Tomcat處理請求的執行緒模型詳解

2022-03-24 19:01:12

一、前言

JAVA後端專案，執行在容器tomcat中，由於現在springboot的內建tomcat容器，其預設設定遮蔽了很多對tomcat的認知，但是對tomcat的學習和認識是比較重要的，所以專門查資料加深了理解，本文主要討論在springboot整合下的tomcat9的請求過程，執行緒模型為NIO。

二、tomcat結構

找了張結構圖，每個模組的意思和作用就不詳解了，可以搜其他文章

三、探討tomcat是如何處理請求

自己畫了一個connector的結構

1、初始化

在springboot啟動後，org.springframework.context.support.AbstractApplicationContext#finishRefresh，這裡進去呼叫org.springframework.boot.web.servlet.context.WebServerStartStopLifecycle.start()方法啟動TomcatWebServer，初始化tomcat。

通過這樣的呼叫鏈到達org.apache.tomcat.util.net.NioEndpoint#startInternal(),進行初始化Endpoint中的Acceptor和Poller，這兩者都實現了Runnable介面，初始化後就通過執行緒start啟動了。

2、如何處理使用者端請求

Acceptor：接收器，作用是接受scoket網路請求，並呼叫setSocketOptions()封裝成為NioSocketWrapper，並註冊到Poller的events中。注意檢視run方法org.apache.tomcat.util.net.Acceptor#run

 @Override
    public void run() {
        int errorDelay = 0;
        try {
            // Loop until we receive a shutdown command
            while (!stopCalled) {
                // Loop if endpoint is paused
                while (endpoint.isPaused() && !stopCalled) {
                    state = AcceptorState.PAUSED;
                    try {
                        Thread.sleep(50);
                    } catch (InterruptedException e) {
                        // Ignore
                    }
                }
                if (stopCalled) {
                    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 {
                        // 等待下一個請求進來
                        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 (!stopCalled && !endpoint.isPaused()) {
                        // 註冊socket到Poller，生成PollerEvent事件
                        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);
                    }
                }
            }
        } finally {
            stopLatch.countDown();
        }
        state = AcceptorState.ENDED;
    }

Poller：輪詢器，輪詢是否有事件達到，有請求事件到達後，以NIO的處理方式，查詢Selector取出所有請求，遍歷每個請求的需求，分配給Executor執行緒池執行。檢視org.apache.tomcat.util.net.NioEndpoint.Poller#run()

 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;
                    }
                    // 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;
                }
				//查詢selector取出所有請求
                Iterator<SelectionKey> iterator =
                    keyCount > 0 ? selector.selectedKeys().iterator() : null;
                // Walk through the collection of ready keys and dispatch
                // any active event.
                while (iterator != null && iterator.hasNext()) {
                    SelectionKey sk = iterator.next();
                    iterator.remove();
                    NioSocketWrapper socketWrapper = (NioSocketWrapper) sk.attachment();
                    //處理請求key
                    if (socketWrapper != null) {
                        processKey(sk, socketWrapper);
                    }
                }
                // Process timeouts
                timeout(keyCount,hasEvents);
            }
            getStopLatch().countDown();
        }

請求過程大致如下圖：

總結

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