spring boot 2.x启动过程源码解析__启动Tomcat

对于Spring Boot整个的启动，“SpringBoot源码分析之SpringBoot的启动过程”这篇文章分析得很详细所以一懒就不想写了～这里主要想讨论下Spring Boot里内嵌Tomcat的启动。

主要的入口是“createApplicationContext()”方法，目前看很多书和文章都是提到的“AnnotationConfigEmbeddedWebApplicationContext”类，但是查源码的时候发现这个类只在Spring boot 1.x版本中有，应该是2.x版本有所调整。

这里先梳理一下逻辑：

Spring Boot项目的启动是从“SpringApplication.run(Application.class, args);”这句开始的，而这一句定位到代码中，实际会执行的代码如下：

/**
     * Run the Spring application, creating and refreshing a new
     * {@link ApplicationContext}.
     * @param args the application arguments (usually passed from a Java main method)
     * @return a running {@link ApplicationContext}
     */
    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 {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
20             context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            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;
    }

这里我们只点进20行继续跟踪，可以看到：

/**
     * Strategy method used to create the {@link ApplicationContext}. By default this
     * method will respect any explicitly set application context or application context
     * class before falling back to a suitable default.
     * @return the application context (not yet refreshed)
     * @see #setApplicationContextClass(Class)
     */
    protected ConfigurableApplicationContext createApplicationContext() {
        Class<?> contextClass = this.applicationContextClass;
        if (contextClass == null) {
            try {
                switch (this.webApplicationType) {
                case SERVLET:
                    contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                    break;
                case REACTIVE:
                    contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                    break;
                default:
                    contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
                }
            }
            catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
            }
        }
        return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

继续～注意第14行，不难找到“DEFAULT_SERVLET_WEB_CONTEXT_CLASS”的值为：

"org.springframework.boot."
+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext"

然后我们再来看看相关的类图：

类太多不要紧，我们只需要重点关注上面的几个类：

AnnotationConfigServletWebServerApplicationContext（也就是我们刚刚说的类），ServletWebServerApplicationContext，AbstractApplicationContext。

AnnotationConfigServletWebServerApplicationContext继承自ServletWebServerApplicationContext，而ServletWebServerApplicationContext则重写了AbstractApplicationContext中refresh()方法中提到的提供给子类来重写的onRefresh()方法。（接下来继续一步一步跟踪下方标红的代码）

@Override
protected void onRefresh() {
        super.onRefresh();
        try {
 5             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) {
            ServletWebServerFactory factory = getWebServerFactory();
 6             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();
}

然后我们点进TomcatServletWebServerFactory可以看到具体的实现：

@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);
        return getTomcatWebServer(tomcat);
    }