SpringBoot启动嵌入式Tomcat的实现步骤

public interface ServletWebServerFactory {
    WebServer getWebServer(ServletContextInitializer... initializers);
}

public interface ServletContextInitializer {
    void onStartup(ServletContext servletContext) throws ServletException;
}

public interface WebServerFactoryCustomizer<T extends WebServerFactory> {
    void customize(T factory);
}

private void createWebServer() {
    // WebServer是Spring Boot抽象出来的接口，具体实现类就是不同Web容器
    WebServer webServer = this.webServer;
    ServletContext servletContext = this.getServletContext();
    
    // 若Web容器尚未创建
    if (webServer == null && servletContext == null) {
        // 通过Web容器工厂创建
        ServletWebServerFactory factory = this.getWebServerFactory();
        // 传入一个"SelfInitializer"
        this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});
        
    } else if (servletContext != null) {
        try {
            this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletException var4) {
          ...
        }
    }

    this.initPropertySources();
}

public WebServer getWebServer(ServletContextInitializer... initializers) {
    // 1.实例化一个Tomcat【Server组件】
    Tomcat tomcat = new Tomcat();
    
    // 2. 创建一个临时目录
    File baseDir = this.baseDirectory != null ? this.baseDirectory : this.createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    
    // 3.初始化各种组件
    Connector connector = new Connector(this.protocol);
    tomcat.getService().addConnector(connector);
    this.customizeConnector(connector);
    tomcat.setConnector(connector);
    tomcat.getHost().setAutoDeploy(false);
    this.configureEngine(tomcat.getEngine());
    
    // 4. 创建定制版的"Context"组件
    this.prepareContext(tomcat.getHost(), initializers);
    return this.getTomcatWebServer(tomcat);
}

@Component
public class MyServletRegister implements ServletContextInitializer {

    @Override
    public void onStartup(ServletContext servletContext) {
    
        // Servlet 3.0规范新的API
        ServletRegistration myServlet = servletContext
                .addServlet("HelloServlet", HelloServlet.class);
                
        myServlet.addMapping("/hello");
        
        myServlet.setInitParameter("name", "Hello Servlet");
    }

}

context.addServletContainerInitializer(starter, NO_CLASSES);

@Component
public class MyGeneralCustomizer implements
  WebServerFactoryCustomizer<ConfigurableServletWebServerFactory> {
  
    public void customize(ConfigurableServletWebServerFactory factory) {
        factory.setPort(8081);
        factory.setContextPath("/hello");
     }
}

class TraceValve extends ValveBase {
    @Override
    public void invoke(Request request, Response response) throws IOException, ServletException {

        request.getCoyoteRequest().getMimeHeaders().
        addValue("traceid").setString("1234xxxxabcd");

        Valve next = getNext();
        if (null == next) {
            return;
        }

        next.invoke(request, response);
    }

}

@Component
public class MyTomcatCustomizer implements
        WebServerFactoryCustomizer<TomcatServletWebServerFactory> {

    @Override
    public void customize(TomcatServletWebServerFactory factory) {
        factory.setPort(8081);
        factory.setContextPath("/hello");
        factory.addEngineValves(new TraceValve() );

    }
}