SpringBoot内置Tomcat启动原理源码分析

1、获取SpringBoot内置Tomcat自动配置类：

　　在SpringBoot项目中引入spring-boot-starter-web依赖，就默认使用Tomcat容器，该依赖中引入
spring-boot-starter-tomcat、spring-webmvc，就引入了tomtcat核心依赖和springMvc相关jar包，这样就间接地引入了tomcat。

　　在执行SpringBoot项目启动类的mAIn()方法，启动SpringBoot项目的过程中会加载各个jar包下META-INF/spring.factories的文件，在该文件中包含着自动配置的子路径，在refresh()方法中的
invokeBeanFactoryPostProcessors()中首先会对启动类上的 @SpringBootApplication 注解进行解析，最终调用 AutoConfigurationImportSelector类中的 getAutoConfigurationEntry() 加载 META-INF/spring.factories 文件中的自动配置类，得到自动配置类的全路径,其中 org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration 为tomcat自动配置类。

　　具体加载流程见: springBoot-启动原理 - sun丶牧羊人 - 博客园

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    // 1、得到META-INF/spring.factories文件中配置的所有自动配置类
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    // 移除重复的配置类
    configurations = removeDuplicates(configurations);
    // 获取需要排除的自动配置类，eg:注解属性中的exculde的配置类
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    // 检查需要被排除的配置类，因为有些不是自动配置类，需要抛异常
    checkExcludedClasses(configurations, exclusions);
    // 移除需要排除的配置类
    configurations.removeAll(exclusions);
    // 根据 META-INF/spring-autoconfigure-metadata.properties 中配置的规则过虑掉一部分配置类(根据@ConditionalOnXXX注解进行过滤)
    configurations = getConfigurationClassFilter().filter(configurations);
    // 获取符合条件的配置类后，触发 AutoConfigurationImportEvent 事件
    fireAutoConfigurationImportEvents(configurations, exclusions);
    // 将符合条件和需要排除的配置类封装进 AutoConfigurationEntry 对象中返回
    return new AutoConfigurationEntry(configurations, exclusions);
}

2、
ServletWebServerFactoryAutoConfiguration - tomcat自动配置类分析

3、创建tomcat工厂

@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) {
        // 创建生产tomcat的工厂
        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;
    }

}

　4、创建tomcat容器

　　在SpringBoot启动过程中会调用
AbstractApplicationContext.refresh() 方法，在该方法会调用onRefresh()方法，这个方法是个模板方法，最终会交给子类实现，在使用内置tomcat的SpringBoot项目中，最终会调用 ServletWebServerApplicationContext 实现(AbstractApplicationContext是GenericWebApplicationContext，ServletWebServerApplicationContext 是GenericWebApplicationContext)，最终调用ServletWebServerApplicationContext 的createWebServer()方法创建 webServer。

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
protected void onRefresh() {
    // 调用 GenericWebApplicationContext类的 onRefresh() 方法，
    super.onRefresh();
    try {
        // 获取嵌入式的Servlet容器工厂(TomcatServletWebServerFactory)，并通过工厂来获取Servlet容器
        createWebServer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
        // 获取 Servlet容器工厂
        ServletWebServerFactory factory = getWebServerFactory();
        createWebServer.tag("factory", factory.getClass().toString());
        // 通过Servlet容器工厂加载tomcat并启动tomcat  
        this.webServer = factory.getWebServer(getSelfInitializer());
        createWebServer.end();
        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();
}

在 TomcatServletWebServerFactory 类中
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
    if (this.disableMBeanRegistry) {
        Registry.disableRegistry();
    }
    // 实例化tomcat
    Tomcat tomcat = new Tomcat();
    File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
    // 设置tomcat临时工作目录
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    // 默认使用 org.Apache.coyote.http11.Http11NioProtocol 实例化Connector
    Connector connector = new Connector(this.protocol);
    connector.setThrowOnFailure(true);
    // 给service添加Connector
    tomcat.getService().addConnector(connector);
    customizeConnector(connector);
    tomcat.setConnector(connector);
    // 关闭热部署
    tomcat.getHost().setAutoDeploy(false);
    // 配置 Engine
    configureEngine(tomcat.getEngine());
    for (Connector additionalConnector : this.additionalTomcatConnectors) {
        tomcat.getService().addConnector(additionalConnector);
    }
    prepareContext(tomcat.getHost(), initializers);
    // 实例化 TomcatWebServer，将 DispatcherServlet 以及一些Filter添加到Tomcat中
    return getTomcatWebServer(tomcat);
}

protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
    return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
}

//TomcatWebServer类的构造方法
public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;
    initialize();
}

private void initialize() throws WebServerException {
    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
            //利用LifecycleBase对这一套容器（engine，host，context及wrapper）进行启动并发布configure_start、
            // before_init、after_start的lifecycleEvent等事件给相应的监听器
            // 本方法并没有启动tomcat
            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) {
            stopSilently();
            destroySilently();
            throw new WebServerException("Unable to start embedded Tomcat", ex);
        }
    }
}

// Tomcat类
public void start() throws LifecycleException {
    getServer();
    server.start();
}
// LifecycleBase
public final synchronized void start() throws LifecycleException {

    if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||
            LifecycleState.STARTED.equals(state)) {

        if (log.isDebugEnabled()) {
            Exception e = new LifecycleException();
            log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e);
        } else if (log.isInfoEnabled()) {
            log.info(sm.getString("lifecycleBase.alreadyStarted", toString()));
        }

        return;
    }

    if (state.equals(LifecycleState.NEW)) {
        init();
    } else if (state.equals(LifecycleState.FAILED)) {
        stop();
    } else if (!state.equals(LifecycleState.INITIALIZED) &&
            !state.equals(LifecycleState.STOPPED)) {
        invalidTransition(Lifecycle.BEFORE_START_EVENT);
    }

    try {
        setStateInternal(LifecycleState.STARTING_PREP, null, false);
        startInternal();
        if (state.equals(LifecycleState.FAILED)) {
            // This is a 'controlled' failure. The component put itself into the
            // FAILED state so call stop() to complete the clean-up.
            stop();
        } else if (!state.equals(LifecycleState.STARTING)) {
            // Shouldn't be necessary but acts as a check that sub-classes are
            // doing what they are supposed to.
            invalidTransition(Lifecycle.AFTER_START_EVENT);
        } else {
            setStateInternal(LifecycleState.STARTED, null, false);
        }
    } catch (Throwable t) {
        // This is an 'uncontrolled' failure so put the component into the
        // FAILED state and throw an exception.
        handleSubClassException(t, "lifecycleBase.startFail", toString());
    }
}

　5、启动tomcat容器

　　创建webServer后在refresh()方法中的finishRefresh()完成tomcat的启动。

　　最终会调用 TomcatWebServer中的start()方法启动tomcat容器。

protected void finishRefresh() {
    // Clear context-level resource caches (such as ASM metadata from scanning).
    clearResourceCaches();

    // Initialize lifecycle processor for this context.
    initLifecycleProcessor();

    // Propagate refresh to lifecycle processor first.
    // 启动tomcat
    getLifecycleProcessor().onRefresh();

    // Publish the final event.
    publishEvent(new ContextRefreshedEvent(this));

    // Participate in LiveBeansView MBean, if active.
    if (!NativeDetector.inNativeImage()) {
        LiveBeansView.registerApplicationContext(this);
    }
}

@Override
public void onRefresh() {
    startBeans(true);
    this.running = true;
}

private void startBeans(boolean autoStartupOnly) {
    Map<String, Lifecycle> lifecycleBeans = getLifecycleBeans();
    Map<Integer, LifecycleGroup> phases = new TreeMap<>();

    lifecycleBeans.forEach((beanName, bean) -> {
        if (!autoStartupOnly || (bean instanceof SmartLifecycle && ((SmartLifecycle) bean).isAutoStartup())) {
            int phase = getPhase(bean);
            phases.computeIfAbsent(
                    phase,
                    p -> new LifecycleGroup(phase, this.timeoutPerShutdownPhase, lifecycleBeans, autoStartupOnly)
            ).add(beanName, bean);
        }
    });
    if (!phases.isEmpty()) {
        // 遍历启动
        phases.values().forEach(LifecycleGroup::start);
    }
}

public void start() {
    if (this.members.isEmpty()) {
        return;
    }
    if (logger.isDebugEnabled()) {
        logger.debug("Starting beans in phase " + this.phase);
    }
    Collections.sort(this.members);
    for (LifecycleGroupMember member : this.members) {
        doStart(this.lifecycleBeans, member.name, this.autoStartupOnly);
    }
}

private void doStart(Map<String, ? extends Lifecycle> lifecycleBeans, String beanName, boolean autoStartupOnly) {
    Lifecycle bean = lifecycleBeans.remove(beanName);
    if (bean != null && bean != this) {
        String[] dependenciesForBean = getBeanFactory().getDependenciesForBean(beanName);
        for (String dependency : dependenciesForBean) {
            doStart(lifecycleBeans, dependency, autoStartupOnly);
        }
        if (!bean.isRunning() &&
                (!autoStartupOnly || !(bean instanceof SmartLifecycle) || ((SmartLifecycle) bean).isAutoStartup())) {
            if (logger.isTraceEnabled()) {
                logger.trace("Starting bean '" + beanName + "' of type [" + bean.getClass().getName() + "]");
            }
            try {
                bean.start();
            }
            catch (Throwable ex) {
                throw new ApplicationContextException("Failed to start bean '" + beanName + "'", ex);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Successfully started bean '" + beanName + "'");
            }
        }
    }
}

//WebServerStartStopLifecycle 中start() 方法的实现
public void start() {
    this.weServerManager.start();
    this.running = true;
}

void start() {
    this.handler.initializeHandler();
    this.webServer.start();
    this.applicationContext
            .publishEvent(new ReactiveWebServerInitializedEvent(this.webServer, this.applicationContext));
}
// 完成对tomcat的启动
@Override
public void start() throws WebServerException {
    synchronized (this.monitor) {
        if (this.started) {
            return;
        }
        try {
            addPreviouslyRemovedConnectors();
            Connector connector = this.tomcat.getConnector();
            if (connector != null && this.autoStart) {
                performDeferredLoadOnStartup();
            }
            checkThatConnectorsHaveStarted();
            this.started = true;
            logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"
                    + getContextPath() + "'");
        }
        catch (ConnectorStartFailedException ex) {
            stopSilently();
            throw ex;
        }
        catch (Exception ex) {
            PortInUseException.throwIfPortBindingException(ex, () -> this.tomcat.getConnector().getPort());
            throw new WebServerException("Unable to start embedded Tomcat server", ex);
        }
        finally {
            Context context = findContext();
            ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
        }
    }
}

同博客: SpringBoot内置Tomcat启动原理源码分析 - sun丶牧羊人 - 博客园