Springboot之自动注册DispatcherServlet

注意:Springboot的版本是2.0.5.release。

Springboot中我们引入spring-boot-starter-web依赖后，web就自动配置好了，在web.xml的年代，我们需要在web.xml中手动配置DispatcherServlet，但是Springboot中不需要，Springboot是如何替我们做好这一切的呢?

我们来看下DispatcherServletAutoConfiguration，这个类在Springboot-autoconfig包中，如下List-1

List-1

@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;
}

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

@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;
}

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

实例化DispatcherServlet，之后注册到Spring容器中。
实例化DispatcherServletRegistrationBean，并将DispatcherServlet传入到构造方法法中，注册到Spring容器中。

所以说，在Springboot中，有个DispatcherServlet的bean，我们可以写个单元测试验证从BeanFactory中获取DispatcherServlet这个bean，接下来看DispatcherServletRegistrationBean。

如图1所示，DispatcherServletRegistrationBean继承了ServletContextInitializer——见List-2，其中onStartUp的参数ServletContext是Servlet里面的。

RegistrationBean实现了ServletContextInitializer，之后调用register方法，这个是抽象方法，由子类DynamicRegistrationBean实现，DynamicRegistrationBean再将ServletContext用方法addRegistration传递给子类ServletRegistrationBean——见List-3，List-3中用addServlet方法加入的就是DispatcherServletRegistrationBean传递到父类ServletRegistrationBean中的。这样Springboot利用Servlet3.0+的特性，自动注册DispatcherServlet到ServletContext中。

List-2

@FunctionalInterface
public interface ServletContextInitializer {

    /**
     * Configure the given {@link ServletContext} with any servlets, filters, listeners
     * context-params and attributes necessary for initialization.
     * @param servletContext the {@code ServletContext} to initialize
     * @throws ServletException if any call against the given {@code ServletContext}
     * throws a {@code ServletException}
     */
    void onStartup(ServletContext servletContext) throws ServletException;
}

List-3

@Override
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);
}

@Override
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);
}

有个问题，实现了ServletContextInitializer的实例，什么时候会调用onStartup方法呢？来看ServletContextInitializerBeans，这个类在Springboot中，如List-4中:

实例化的时候会从BeanFactory中获取所有的ServletContextInitializer——在getOrderedBeansOfType方法中，之后用addServletContextInitializerBean方法，将获取到的ServletContextInitializer类型的Bean，添加到属性initializers中。
这个地方可以看到，实现了ServletContextInitializer的不止是Servlet类型的，还有Listener、Filter类型的，为什么呢，因为他们都需要动态添加到web容器中，即需要ServletContext。

List-4

public class ServletContextInitializerBeans
        extends AbstractCollection<ServletContextInitializer> {

    private static final String DISPATCHER_SERVLET_NAME = "dispatcherServlet";

    private static final Log logger = LogFactory
            .getLog(ServletContextInitializerBeans.class);

    /**
     * Seen bean instances or bean names.
     */
    private final Set<Object> seen = new HashSet<>();

    private final MultiValueMap<Class<?>, ServletContextInitializer> initializers;

    private List<ServletContextInitializer> sortedList;

    public ServletContextInitializerBeans(ListableBeanFactory beanFactory) {
        this.initializers = new LinkedMultiValueMap<>();
        addServletContextInitializerBeans(beanFactory);
        addAdaptableBeans(beanFactory);
        List<ServletContextInitializer> sortedInitializers = this.initializers.values()
                .stream()
                .flatMap((value) -> value.stream()
                        .sorted(AnnotationAwareOrderComparator.INSTANCE))
                .collect(Collectors.toList());
        this.sortedList = Collections.unmodifiableList(sortedInitializers);
    }

    private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
        for (Entry<String, ServletContextInitializer> initializerBean : getOrderedBeansOfType(
                beanFactory, ServletContextInitializer.class)) {
            addServletContextInitializerBean(initializerBean.getKey(),
                    initializerBean.getValue(), beanFactory);
        }
    }

    private void addServletContextInitializerBean(String beanName,
            ServletContextInitializer initializer, ListableBeanFactory beanFactory) {
        if (initializer instanceof ServletRegistrationBean) {
            Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
            addServletContextInitializerBean(Servlet.class, beanName, initializer,
                    beanFactory, source);
        }
        else if (initializer instanceof FilterRegistrationBean) {
            Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();
            addServletContextInitializerBean(Filter.class, beanName, initializer,
                    beanFactory, source);
        }
        else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {
            String source = ((DelegatingFilterProxyRegistrationBean) initializer)
                    .getTargetBeanName();
            addServletContextInitializerBean(Filter.class, beanName, initializer,
                    beanFactory, source);
        }
        else if (initializer instanceof ServletListenerRegistrationBean) {
            EventListener source = ((ServletListenerRegistrationBean<?>) initializer)
                    .getListener();
            addServletContextInitializerBean(EventListener.class, beanName, initializer,
                    beanFactory, source);
        }
        else {
            addServletContextInitializerBean(ServletContextInitializer.class, beanName,
                    initializer, beanFactory, initializer);
        }
    }
...

接着引出一个问题，ServletContextInitializerBeans这个在哪被调用呢，在ServletWebServerApplicationContext中，在Servlet类型的Sprringboot Web应用中，ApplicationContext是AnnotationConfigServletWebServerApplicationContext，而ServletWebServerApplicationContext正是其父类。

ServletWebServerApplicationContext中，方法onRefresh()-->createWebServer()-->getSelfInitializer()-->selfInitialize()-->getServletContextInitializerBeans()-->new ServletContextInitializerBeans(getBeanFactory())。

ServletWebServerApplicationContext的onRefresh方法覆盖了AbstractApplicationContext的onRefresh方法，AbstractApplicationContext中，方法onRefresh被方法refresh调用。SpringApplication的run()-->refreshContext()-->refresh()-->AbstractApplicationContext的refresh()。

通过上面的分析可以看出，Springboot利用SpringFramework的特性，将DispatcherServlet、Filter或者Listener通过ServletContextInitializer的ServletContext，添加到tomcat之类的web容器中，这些都发生在Springboot启动的过程中。该接口的实现类不会被SpringServletContainerInitializer识别因此不会被Servlet容器自动执行。
ServletContextInitializers主要被Spring管理而不是Servlet容器。

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