花了一個(gè)禮拜的時(shí)間赊窥，大致走讀了一下springboot內(nèi)嵌tomcat的代碼逝钥，為下一步自己實(shí)現(xiàn)一個(gè)web容器做知識(shí)儲(chǔ)備隘弊。這里對(duì)走讀的代碼做一個(gè)大致的記錄乞而。

springboot內(nèi)嵌tomcat

我們知道送悔，springboot自動(dòng)裝配的相關(guān)功能封裝在spring-boot-autoconfigure包中。這里通過(guò)走讀spring-boot的2.4.0-M版本的代碼來(lái)大致看一下tomcat的啟動(dòng)過(guò)程爪模。
首先欠啤，寫(xiě)一個(gè)簡(jiǎn)單的demo，調(diào)用springboot的run方法屋灌，一步一步來(lái)看tomcat如何啟動(dòng)的洁段。

@SpringBootApplication
public class TestApplication {
    public static void main(String[] args) {
        SpringApplication.run(TestApplication.class);
    }
}

查看run方法里做了什么。

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            //封裝參數(shù)
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            //準(zhǔn)備springboot運(yùn)行相關(guān)的環(huán)境
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            //確定spring的上下文信息共郭，這里是webservlet類型眉撵，通過(guò)反射得到AnnotationConfigServletWebServerApplicationContext
            context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class<?>[] { ConfigurableApplicationContext.class }, context);
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            //這里真正調(diào)到spring里，進(jìn)行bean的初始化
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

這里落塑，初始化的context是一個(gè)AnnotationConfigServletWebServerApplicationContext。我們看一下該類的繼承關(guān)系罐韩。

AnnotationConfigServletWebServerApplicationContext.png

本篇日記重點(diǎn)看的就是refreshContext(context)憾赁，這個(gè)真正的進(jìn)入spring bean的處理。也在這個(gè)方法里散吵，進(jìn)行了tomcat的初始化及啟動(dòng)龙考。繼續(xù)走讀這個(gè)方法的代碼。一步一步跟下去矾睦，就進(jìn)入了AbstractApplicationContext的refresh方法中。方法的代碼如下：

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

通過(guò)spring的注釋，可以大致猜測(cè)一膨，關(guān)于tomcat的初始化诞仓，應(yīng)該在方法onRefresh中實(shí)現(xiàn)。當(dāng)然赁温，這里是通過(guò)debug源碼坛怪，看調(diào)用棧信息能確切的知道這個(gè)方法就是tomcat初始化的入口淤齐。這個(gè)方法實(shí)現(xiàn)在子類里。上面在創(chuàng)建spring的context時(shí)袜匿，我們看到更啄，這里創(chuàng)建的是一個(gè)ServletWebServerApplicationContext，onRefresh就在該類中實(shí)現(xiàn)居灯。進(jìn)入該方法祭务，看到如下代碼：

    @Override
    protected void onRefresh() {
        super.onRefresh();
        try {
            createWebServer();
        }
        catch (Throwable ex) {
            throw new ApplicationContextException("Unable to start web server", ex);
        }
    }

createWebServer就是創(chuàng)建webserver的代碼。代碼：

private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        if (webServer == null && servletContext == null) {
            ServletWebServerFactory factory = getWebServerFactory();
            this.webServer = factory.getWebServer(getSelfInitializer());
            getBeanFactory().registerSingleton("webServerGracefulShutdown",
                    new WebServerGracefulShutdownLifecycle(this.webServer));
            getBeanFactory().registerSingleton("webServerStartStop",
                    new WebServerStartStopLifecycle(this, this.webServer));
        }
        else if (servletContext != null) {
            try {
                getSelfInitializer().onStartup(servletContext);
            }
            catch (ServletException ex) {
                throw new ApplicationContextException("Cannot initialize servlet context", ex);
            }
        }
        initPropertySources();
    }

ServletWebServerFactory為自動(dòng)裝配進(jìn)來(lái)怪嫌，此時(shí)在BeanFactory中已經(jīng)存在了义锥。具體代碼配置在spring-boot-autoconfigure包的META-INF/spring.factories中。

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
......
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\

ServletWebServerFactoryAutoConfiguration中import了一些類喇勋，其中impot了EmbeddedTomcat缨该。

@Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedTomcat {

        @Bean
        TomcatServletWebServerFactory tomcatServletWebServerFactory(
                ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
                ObjectProvider<TomcatContextCustomizer> contextCustomizers,
                ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
            TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
            factory.getTomcatConnectorCustomizers()
                    .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatContextCustomizers()
                    .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatProtocolHandlerCustomizers()
                    .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

在這里生成了一個(gè)TomcatServletWebServerFactory的bean。
再次回到createWebServer中川背，this.webServer = factory.getWebServer(getSelfInitializer());從factory中生產(chǎn)出一個(gè)webserver贰拿。我們看這個(gè)webServer是如何生產(chǎn)出來(lái)的。

public WebServer getWebServer(ServletContextInitializer... initializers) {
        if (this.disableMBeanRegistry) {
            Registry.disableRegistry();
        }
        Tomcat tomcat = new Tomcat();
        File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
        tomcat.setBaseDir(baseDir.getAbsolutePath());
        Connector connector = new Connector(this.protocol);
        connector.setThrowOnFailure(true);
        tomcat.getService().addConnector(connector);
        customizeConnector(connector);
        tomcat.setConnector(connector);
        tomcat.getHost().setAutoDeploy(false);
        configureEngine(tomcat.getEngine());
        for (Connector additionalConnector : this.additionalTomcatConnectors) {
            tomcat.getService().addConnector(additionalConnector);
        }
        prepareContext(tomcat.getHost(), initializers);
        return getTomcatWebServer(tomcat);
    }

上面這段代碼熄云，就正式進(jìn)入到了tomcat的領(lǐng)域了膨更。為了能更好的閱讀代碼，我們需要了解tomcat的體系結(jié)構(gòu)缴允。這里通過(guò)代碼的閱讀和分析荚守，總結(jié)了一張結(jié)構(gòu)圖。

tomcat結(jié)構(gòu).png