由於SpringBoot預設是以jar包的方式啟動嵌入式的Servlet容器來啟動SpringBoot的web應用，沒有web.xml檔案。

故而採用其他方式來註冊三大元件：程式碼註冊和註解註冊。

【1】ServletRegistrationBean註冊Servlet

檢視ServletRegistrationBean原始碼如下：

/**
 * A {@link ServletContextInitializer} to register {@link Servlet}s in a Servlet 3.0+
 * container. Similar to the {@link ServletContext#addServlet(String, Servlet)
 * registration} features provided by {@link ServletContext} but with a Spring Bean
 * friendly design.
 * <p>
 * The {@link #setServlet(Servlet) servlet} must be specified before calling
 * {@link #onStartup}. URL mapping can be configured used {@link #setUrlMappings} or
 * omitted when mapping to '/*' (unless
 * {@link #ServletRegistrationBean(Servlet, boolean, String...) alwaysMapUrl} is set to
 * {@code false}). The servlet name will be deduced if not specified.
 *
 * @author
 
 Phillip Webb
 * @since 1.4.0
 * @see ServletContextInitializer
 * @see ServletContext#addServlet(String, Servlet)
 */
public class ServletRegistrationBean extends RegistrationBean {

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

    private static final String[] DEFAULT_MAPPINGS = { "/*"
 
 };

    private Servlet servlet;

    private Set<String> urlMappings = new LinkedHashSet<String>();

    private boolean alwaysMapUrl = true;

    private int loadOnStartup = -1;

    private MultipartConfigElement multipartConfig;
    //...
}

註冊示例如下：

 @Bean
 public ServletRegistrationBean myServlet
 
(){
      ServletRegistrationBean registrationBean = new ServletRegistrationBean(new MyServlet(),"/myServlet");
      registrationBean.setLoadOnStartup(1);
      return registrationBean;
  }

【2】FilterRegistrationBean註冊Filter

檢視FilterRegistrationBean原始碼如下：

/**
 * A {@link ServletContextInitializer} to register {@link Filter}s in a Servlet 3.0+
 * container. Similar to the {@link ServletContext#addFilter(String, Filter) registration}
 * features provided by {@link ServletContext} but with a Spring Bean friendly design.
 * <p>
 * The {@link #setFilter(Filter) Filter} must be specified before calling
 * {@link #onStartup(ServletContext)}. Registrations can be associated with
 * {@link #setUrlPatterns URL patterns} and/or servlets (either by {@link #setServletNames
 * name} or via a {@link #setServletRegistrationBeans ServletRegistrationBean}s. When no
 * URL pattern or servlets are specified the filter will be associated to '/*'. The filter
 * name will be deduced if not specified.
 *
 * @author Phillip Webb
 * @since 1.4.0
 * @see ServletContextInitializer
 * @see ServletContext#addFilter(String, Filter)
 * @see DelegatingFilterProxyRegistrationBean
 */
public class FilterRegistrationBean extends AbstractFilterRegistrationBean {

    /**
     * Filters that wrap the servlet request should be ordered less than or equal to this.
     */
    public static final int REQUEST_WRAPPER_FILTER_MAX_ORDER = AbstractFilterRegistrationBean.REQUEST_WRAPPER_FILTER_MAX_ORDER;

    private Filter filter;
    //...
}

註冊示例如下：

@Bean
public FilterRegistrationBean myFilter(){
     FilterRegistrationBean registrationBean = new FilterRegistrationBean();
     registrationBean.setFilter(new MyFilter());
     registrationBean.setUrlPatterns(Arrays.asList("/hello","/myServlet"));
     return registrationBean;
 }

【3】ServletListenerRegistrationBean註冊Listener

檢視ServletListenerRegistrationBean原始碼如下：

/**
 * A {@link ServletContextInitializer} to register {@link EventListener}s in a Servlet
 * 3.0+ container. Similar to the {@link ServletContext#addListener(EventListener)
 * registration} features provided by {@link ServletContext} but with a Spring Bean
 * friendly design.
 *
 * This bean can be used to register the following types of listener:
 * <ul>
 * <li>{@link ServletContextAttributeListener}</li>
 * <li>{@link ServletRequestListener}</li>
 * <li>{@link ServletRequestAttributeListener}</li>
 * <li>{@link HttpSessionAttributeListener}</li>
 * <li>{@link HttpSessionListener}</li>
 * <li>{@link ServletContextListener}</li>
 * </ul>
 *
 * @param <T> the type of listener
 * @author Dave Syer
 * @author Phillip Webb
 * @since 1.4.0
 */
public class ServletListenerRegistrationBean<T extends EventListener>
        extends RegistrationBean {

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

    private static final Set<Class<?>> SUPPORTED_TYPES;

    static {
        Set<Class<?>> types = new HashSet<Class<?>>();
        types.add(ServletContextAttributeListener.class);
        types.add(ServletRequestListener.class);
        types.add(ServletRequestAttributeListener.class);
        types.add(HttpSessionAttributeListener.class);
        types.add(HttpSessionListener.class);
        types.add(ServletContextListener.class);
        SUPPORTED_TYPES = Collections.unmodifiableSet(types);
    }

    private T listener;
    //...
}

註冊Listener示例如下：

 @Bean
    public ServletListenerRegistrationBean myListener(){
        ServletListenerRegistrationBean<MyListener> registrationBean = new ServletListenerRegistrationBean<>(new MyListener());
        return registrationBean;
    }

【4】SpringMVC的DispatcherServlet預設註冊機制

SpringBoot幫我們自動SpringMVC的時候，自動的註冊SpringMVC的前端控制器：DIspatcherServlet。

檢視DispatcherServletAutoConfiguration原始碼如下：

/**
 * {@link EnableAutoConfiguration Auto-configuration} for the Spring
 * {@link DispatcherServlet}. Should work for a standalone application where an embedded
 * servlet container is already present and also for a deployable application using
 * {@link SpringBootServletInitializer}.
 *
 * @author Phillip Webb
 * @author Dave Syer
 * @author Stephane Nicoll
 * @author Brian Clozel
 */
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(EmbeddedServletContainerAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {

    /*
     * 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";

    @Configuration
    @Conditional(DefaultDispatcherServletCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    protected static class DispatcherServletConfiguration {

        private final WebMvcProperties webMvcProperties;

        public DispatcherServletConfiguration(WebMvcProperties webMvcProperties) {
            this.webMvcProperties = webMvcProperties;
        }

        @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
        @ConditionalOnBean(MultipartResolver.class)
        @ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
        public MultipartResolver multipartResolver(MultipartResolver resolver) {
            // Detect if the user has created a MultipartResolver but named it incorrectly
            return resolver;
        }

    }

    @Configuration
    @Conditional(DispatcherServletRegistrationCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    @Import(DispatcherServletConfiguration.class)
    protected static class DispatcherServletRegistrationConfiguration {

        private final ServerProperties serverProperties;

        private final WebMvcProperties webMvcProperties;

        private final MultipartConfigElement multipartConfig;

        public DispatcherServletRegistrationConfiguration(
                ServerProperties serverProperties, WebMvcProperties webMvcProperties,
                ObjectProvider<MultipartConfigElement> multipartConfigProvider) {
            this.serverProperties = serverProperties;
            this.webMvcProperties = webMvcProperties;
            this.multipartConfig = multipartConfigProvider.getIfAvailable();
        }
        // 這裡註冊DispatcherServlet
        @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        @ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public ServletRegistrationBean dispatcherServletRegistration(
                DispatcherServlet dispatcherServlet) {
            ServletRegistrationBean registration = new ServletRegistrationBean(
                    dispatcherServlet, this.serverProperties.getServletMapping());
            registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            registration.setLoadOnStartup(
                    this.webMvcProperties.getServlet().getLoadOnStartup());
            if (this.multipartConfig != null) {
                registration.setMultipartConfig(this.multipartConfig);
            }
            return registration;
        }

    }

    @Order(Ordered.LOWEST_PRECEDENCE - 10)
    private static class DefaultDispatcherServletCondition extends SpringBootCondition {

        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            ConditionMessage.Builder message = ConditionMessage
                    .forCondition("Default DispatcherServlet");
            ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
            List<String> dispatchServletBeans = Arrays.asList(beanFactory
                    .getBeanNamesForType(DispatcherServlet.class, false, false));
            if (dispatchServletBeans.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome.noMatch(message.found("dispatcher servlet bean")
                        .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            if (beanFactory.containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome
                        .noMatch(message.found("non dispatcher servlet bean")
                                .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            if (dispatchServletBeans.isEmpty()) {
                return ConditionOutcome
                        .match(message.didNotFind("dispatcher servlet beans").atAll());
            }
            return ConditionOutcome.match(message
                    .found("dispatcher servlet bean", "dispatcher servlet beans")
                    .items(Style.QUOTE, dispatchServletBeans)
                    .append("and none is named " + DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
        }

    }

    @Order(Ordered.LOWEST_PRECEDENCE - 10)
    private static class DispatcherServletRegistrationCondition
            extends SpringBootCondition {

        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
            ConditionOutcome outcome = checkDefaultDispatcherName(beanFactory);
            if (!outcome.isMatch()) {
                return outcome;
            }
            return checkServletRegistration(beanFactory);
        }

        private ConditionOutcome checkDefaultDispatcherName(
                ConfigurableListableBeanFactory beanFactory) {
            List<String> servlets = Arrays.asList(beanFactory
                    .getBeanNamesForType(DispatcherServlet.class, false, false));
            boolean containsDispatcherBean = beanFactory
                    .containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            if (containsDispatcherBean
                    && !servlets.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome
                        .noMatch(startMessage().found("non dispatcher servlet")
                                .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            return ConditionOutcome.match();
        }

        private ConditionOutcome checkServletRegistration(
                ConfigurableListableBeanFactory beanFactory) {
            ConditionMessage.Builder message = startMessage();
            List<String> registrations = Arrays.asList(beanFactory
                    .getBeanNamesForType(ServletRegistrationBean.class, false, false));
            boolean containsDispatcherRegistrationBean = beanFactory
                    .containsBean(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME);
            if (registrations.isEmpty()) {
                if (containsDispatcherRegistrationBean) {
                    return ConditionOutcome
                            .noMatch(message.found("non servlet registration bean").items(
                                    DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
                }
                return ConditionOutcome
                        .match(message.didNotFind("servlet registration bean").atAll());
            }
            if (registrations
                    .contains(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)) {
                return ConditionOutcome.noMatch(message.found("servlet registration bean")
                        .items(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
            }
            if (containsDispatcherRegistrationBean) {
                return ConditionOutcome
                        .noMatch(message.found("non servlet registration bean").items(
                                DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
            }
            return ConditionOutcome.match(message.found("servlet registration beans")
                    .items(Style.QUOTE, registrations).append("and none is named "
                            + DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
        }

        private ConditionMessage.Builder startMessage() {
            return ConditionMessage.forCondition("DispatcherServlet Registration");
        }

    }

}

【5】Servlet3.0下注冊三大元件的方式

在上面的原始碼註釋中，頻繁見到如下字樣：

 A {@link ServletContextInitializer} to register {@link Filter}s in a Servlet 3.0+
 * container. Similar to the {@link ServletContext#addFilter(String, Filter) registration}
 * features provided by {@link ServletContext} but with a Spring Bean friendly design.

即，SpringBoot提供的三大元件新增方式只是一種更為友好的設計方式，和Servlet3.0中使用ServletContext#addFilter方式一樣！

參考部落格如下：

ServletContainerInitializer並註冊三大元件：部落格地址

Servlet、Filter和Listener(註解註冊)：部落格地址

【6】SpringBoot下使用註冊註冊三大元件

該種方式與【5】中Servlet、Filter和Listener(註解註冊)是一致的，只是需要在主配置類上新增@ServletComponentScan註冊。

示例如下：

@SpringBootApplication
@ServletComponentScan
public class SpringBootApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootApplication.class, args);
    }

    @Bean
    public ViewResolver myViewReolver(){
        return new MyViewResolver();
    }

    public static class MyViewResolver implements ViewResolver{

        @Override
        public View resolveViewName(String viewName, Locale locale) throws Exception {
            return null;
        }
    }
}

然後就可以在Servlet、Filter和Listener上使用註解進行註冊！