Tomcat 容器的初始化和启动

├─onRefresh();   Spring 核心方法中的
│ ├─ServletWebServerApplicationContext.onRefresh();     实际上是这个 onRefresh();

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
        implements ConfigurableWebServerApplicationContext {
    @Override
    protected void onRefresh() {
        super.onRefresh();
        try {
            // 这里创建了 Tomcat 对象并启动。
            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");
            // 默认使用的是 TomcatServletWebServerFactory
            ServletWebServerFactory factory = getWebServerFactory();
            createWebServer.tag("factory", factory.getClass().toString());
            // 主要看这里，创建 Tomcat对象并启动
            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 {
                /**
                 * 这里调用了钩子方法，调用 所有实现了 ServletContextInitializer 类的 onStartup 方法
                 * 在 DispatcherServletAutoConfiguration 自动配置的时候，
                 * 首先, 定义了一个 @Bean DispatcherServlet;
                 * 然后, 定义了一个 @Bean DispatcherServletRegistrationBean, 用来添加 DispatcherServlet 到 ServletContext;
                 * DispatcherServletRegistrationBean extends ServletRegistrationBean
                 * ServletRegistrationBean extends DynamicRegistrationBean
                 * DynamicRegistrationBean extends RegistrationBean
                 * RegistrationBean implements ServletContextInitializer
                 * 至此，就可以解释的通，在这儿，tomcat 创建后，就可以通过 onStartup 注册 Servlet 组件到 ServletContext。
                 * 同理，Filter, Listener 一样。
                 * 即这里的钩子方法将所有的Spring容器中实例化的 Servlet, Filter, listener 全都添加到 ServletContext 中了。
                 */
                getSelfInitializer().onStartup(servletContext);
            }
            catch (ServletException ex) {
                throw new ApplicationContextException("Cannot initialize servlet context", ex);
            }
        }
        initPropertySources();
    }
}

public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory
		implements ConfigurableTomcatWebServerFactory, ResourceLoaderAware {
    @Override
    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);
        // 主要看这里，创建 Tomcat对象并启动
        return getTomcatWebServer(tomcat);
    }

    protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
        // 主要看这里，创建 Tomcat对象并启动
        return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
    }
}

public class TomcatWebServer implements WebServer {
    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;
        // 主要看这里，启动 tomcat
        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
                // 这里启动 tomcat 容器
                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);
            }
        }
    }
}