springboot中如何启动tomcat

springboot启动时，会自动识别出当前环境是否是web环境还是非web环境。

默认的web环境的context(DEFAULT_WEB_CONTEXT_CLASS)：org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext
默认的非web环境的context(DEFAULT_CONTEXT_CLASS):org.springframework.context.annotation.AnnotationConfigApplicationContext

以下代码为springboot启动类springApplication创建context代码:

ConfigurableApplicationContext org.springframework.boot.SpringApplication.createApplicationContext():

    /**
     * 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_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);
    }

那么我们来详细看下默认的web环境的AnnotationConfigServletWebServerApplicationContext的具体实现：

public class AnnotationConfigServletWebServerApplicationContext extends ServletWebServerApplicationContext implements AnnotationConfigRegistry

发现AnnotationConfigServletWebServerApplicationContext继承了org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext，

我们看AnnotationConfigServletWebServerApplicationContext的代码，发现并没有和webServer(tomcat,jetty....)相关的代码。那么我们继续来看下父类ServletWebServerApplicationContext的实现:

ServletWebServerApplicationContext:发现这个类是boot包下的,继承了GenericWebApplicationContext是spring-web这个jar包，推断springboot嵌入webServer的逻辑很有可能是在这个ServletWebServerApplicationContext中

类结构:public class ServletWebServerApplicationContext extends GenericWebApplicationContext implements ConfigurableWebServerApplicationContext

我们终于在org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext看到了和webServer相关的代码:

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

@Override
    protected void finishRefresh() {
        super.finishRefresh();
        WebServer webServer = startWebServer();
        if (webServer != null) {
            publishEvent(new ServletWebServerInitializedEvent(webServer, this));
        }
    }

其中onRefresh方法会在void org.springframework.context.support.AbstractApplicationContext.refresh() 中会在context注册BeanFactoryPostProcessor、执行BeanFactoryPostProcessor、注册BeanPostProcessor之后调用，

而finishRefresh方法会在void org.springframework.context.support.AbstractApplicationContext.finishRefresh()中会在context完成各种初始化(初始化bean)后被调用到，从而在context启动完成后，启动WebServer(tomcat、jetty)...

接下来我们看下创建WebServer和启动WebServer的逻辑:

private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        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();
    }

private WebServer startWebServer() {
        WebServer webServer = this.webServer;
        if (webServer != null) {
            webServer.start();
        }
        return webServer;
    }