[Getting to give up -SpringBoot] SpringBoot source code analysis - Request Process

Foreword

Previously [from entry to abandon -SpringBoot] SpringBoot source code analysis -WebServer to SpringBoot embedded Tomcat as an example to understand the process of starting the webserver.

This paper will analyze a request under the SpringBoot, the return received from those who have gone through the process.

Acceptor

When the connector above last start, will start acceptor thread is waiting to receive requests.

Acceptor::run

public void run() {

    int errorDelay = 0;

    // Loop until we receive a shutdown command
    while (endpoint.isRunning()) {

        // Loop if endpoint is paused
        while (endpoint.isPaused() && endpoint.isRunning()) {
            state = AcceptorState.PAUSED;
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                // Ignore
            }
        }

        if (!endpoint.isRunning()) {
            break;
        }
        state = AcceptorState.RUNNING;

        try {
            //if we have reached max connections, wait
            endpoint.countUpOrAwaitConnection();

            // Endpoint might have been paused while waiting for latch
            // If that is the case, don't accept new connections
            if (endpoint.isPaused()) {
                continue;
            }

            U socket = null;
            try {
                // Accept the next incoming connection from the server
                // socket
                socket = endpoint.serverSocketAccept();
            } catch (Exception ioe) {
                // We didn't get a socket
                endpoint.countDownConnection();
                if (endpoint.isRunning()) {
                    // Introduce delay if necessary
                    errorDelay = handleExceptionWithDelay(errorDelay);
                    // re-throw
                    throw ioe;
                } else {
                    break;
                }
            }
            // Successful accept, reset the error delay
            errorDelay = 0;

            // Configure the socket
            if (endpoint.isRunning() && !endpoint.isPaused()) {
                // setSocketOptions() will hand the socket off to
                // an appropriate processor if successful
                if (!endpoint.setSocketOptions(socket)) {
                    endpoint.closeSocket(socket);
                }
            } else {
                endpoint.destroySocket(socket);
            }
        } catch (Throwable t) {
            ExceptionUtils.handleThrowable(t);
            String msg = sm.getString("endpoint.accept.fail");
            // APR specific.
            // Could push this down but not sure it is worth the trouble.
            if (t instanceof Error) {
                Error e = (Error) t;
                if (e.getError() == 233) {
                    // Not an error on HP-UX so log as a warning
                    // so it can be filtered out on that platform
                    // See bug 50273
                    log.warn(msg, t);
                } else {
                    log.error(msg, t);
                }
            } else {
                    log.error(msg, t);
            }
        }
    }
    state = AcceptorState.ENDED;
}

acceptor will always listen port, waiting for a connection

setSocketOptions

@Override
protected boolean setSocketOptions(SocketChannel socket) {
    // Process the connection
    try {
        //disable blocking, APR style, we are gonna be polling it
        socket.configureBlocking(false);
        Socket sock = socket.socket();
        socketProperties.setProperties(sock);

        NioChannel channel = nioChannels.pop();
        if (channel == null) {
            SocketBufferHandler bufhandler = new SocketBufferHandler(
                    socketProperties.getAppReadBufSize(),
                    socketProperties.getAppWriteBufSize(),
                    socketProperties.getDirectBuffer());
            if (isSSLEnabled()) {
                channel = new SecureNioChannel(socket, bufhandler, selectorPool, this);
            } else {
                channel = new NioChannel(socket, bufhandler);
            }
        } else {
            channel.setIOChannel(socket);
            channel.reset();
        }
        getPoller0().register(channel);
    } catch (Throwable t) {
        ExceptionUtils.handleThrowable(t);
        try {
            log.error(sm.getString("endpoint.socketOptionsError"), t);
        } catch (Throwable tt) {
            ExceptionUtils.handleThrowable(tt);
        }
        // Tell to close the socket
        return false;
    }
    return true;
}

• NioChannel: The captured socket encapsulated NioChannel

register

public void register(final NioChannel socket) {
    socket.setPoller(this);
    NioSocketWrapper ka = new NioSocketWrapper(socket, NioEndpoint.this);
    socket.setSocketWrapper(ka);
    ka.setPoller(this);
    ka.setReadTimeout(getConnectionTimeout());
    ka.setWriteTimeout(getConnectionTimeout());
    ka.setKeepAliveLeft(NioEndpoint.this.getMaxKeepAliveRequests());
    ka.setSecure(isSSLEnabled());
    PollerEvent r = eventCache.pop();
    ka.interestOps(SelectionKey.OP_READ);//this is what OP_REGISTER turns into.
    if ( r==null) r = new PollerEvent(socket,ka,OP_REGISTER);
    else r.reset(socket,ka,OP_REGISTER);
    addEvent(r);
}

• The packaged into a channel SynchronizedQueue PollerEvent posted to queue, waiting for consumption poller

Poller::run

@Override
public void run() {
    // Loop until destroy() is called
    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 select
                    keyCount = selector.selectNow();
                } else {
                    keyCount = selector.select(selectorTimeout);
                }
                wakeupCounter.set(0);
            }
            if (close) {
                events();
                timeout(0, false);
                try {
                    selector.close();
                } catch (IOException ioe) {
                    log.error(sm.getString("endpoint.nio.selectorCloseFail"), ioe);
                }
                break;
            }
        } catch (Throwable x) {
            ExceptionUtils.handleThrowable(x);
            log.error(sm.getString("endpoint.nio.selectorLoopError"), x);
            continue;
        }
        //either we timed out or we woke up, process events first
        if ( keyCount == 0 ) hasEvents = (hasEvents | events());

        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();
            NioSocketWrapper attachment = (NioSocketWrapper)sk.attachment();
            // Attachment may be null if another thread has called
            // cancelledKey()
            if (attachment == null) {
                iterator.remove();
            } else {
                iterator.remove();
                processKey(sk, attachment);
            }
        }//while

        //process timeouts
        timeout(keyCount,hasEvents);
    }//while

    getStopLatch().countDown();
}

• events: Poller Event queue processing, direct call PollerEvent :: run. If it returns true then the event has been processed, and false explanation queue is empty
• processKey: request logic processing, the socket to processSocket process.
• SocketProcessor: inherited from SocketProcessorBase, call processing logic run method starts request.

Source: https://blog.csdn.net/TVwR8OfV0P/article/details/78851949

SocketProcessorBase run method is not very familiar? In the database connection process analysis from entry to abandon -MySQL [] - client , we breakpoint debugging process a query request, the call stack entry is this method.

SocketProcessor::run

@Override
protected void doRun() {
    NioChannel socket = socketWrapper.getSocket();
    SelectionKey key = socket.getIOChannel().keyFor(socket.getPoller().getSelector());

    try {
        int handshake = -1;

        try {
            if (key != null) {
                if (socket.isHandshakeComplete()) {
                    // No TLS handshaking required. Let the handler
                    // process this socket / event combination.
                    handshake = 0;
                } else if (event == SocketEvent.STOP || event == SocketEvent.DISCONNECT ||
                        event == SocketEvent.ERROR) {
                    // Unable to complete the TLS handshake. Treat it as
                    // if the handshake failed.
                    handshake = -1;
                } else {
                    handshake = socket.handshake(key.isReadable(), key.isWritable());
                    // The handshake process reads/writes from/to the
                    // socket. status may therefore be OPEN_WRITE once
                    // the handshake completes. However, the handshake
                    // happens when the socket is opened so the status
                    // must always be OPEN_READ after it completes. It
                    // is OK to always set this as it is only used if
                    // the handshake completes.
                    event = SocketEvent.OPEN_READ;
                }
            }
        } catch (IOException x) {
            handshake = -1;
            if (log.isDebugEnabled()) log.debug("Error during SSL handshake",x);
        } catch (CancelledKeyException ckx) {
            handshake = -1;
        }
        if (handshake == 0) {
            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, key);
            }
        } else if (handshake == -1 ) {
            close(socket, key);
        } else if (handshake == SelectionKey.OP_READ){
            socketWrapper.registerReadInterest();
        } else if (handshake == SelectionKey.OP_WRITE){
            socketWrapper.registerWriteInterest();
        }
    } catch (CancelledKeyException cx) {
        socket.getPoller().cancelledKey(key);
    } catch (VirtualMachineError vme) {
        ExceptionUtils.handleThrowable(vme);
    } catch (Throwable t) {
        log.error(sm.getString("endpoint.processing.fail"), t);
        socket.getPoller().cancelledKey(key);
    } finally {
        socketWrapper = null;
        event = null;
        //return to cache
        if (running && !paused) {
            processorCache.push(this);
        }
    }
}

• Determining whether the handshaking socket and socket according to the state or the close link processing request
• ConnectionHandler::process：调用AbstractProcessorLight::process，判断socket的状态及协议是否需要upgrade（如HTTP 2，websocket等基于HTTP的协议），选择对应的Processor处理请求。

Http11Processor::service

public SocketState service(SocketWrapperBase<?> socketWrapper)
    throws IOException {
    RequestInfo rp = request.getRequestProcessor();
    rp.setStage(org.apache.coyote.Constants.STAGE_PARSE);

    // Setting up the I/O
    setSocketWrapper(socketWrapper);
    inputBuffer.init(socketWrapper);
    outputBuffer.init(socketWrapper);

    // Flags
    keepAlive = true;
    openSocket = false;
    readComplete = true;
    boolean keptAlive = false;
    SendfileState sendfileState = SendfileState.DONE;

    while (!getErrorState().isError() && keepAlive && !isAsync() && upgradeToken == null &&
            sendfileState == SendfileState.DONE && !protocol.isPaused()) {

        // Parsing the request header
        try {
            if (!inputBuffer.parseRequestLine(keptAlive, protocol.getConnectionTimeout(),
                    protocol.getKeepAliveTimeout())) {
                if (inputBuffer.getParsingRequestLinePhase() == -1) {
                    return SocketState.UPGRADING;
                } else if (handleIncompleteRequestLineRead()) {
                    break;
                }
            }

            if (protocol.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(protocol.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 (!protocol.getDisableUploadTimeout()) {
                    socketWrapper.setReadTimeout(protocol.getConnectionUploadTimeout());
                }
            }
        } catch (IOException e) {
            if (log.isDebugEnabled()) {
                log.debug(sm.getString("http11processor.header.parse"), e);
            }
            setErrorState(ErrorState.CLOSE_CONNECTION_NOW, e);
            break;
        } catch (Throwable t) {
            ExceptionUtils.handleThrowable(t);
            UserDataHelper.Mode logMode = userDataHelper.getNextMode();
            if (logMode != null) {
                String message = sm.getString("http11processor.header.parse");
                switch (logMode) {
                    case INFO_THEN_DEBUG:
                        message += sm.getString("http11processor.fallToDebug");
                        //$FALL-THROUGH$
                    case INFO:
                        log.info(message, t);
                        break;
                    case DEBUG:
                        log.debug(message, 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 = protocol.getUpgradeProtocol(requestedProtocol);
            if (upgradeProtocol != null) {
                if (upgradeProtocol.accept(request)) {
                    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(
                                    socketWrapper, 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 {
                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);
            }
        }

        int maxKeepAliveRequests = protocol.getMaxKeepAliveRequests();
        if (maxKeepAliveRequests == 1) {
            keepAlive = false;
        } else if (maxKeepAliveRequests > 0 &&
                socketWrapper.decrementKeepAlive() <= 0) {
            keepAlive = false;
        }

        // Process the request in the adapter
        if (getErrorState().isIoAllowed()) {
            try {
                rp.setStage(org.apache.coyote.Constants.STAGE_SERVICE);
                getAdapter().service(request, response);
                // Handle when the response was committed before a serious
                // error occurred.  Throwing a ServletException should both
                // set the status to 500 and set the errorException.
                // If we fail here, then the response is likely already
                // committed, so we can't try and set headers.
                if(keepAlive && !getErrorState().isError() && !isAsync() &&
                        statusDropsConnection(response.getStatus())) {
                    setErrorState(ErrorState.CLOSE_CLEAN, null);
                }
            } catch (InterruptedIOException e) {
                setErrorState(ErrorState.CLOSE_CONNECTION_NOW, e);
            } catch (HeadersTooLargeException e) {
                log.error(sm.getString("http11processor.request.process"), e);
                // The response should not have been committed but check it
                // anyway to be safe
                if (response.isCommitted()) {
                    setErrorState(ErrorState.CLOSE_NOW, e);
                } else {
                    response.reset();
                    response.setStatus(500);
                    setErrorState(ErrorState.CLOSE_CLEAN, e);
                    response.setHeader("Connection", "close"); // TODO: Remove
                }
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
                log.error(sm.getString("http11processor.request.process"), t);
                // 500 - Internal Server Error
                response.setStatus(500);
                setErrorState(ErrorState.CLOSE_CLEAN, t);
                getAdapter().log(request, response, 0);
            }
        }

        // Finish the handling of the request
        rp.setStage(org.apache.coyote.Constants.STAGE_ENDINPUT);
        if (!isAsync()) {
            // If this is an async request then the request ends when it has
            // been completed. The AsyncContext is responsible for calling
            // endRequest() in that case.
            endRequest();
        }
        rp.setStage(org.apache.coyote.Constants.STAGE_ENDOUTPUT);

        // If there was an error, make sure the request is counted as
        // and error, and update the statistics counter
        if (getErrorState().isError()) {
            response.setStatus(500);
        }

        if (!isAsync() || getErrorState().isError()) {
            request.updateCounters();
            if (getErrorState().isIoAllowed()) {
                inputBuffer.nextRequest();
                outputBuffer.nextRequest();
            }
        }

        if (!protocol.getDisableUploadTimeout()) {
            int connectionTimeout = protocol.getConnectionTimeout();
            if(connectionTimeout > 0) {
                socketWrapper.setReadTimeout(connectionTimeout);
            } else {
                socketWrapper.setReadTimeout(0);
            }
        }

        rp.setStage(org.apache.coyote.Constants.STAGE_KEEPALIVE);

        sendfileState = processSendfile(socketWrapper);
    }

    rp.setStage(org.apache.coyote.Constants.STAGE_ENDED);

    if (getErrorState().isError() || protocol.isPaused()) {
        return SocketState.CLOSED;
    } else if (isAsync()) {
        return SocketState.LONG;
    } else if (isUpgrade()) {
        return SocketState.UPGRADING;
    } else {
        if (sendfileState == SendfileState.PENDING) {
            return SocketState.SENDFILE;
        } else {
            if (openSocket) {
                if (readComplete) {
                    return SocketState.OPEN;
                } else {
                    return SocketState.LONG;
                }
            } else {
                return SocketState.CLOSED;
            }
        }
    }
}

• 初始化各项配置及初始值
• 解析request的header信息，判断是否是upgrade协议，是的话设置http header信息
• prepareRequest：校验协议、校验header（如user-agent、Connection、host等）设置filters
• getAdapter().service：getAdapter获取的是在Connector::initInternal中设置的CoyoteAdapter

CoyoteAdapter::service

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 (request == null) {
        // Create objects
        request = connector.createRequest();
        request.setCoyoteRequest(req);
        response = connector.createResponse();
        response.setCoyoteResponse(res);

        // Link objects
        request.setResponse(response);
        response.setRequest(request);

        // Set as notes
        req.setNote(ADAPTER_NOTES, request);
        res.setNote(ADAPTER_NOTES, response);

        // Set query string encoding
        req.getParameters().setQueryStringCharset(connector.getURICharset());
    }

    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
        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);
        }
        if (request.isAsync()) {
            async = true;
            ReadListener readListener = req.getReadListener();
            if (readListener != null && request.isFinished()) {
                // Possible the all data may have been read during service()
                // method so this needs to be checked here
                ClassLoader oldCL = null;
                try {
                    oldCL = request.getContext().bind(false, null);
                    if (req.sendAllDataReadEvent()) {
                        req.getReadListener().onAllDataRead();
                    }
                } finally {
                    request.getContext().unbind(false, oldCL);
                }
            }

            Throwable throwable =
                    (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);

            // If an async request was started, is not going to end once
            // this container thread finishes and an error occurred, trigger
            // the async error process
            if (!request.isAsyncCompleting() && throwable != null) {
                request.getAsyncContextInternal().setErrorState(throwable, true);
            }
        } else {
            request.finishRequest();
            response.finishResponse();
        }

    } catch (IOException e) {
        // Ignore
    } finally {
        AtomicBoolean error = new AtomicBoolean(false);
        res.action(ActionCode.IS_ERROR, error);

        if (request.isAsyncCompleting() && error.get()) {
            // Connection will be forcibly closed which will prevent
            // completion happening at the usual point. Need to trigger
            // call to onComplete() here.
            res.action(ActionCode.ASYNC_POST_PROCESS,  null);
            async = false;
        }

        // Access log
        if (!async && postParseSuccess) {
            // Log only if processing was invoked.
            // If postParseRequest() failed, it has already logged it.
            Context context = request.getContext();
            Host host = request.getHost();
            // If the context is null, it is likely that the endpoint was
            // shutdown, this connection closed and the request recycled in
            // a different thread. That thread will have updated the access
            // log so it is OK not to update the access log here in that
            // case.
            // The other possibility is that an error occurred early in
            // processing and the request could not be mapped to a Context.
            // Log via the host or engine in that case.
            long time = System.currentTimeMillis() - req.getStartTime();
            if (context != null) {
                context.logAccess(request, response, time, false);
            } else if (response.isError()) {
                if (host != null) {
                    host.logAccess(request, response, time, false);
                } else {
                    connector.getService().getContainer().logAccess(
                            request, response, time, false);
                }
            }
        }

        req.getRequestProcessor().setWorkerThreadName(null);

        // Recycle the wrapper request and response
        if (!async) {
            updateWrapperErrorCount(request, response);
            request.recycle();
            response.recycle();
        }
    }
}

• 初始化request和response，如果没有则新new
• postParseRequest：保证在header解析后的request能被Container正常解析、使用。比如权限认证、字符编码、版本、session等的转换和校验
• invoke：逐级调用StandardEngineValve、StandardHostValve、StandardHostValve、StandardWrapperValve的invoke实现
• doFilter：StandardWrapperValve调用doFilter，会执行定义的Filter过滤器，执行完毕后执行HttpServlet::service

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);
    }
}

• 根据请求的method做对应的处理逻辑，其实doXXX底层都是调用的SpringMVC的FrameworkServlet::processRequest
• FrameworkServlet::processRequest对request做简单处理后，调用了doService，实现类为DispatcherServlet

DispatcherServlet::doService

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    logRequest(request);

    // Keep a snapshot of the request attributes in case of an include,
    // to be able to restore the original attributes after the include.
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        attributesSnapshot = new HashMap<>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
    }

    // Make framework objects available to handlers and view objects.
    request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

    if (this.flashMapManager != null) {
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }
        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
    }

    try {
        doDispatch(request, response);
    }
    finally {
        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }
}

• 主要调用方法为doDispatch

DispatcherServlet::doDispatch

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

    try {
        ModelAndView mv = null;
        Exception dispatchException = null;

        try {
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // Determine handler for the current request.
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // Determine handler adapter for the current request.
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }

            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // Actually invoke the handler.
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }

            applyDefaultViewName(processedRequest, mv);
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        }
        catch (Exception ex) {
            dispatchException = ex;
        }
        catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    }
    catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
    }
    finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

• getHandler：根据requestURI通过HandlerMapping(这里是requestMappingHandlerMapping)获取到处理此请求的handler以及拦截器（requestMapping对应的类和方法）
• getHandlerAdapter：获取能执行这个Handler的Adapter
• applyPreHandle：处理拦截器的前置拦截。
• ha.handle：通过获取到的Adapter（这里是RequestMappingHandlerAdapter）执行Controller方法，得到ModelAndView
• processDispatchResult：直接调用render方法，将返回的ModelAndView交给ViewResolver视图解析器找到指定的视图（view），赋值给response返回给客户端显示。

RequestMappingHandlerAdapter::handleInternal

protected ModelAndView handleInternal(HttpServletRequest request,
        HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

    ModelAndView mav;
    checkRequest(request);

    // Execute invokeHandlerMethod in synchronized block if required.
    if (this.synchronizeOnSession) {
        HttpSession session = request.getSession(false);
        if (session != null) {
            Object mutex = WebUtils.getSessionMutex(session);
            synchronized (mutex) {
                mav = invokeHandlerMethod(request, response, handlerMethod);
            }
        }
        else {
            // No HttpSession available -> no mutex necessary
            mav = invokeHandlerMethod(request, response, handlerMethod);
        }
    }
    else {
        // No synchronization on session demanded at all...
        mav = invokeHandlerMethod(request, response, handlerMethod);
    }

    if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
        if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
            applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
        }
        else {
            prepareResponse(response);
        }
    }

    return mav;
}

• invokeHandlerMethod：执行Handler方法获取到ModelAndView，调用ServletInvocableHandlerMethod::invokeAndHandle

ServletInvocableHandlerMethod::invokeAndHandle

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    setResponseStatus(webRequest);

    if (returnValue == null) {
        if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
            mavContainer.setRequestHandled(true);
            return;
        }
    }
    else if (StringUtils.hasText(getResponseStatusReason())) {
        mavContainer.setRequestHandled(true);
        return;
    }

    mavContainer.setRequestHandled(false);
    Assert.state(this.returnValueHandlers != null, "No return value handlers");
    try {
        this.returnValueHandlers.handleReturnValue(
                returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    }
    catch (Exception ex) {
        if (logger.isTraceEnabled()) {
            logger.trace(formatErrorForReturnValue(returnValue), ex);
        }
        throw ex;
    }
}

• invokeForRequest: 调用InvocableHandlerMethod::doInvoke 通过反射的方式执行requestMapping对应的Controller方法
• this.returnValueHandlers.handleReturnValue：将执行完的returnValue设置到response中，我们常用的json返回值就是在这里设置的。

总结

通过对源码的分析我们了解了请求在Tomcat中的流转情况，以及SpringMVC如何完成一个请求的处理。
一个请求在SpringMVC中的生命周期如下图所示：

1、用户发送请求，浏览器acceptor监听到请求，投递至PollerEvent队列，Poller进行消费，交给SocketProcessor（DispatcherServlet）处理。

2、分发器DispatcherServlet向映射器HandlerMapping发请求，根据requestURL获取Handler

3、映射器返回HandlerExecutionChain，包含了需要处理的Handler及HandlerInterceptor拦截器

4、根据Handler获取对应的适配器

5、适配器执行具体的Handler（即Controller的方法）

6、Handler执行完返回ModelAndModel

7、HandlerAdapter将ModelAndModel返回给分发器DispatcherServlet

8、DispatcherServlet将ModelAndView传给ViewReslover视图解析器

9、ViewReslover解析后返回具体的View模板

10、渲染视图，将数据填充至视图中

11、将渲染好的视图返回给用户，在浏览器客户端展现

• 参考：https://www.cnblogs.com/xiaoxi/p/6164383.html

更多文章见：https://nc2era.com