一、方法功能概述

// Initialize other special beans in specific context subclasses.
// 初始化特定上下文子类中的其他特殊bean。
onRefresh();

初始化特定上下文子类中的其他特殊bean。这是个模板方法，实现是空的，交由子类实现，我们看下其子类ServletWebServerApplicationContext的实现，借此可以看看Springboot项目是如何自动启动tomcat的

二、整体代码展示

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

点击createWebServer进去

//ServletWebServerApplicationContext
private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        //从容器里拿到ServletWebServerFactory
        ServletWebServerFactory factory = getWebServerFactory();
        //是用ServletWebServerFactory来创建一个webServer
        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();
}

点击getWebServer进去，可以看到有多个子类实现，对应的不同的webServer容器

我们选择TomcatServletWebServerFactory进去

//TomcatServletWebServerFactory
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
    //直接new一个tomcat,但还需要启动tomcat,我们继续往下看
    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);
    //把tomcat实例作为参数传给TomcatWebServer
    return getTomcatWebServer(tomcat);
}

点击getTomcatWebServer进去

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

继续点看其构造函数

//TomcatWebServer
public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    initialize();
}

可以看到其构造函数中有个initialize的方法，点进去

//TomcatWebServer
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())) {
                    removeServiceConnectors();
                }
            });

            // 此处启动了tomcat
            this.tomcat.start();

            rethrowDeferredStartupExceptions();

            try {
                ContextBindings.bindClassLoader(context, context.getNamingToken(),
                        getClass().getClassLoader());
            }
            catch (NamingException ex) {
                
            }

            startDaemonAwaitThread();
        }
        catch (Exception ex) {
            stopSilently();
            throw new WebServerException("Unable to start embedded Tomcat", ex);
        }
    }
}

可以看到在第16行，启动了tomcat