the filter chain for this request
    // NOTE: This also calls the servlet's service() method
    Container container = this.container;
    try {
        if ((servlet != null) && (filterChain != null)) {
            // Swallow output if needed
            if (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 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();
    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 (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 problem
            continue;
        }
        filterChain.addFilter(filterConfig);
    }

    // Add filters that match on servlet name second
    for (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 problem
            continue;
        }
        filterChain.addFilter(filterConfig);
    }

    // Return the completed filter chain
    return 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>() {
                    @Override
                    public 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 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 {
                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 {
            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。文章来源地址https://www.toymoban.com/news/detail-654400.html

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