springboot中如何注册servlet和filter

法1

通过@web*开头的注解

    @WebServlet
    public class HalloServlet extends HttpServlet{}

---

    @WebFilter("/hello/*")
    public class HelloFilter implements Filter {}

---    

    //启动类中需要通过@ServletComponentScan扫描这些注解
    @SpringBootApplication
    @ServletComponentScan
    public class SpringBootServletApplication {
       public static void main(String[] args) {
           SpringApplication.run(SpringBootServletApplication.class, args);
       }
    }

法2

通过RegistrationBean

    @Bean
    public ServletRegistrationBean helloServlet() {
        ServletRegistrationBean helloServlet = new ServletRegistrationBean();
        helloServlet.addUrlMappings("/hello");
        helloServlet.setServlet(new HelloServlet());
        return helloServlet;
    }

    @Bean
    public FilterRegistrationBean helloFilter() {
        FilterRegistrationBean helloFilter = new FilterRegistrationBean();
        helloFilter.addUrlPatterns("/hello/*");
        helloFilter.setFilter(new HelloFilter());
        return helloWorldFilter;
    }

springboot如何加载DispatcherServlet？

我们只讨论springboot环境下使用内嵌tomcat容器的情况。
springboot版本 2.0.2.RELEASE

DispatcherServlet初始化的位置

// DispatcherServletAutoConfiguration.class


/*
 * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
 */
public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public DispatcherServlet dispatcherServlet() {
    DispatcherServlet dispatcherServlet = new DispatcherServlet();
    dispatcherServlet.setDispatchOptionsRequest(
            this.webMvcProperties.isDispatchOptionsRequest());
    dispatcherServlet.setDispatchTraceRequest(
            this.webMvcProperties.isDispatchTraceRequest());
    dispatcherServlet.setThrowExceptionIfNoHandlerFound(
            this.webMvcProperties.isThrowExceptionIfNoHandlerFound());
    return dispatcherServlet;
}

同时也需要关注下DispatcherServlet的RigistrationBean

// DispatcherServletAutoConfiguration.class

/*
 * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
 */
public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

/*
 * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
 */
public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

@Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
@ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
public ServletRegistrationBean<DispatcherServlet> dispatcherServletRegistration(
        DispatcherServlet dispatcherServlet) {
    ServletRegistrationBean<DispatcherServlet> registration = new ServletRegistrationBean<>(
            dispatcherServlet,
            this.serverProperties.getServlet().getServletMapping());
    registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
    registration.setLoadOnStartup(
            this.webMvcProperties.getServlet().getLoadOnStartup());
    if (this.multipartConfig != null) {
        registration.setMultipartConfig(this.multipartConfig);
    }
    return registration;
}

可以看出，ServletRegistrationBean中的servlet属性即为上面创建的DispatcherServlet，现在 RegistrationBean准备好了，那么是在什么地方被使用的呢？

DispatcherServlet的加载流程

TomcatStarter
首先在tomcatstarter中 会有整体对ServletContextInitializer的onStartup方法的调用。

// TomcatStarter.class

@Override
public void onStartup(Set<Class<?>> classes, ServletContext servletContext)
        throws ServletException {
    try {
        for (ServletContextInitializer initializer : this.initializers) {
            initializer.onStartup(servletContext);
        }
    }
    catch (Exception ex) {
        this.startUpException = ex;
        // Prevent Tomcat from logging and re-throwing when we know we can
        // deal with it in the main thread, but log for information here.
        if (logger.isErrorEnabled()) {
            logger.error("Error starting Tomcat context. Exception: "
                    + ex.getClass().getName() + ". Message: " + ex.getMessage());
        }
    }
}

image.png

通过断点信息，并没有在其中发现RegisterBean的相关信息
为了真正理解springboot如何加载servlet，还得研究下ServletWebServerApplicationContext，来看看ServletWebServerApplicationContext的逻辑流程：

//ServletWebServerApplicationContext.class

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

// onRefresh方法中 调用了createWebServer()
private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        ServletWebServerFactory factory = getWebServerFactory();
        //下一层的入口 
        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();
}

// 然后是下一层的入口getSelfInitializer()，我们在TomcatStarter中看到的其实就是这里返回的匿名类
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
}

private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(
            beanFactory);
    WebApplicationContextUtils.registerWebApplicationScopes(beanFactory,
            getServletContext());
    existingScopes.restore();
    WebApplicationContextUtils.registerEnvironmentBeans(beanFactory,
            getServletContext());
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

// 在selfInitialize()中，转而调用getServletContextInitializerBeans()
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory());
}

现在流程转到了ServletContextInitializerBeans中，那么ServletContextInitializerBeans做了什么呢

//ServletContextInitializerBeans.class

// 在构造方法中，调用了addServletContextInitializerBeans()方法，看起来离我们的目标不远了
public ServletContextInitializerBeans(ListableBeanFactory beanFactory) {
    this.initializers = new LinkedMultiValueMap<>();
    addServletContextInitializerBeans(beanFactory);
    addAdaptableBeans(beanFactory);
    List<ServletContextInitializer> sortedInitializers = new ArrayList<>();
    this.initializers.values().forEach((contextInitializers) -> {
        AnnotationAwareOrderComparator.sort(contextInitializers);
        sortedInitializers.addAll(contextInitializers);
    });
    this.sortedList = Collections.unmodifiableList(sortedInitializers);
}

//addServletContextInitializerBeans()方法会去容器中寻找注册过的ServletContextInitializer
private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
    for (Entry<String, ServletContextInitializer> initializerBean : getOrderedBeansOfType(
            beanFactory, ServletContextInitializer.class)) {
        addServletContextInitializerBean(initializerBean.getKey(),
                initializerBean.getValue(), beanFactory);
    }
}

image.png

到此，整个流程就串起来了。

• 首先，以ServletWebServerApplicationContext中的onRefresh()方法为起点触发配置了一个匿名的ServletContextInitializer。
• 这个匿名的ServletContextInitializer的onStart方法会去容器中找到所有的RegisterBean并加载到ServletContext中，ServletContainer 其实就是 servlet filter listener 的汇总。
• 这个匿名的ServletContextInitializer最终传递给TomcatStarter，由TomcatStarter的onStartup方法触发ServletContextInitializer 的 onStartup 方法，完成装配。

后记

是哪里调用的TomcatStarter的onStart方法呢

在TomcatServletWebServerFactory中可以看到TomcatStart是被new出来的

// TomcatServletWebServerFactory.class

protected void configureContext(Context context,
        ServletContextInitializer[] initializers) {
    TomcatStarter starter = new TomcatStarter(initializers);
    if (context instanceof TomcatEmbeddedContext) {
        // Should be true
        ((TomcatEmbeddedContext) context).setStarter(starter);
    }
    context.addServletContainerInitializer(starter, NO_CLASSES);
    ......
}

那么 TomcatServletWebServerFactory 又是如何被创建的呢，经过寻找，是在ServletWebServerFactoryConfiguration中

//ServletWebServerFactoryConfiguration.class

@Configuration
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {

    @Bean
    public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
        return new TomcatServletWebServerFactory();
    }

}

满足上面的条件，就会触发springboot的自动配置。

后后记

ServletRegistrationBean的onStract方法干了什么呢？

image.png

可以看到ServletRegistrationBean是继承自RegistartionBean的,在RegistartionBean的onStart方法中，调用了register方法。

// RegistartionBean.class

@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
    String description = getDescription();
    if (!isEnabled()) {
        logger.info(StringUtils.capitalize(description)
                + " was not registered (disabled)");
        return;
    }
    register(description, servletContext);
}

register方法被DynamicRegistrationBean实现,在register方法中，转而调用了addRegistration方法。

// DynamicRegistrationBean.class

@Override
protected final void register(String description, ServletContext servletContext) {
    D registration = addRegistration(description, servletContext);
    if (registration == null) {
        logger.info(StringUtils.capitalize(description) + " was not registered "
                + "(possibly already registered?)");
        return;
    }
    configure(registration);
}

该方法最终被ServletRegistrationBean实现。

//ServletRegistrationBean.class

protected ServletRegistration.Dynamic addRegistration(String description,
        ServletContext servletContext) {
    String name = getServletName();
    logger.info("Servlet " + name + " mapped to " + this.urlMappings);
    return servletContext.addServlet(name, this.servlet);
}

可以看到 ，最终其实就是将servlet放入servletContext中。