在读本篇文章之前如果你读过这篇文章SpringBoot自动装配原理解析应该会更加轻松
准备工作

我们知道SpringBoot的自动装配的秘密在org.springframework.boot.autoconfigure包下的spring.factories文件中，而嵌入Tomcat的原理就在这个文件中加载的一个配置类：org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration

@Configuration@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)@ConditionalOnClass(ServletRequest.class)@ConditionalOnWebApplication(type = Type.SERVLET)@EnableConfigurationProperties(ServerProperties.class)@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })public class ServletWebServerFactoryAutoConfiguration {    @Bean    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(            ServerProperties serverProperties) {        return new ServletWebServerFactoryCustomizer(serverProperties);    }    @Bean    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(            ServerProperties serverProperties) {        return new TomcatServletWebServerFactoryCustomizer(serverProperties);    }    /**     * Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via     * {@link ImportBeanDefinitionRegistrar} for early registration.     */    public static class BeanPostProcessorsRegistrar            implements ImportBeanDefinitionRegistrar, BeanFactoryAware {        private ConfigurableListableBeanFactory beanFactory;        @Override        public void setBeanFactory(BeanFactory beanFactory) throws BeansException {            if (beanFactory instanceof ConfigurableListableBeanFactory) {                this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;            }        }        @Override        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,                BeanDefinitionRegistry registry) {            if (this.beanFactory == null) {                return;            }            registerSyntheticBeanIfMissing(registry,                    "webServerFactoryCustomizerBeanPostProcessor",                    WebServerFactoryCustomizerBeanPostProcessor.class);            registerSyntheticBeanIfMissing(registry,                    "errorPageRegistrarBeanPostProcessor",                    ErrorPageRegistrarBeanPostProcessor.class);        }        private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,                String name, Class<?> beanClass) {            if (ObjectUtils.isEmpty(                    this.beanFactory.getBeanNamesForType(beanClass, true, false))) {                RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);                beanDefinition.setSynthetic(true);                registry.registerBeanDefinition(name, beanDefinition);            }        }    }}

首先看一下上方的几个注解

  1. @AutoConfigureOrder这个注解是决定配置类的加载顺序的，当注解里的值越小越先加载，而Ordered.HIGHEST_PRECEDENCE的值是Integer.MIN_VALUE也就是说这个类肯定是最先加载的那一批
  2. @ConditionalOnXXX在之前的文章中已经无数次提到了，就不再阐述了
  3. @EnableConfigurationProperties开启ServerProperties类的属性值配置。而这个类里面包含的就是Web服务的配置
@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)public class ServerProperties {    private Integer port;    private InetAddress address;    @NestedConfigurationProperty    private final ErrorProperties error = new ErrorProperties();    private Boolean useForwardHeaders;    private String serverHeader;    private int maxHttpHeaderSize = 0; // bytes    private Duration connectionTimeout;    @NestedConfigurationProperty    private Ssl ssl;    @NestedConfigurationProperty    private final Compression compression = new Compression();    @NestedConfigurationProperty    private final Http2 http2 = new Http2();    private final Servlet servlet = new Servlet();    private final Tomcat tomcat = new Tomcat();    private final Jetty jetty = new Jetty();    private final Undertow undertow = new Undertow();}

这个类的代码太多了，这里就不一一贴出来了，我们平常在application.properties中配置的server.xxx就是这个类中属性

  1. @Import引入了4个类，看都是什么吧

    1. BeanPostProcessorsRegistrar
public static class BeanPostProcessorsRegistrar            implements ImportBeanDefinitionRegistrar, BeanFactoryAware {        @Override        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,                BeanDefinitionRegistry registry) {            if (this.beanFactory == null) {                return;            }            registerSyntheticBeanIfMissing(registry,                    "webServerFactoryCustomizerBeanPostProcessor",                    WebServerFactoryCustomizerBeanPostProcessor.class);            registerSyntheticBeanIfMissing(registry,                    "errorPageRegistrarBeanPostProcessor",                    ErrorPageRegistrarBeanPostProcessor.class);        }        private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,                String name, Class<?> beanClass) {            if (ObjectUtils.isEmpty(                    this.beanFactory.getBeanNamesForType(beanClass, true, false))) {                RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);                beanDefinition.setSynthetic(true);                registry.registerBeanDefinition(name, beanDefinition);            }        }    }

这个类注册了两个bean：WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor关于这两个bean的作用稍后再详细介绍

  1. EmbeddedTomcat
@Configuration    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)    public static class EmbeddedTomcat {        @Bean        public TomcatServletWebServerFactory tomcatServletWebServerFactory() {            return new TomcatServletWebServerFactory();        }    }

这个类会在存在Tomcat相关jar包时添加一个TomcatServletWebServerFactorybean

其他两个相信大家都知道怎么回事了

  1. 除了这些这个类还注入了两个类ServletWebServerFactoryCustomizer和TomcatServletWebServerFactoryCustomizer

现在前期准备工作已经做好了，看一下这个Tomcat是如何启动的吧

启动

启动入口在ServletWebServerApplicationContext中的onRefresh方法

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

Tomcat的启动就在createWebServer方法里面了

private void createWebServer() {        WebServer webServer = this.webServer;        ServletContext servletContext = getServletContext();          //第一次访问的时候两个对象都为空        if (webServer == null && servletContext == null) {            ServletWebServerFactory factory = getWebServerFactory();            this.webServer = factory.getWebServer(getSelfInitializer());        }        else if (servletContext != null) {            try {                getSelfInitializer().onStartup(servletContext);            }            catch (ServletException ex) {                throw new ApplicationContextException("Cannot initialize servlet context",                        ex);            }        }        initPropertySources();    }

首先看一下getWebServerFactory

protected ServletWebServerFactory getWebServerFactory() {        // 这里获取的beanname就是上方注册的tomcatServletWebServerFactory了        String[] beanNames = getBeanFactory()                .getBeanNamesForType(ServletWebServerFactory.class);        if (beanNames.length == 0) {            throw new ApplicationContextException(                    "Unable to start ServletWebServerApplicationContext due to missing "                            + "ServletWebServerFactory bean.");        }        if (beanNames.length > 1) {            throw new ApplicationContextException(                    "Unable to start ServletWebServerApplicationContext due to multiple "                            + "ServletWebServerFactory beans : "                            + StringUtils.arrayToCommaDelimitedString(beanNames));        }        return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);    }

准备环境里注册的bean现在出来一个了。注意，上方还注册了一个后置处理器EmbeddedServletContainerCustomizerBeanPostProcessor，获取beantomcatServletWebServerFactory的时候就会执行后置处理器的postProcessBeforeInitialization方法

public Object postProcessBeforeInitialization(Object bean, String beanName)            throws BeansException {    if (bean instanceof WebServerFactory) {        postProcessBeforeInitialization((WebServerFactory) bean);    }    return bean;}private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {        LambdaSafe                .callbacks(WebServerFactoryCustomizer.class, getCustomizers(),                        webServerFactory)                .withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)                .invoke((customizer) -> customizer.customize(webServerFactory));    }    private Collection<WebServerFactoryCustomizer<?>> getCustomizers() {        if (this.customizers == null) {            // Look up does not include the parent context            this.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans());            this.customizers.sort(AnnotationAwareOrderComparator.INSTANCE);            this.customizers = Collections.unmodifiableList(this.customizers);        }        return this.customizers;    }    @SuppressWarnings({ "unchecked", "rawtypes" })    private Collection<WebServerFactoryCustomizer<?>> getWebServerFactoryCustomizerBeans() {        return (Collection) this.beanFactory                .getBeansOfType(WebServerFactoryCustomizer.class, false, false).values();    }

这个处理器的作用是获得所有定制器，然后执行定制器的方法

接着往下看

这个时候就可以启动Tomcat了

public WebServer getWebServer(ServletContextInitializer... initializers) {        Tomcat tomcat = new Tomcat();        File baseDir = (this.baseDirectory != null ? this.baseDirectory                : createTempDir("tomcat"));        tomcat.setBaseDir(baseDir.getAbsolutePath());        Connector connector = new Connector(this.protocol);        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);    }protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {        return new TomcatWebServer(tomcat, getPort() >= 0);    }public TomcatWebServer(Tomcat tomcat, boolean autoStart) {        Assert.notNull(tomcat, "Tomcat Server must not be null");        this.tomcat = tomcat;        this.autoStart = autoStart;        initialize();    }private void initialize() throws WebServerException {        TomcatWebServer.logger                .info("Tomcat initialized with port(s): " + getPortsDescription(false));        synchronized (this.monitor) {            try {                addInstanceIdToEngineName();                Context context = findContext();                context.addLifecycleListener((event) -> {                    if (context.equals(event.getSource())                            && Lifecycle.START_EVENT.equals(event.getType())) {                        // Remove service connectors so that protocol binding doesn't                        // happen when the service is started.                        removeServiceConnectors();                    }                });                // Start the server to trigger initialization listeners                this.tomcat.start();                // We can re-throw failure exception directly in the main thread                rethrowDeferredStartupExceptions();                try {                    ContextBindings.bindClassLoader(context, context.getNamingToken(),                            getClass().getClassLoader());                }                catch (NamingException ex) {                    // Naming is not enabled. Continue                }                // Unlike Jetty, all Tomcat threads are daemon threads. We create a                // blocking non-daemon to stop immediate shutdown                startDaemonAwaitThread();            }            catch (Exception ex) {                throw new WebServerException("Unable to start embedded Tomcat", ex);            }        }    }