"目录自定义 Servlet、Filter、Listener Spring 容器中声明 ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean @ServletComponentScan 注解和 @ ...."

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SpringBoot 源码解析（七）----- Spring Boot的核心能力 - SpringBoot如何实现SpringMvc的？

正文

上一篇我们讲了 SpringBoot 中 Tomcat 的启动过程，本篇我们接着讲在 SpringBoot 中如何向 Tomcat 中添加 Servlet、Filter、Listener

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自定义 Servlet、Filter、Listener

Spring 容器中声明 ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean

@Bean
public ServletRegistrationBean customServlet() {
    return new ServletRegistrationBean(new CustomServlet(), "/custom");
}
private static class CustomServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.getWriter().write("receive by custom servlet");
}
}

先自定义一个Servlet，重写service 实现自己的业务逻辑，然后通过 @Bean 注解往 Spring 容器中注入一个ServletRegistrationBean 类型的 bean 实例，并且实例化一个自定义的 Servlet 作为参数，这样就将自定义的 Servlet 加入 Tomcat 中了。

@ServletComponentScan 注解和 @WebServlet、@WebFilter 以及 @WebListener 注解配合使用

@ServletComponentScan 注解启用 ImportServletComponentScanRegistrar 类，是个 ImportBeanDefinitionRegistrar 接口的实现类，会被 Spring 容器所解析。ServletComponentScanRegistrar 内部会解析 @ServletComponentScan 注解，然后会在 Spring 容器中注册 ServletComponentRegisteringPostProcessor，是个 BeanFactoryPostProcessor，会去解析扫描出来的类是不是有 @WebServlet、@WebListener、@WebFilter 这 3 种注解，有的话把这 3 种类型的类转换成 ServletRegistrationBean、FilterRegistrationBean 或者 ServletListenerRegistrationBean，然后让 Spring 容器去解析：

@SpringBootApplication
@ServletComponentScan
public class EmbeddedServletApplication {...}
@WebServlet(urlPatterns = "/simple")
public class SimpleServlet extends HttpServlet {
@Override
</span><span style="color: rgba(0, 0, 255, 1)">protected</span> <span style="color: rgba(0, 0, 255, 1)">void</span> service(HttpServletRequest req, HttpServletResponse resp) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> ServletException, IOException {
    resp.getWriter().write(</span>"receive by SimpleServlet"<span style="color: rgba(0, 0, 0, 1)">);
}

}

在 Spring 容器中声明 Servlet、Filter 或者 Listener

@Bean(name = "dispatcherServlet")
public DispatcherServlet myDispatcherServlet() {
    return new DispatcherServlet();
}

我们发现往 Tomcat 中添加 Servlet、Filter 或者 Listener 还是挺容易的，大家还记得以前 SpringMVC 是怎么配置DispatcherServlet的吗？在 web.xml 中：

<servlet>
    <servlet-name>dispatcher</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>dispatcher</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

和我们 SpringBoot 中配置 Servlet 相比是不是复杂很多，虽然 SpringBoot 中自定义 Servlet 很简单，但是其底层却不简单，下面我们来分析一下其原理

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ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean

我们来看看这几个特殊的类:

ServletRegistrationBean

public class ServletRegistrationBean extends RegistrationBean {
    //存放目标 Servlet 实例
    private Servlet servlet;
    //存放 Servlet 的 urlMapping
    private Set<String> urlMappings;
    private boolean alwaysMapUrl;
    private int loadOnStartup;
    private MultipartConfigElement multipartConfig;
</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> ServletRegistrationBean(Servlet servlet, String... urlMappings) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>(servlet, <span style="color: rgba(0, 0, 255, 1)">true</span><span style="color: rgba(0, 0, 0, 1)">, urlMappings);
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> ServletRegistrationBean(Servlet servlet, <span style="color: rgba(0, 0, 255, 1)">boolean</span><span style="color: rgba(0, 0, 0, 1)"> alwaysMapUrl, String... urlMappings) {
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.urlMappings = <span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> LinkedHashSet();
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.alwaysMapUrl = <span style="color: rgba(0, 0, 255, 1)">true</span><span style="color: rgba(0, 0, 0, 1)">;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.loadOnStartup = -1<span style="color: rgba(0, 0, 0, 1)">;
    Assert.notNull(servlet, </span>"Servlet must not be null"<span style="color: rgba(0, 0, 0, 1)">);
    Assert.notNull(urlMappings, </span>"UrlMappings must not be null"<span style="color: rgba(0, 0, 0, 1)">);
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.servlet =<span style="color: rgba(0, 0, 0, 1)"> servlet;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.alwaysMapUrl =<span style="color: rgba(0, 0, 0, 1)"> alwaysMapUrl;
    </span><span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.urlMappings.addAll(Arrays.asList(urlMappings));
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span> onStartup(ServletContext servletContext) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> ServletException {
    Assert.notNull(</span><span style="color: rgba(0, 0, 255, 1)">this</span>.servlet, "Servlet must not be null"<span style="color: rgba(0, 0, 0, 1)">);
    String name </span>= <span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.getServletName();
    </span><span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.isEnabled()) {
        logger.info(</span>"Servlet " + name + " was not registered (disabled)"<span style="color: rgba(0, 0, 0, 1)">);
    } </span><span style="color: rgba(0, 0, 255, 1)">else</span><span style="color: rgba(0, 0, 0, 1)"> {
        logger.info(</span>"Mapping servlet: '" + name + "' to " + <span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.urlMappings);
        Dynamic added </span>= servletContext.addServlet(name, <span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.servlet);
        </span><span style="color: rgba(0, 0, 255, 1)">if</span> (added == <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">) {
            logger.info(</span>"Servlet " + name + " was not registered (possibly already registered?)"<span style="color: rgba(0, 0, 0, 1)">);
        } </span><span style="color: rgba(0, 0, 255, 1)">else</span><span style="color: rgba(0, 0, 0, 1)"> {
            </span><span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.configure(added);
        }
    }
}

</span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">略</span>

}

在我们例子中我们通过 return new ServletRegistrationBean(new CustomServlet(), "/custom"); 就知道，ServletRegistrationBean 里会存放目标 Servlet 实例和 urlMapping, 并且继承 RegistrationBean 这个类

FilterRegistrationBean

public class FilterRegistrationBean extends AbstractFilterRegistrationBean {
    //存放目标 Filter 对象
    private Filter filter;
</span></strong><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> FilterRegistrationBean() {
    </span><span style="color: rgba(0, 0, 255, 1)">super</span>(<span style="color: rgba(0, 0, 255, 1)">new</span> ServletRegistrationBean[0<span style="color: rgba(0, 0, 0, 1)">]);
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> FilterRegistrationBean(Filter filter, ServletRegistrationBean... servletRegistrationBeans) {
    </span><span style="color: rgba(0, 0, 255, 1)">super</span><span style="color: rgba(0, 0, 0, 1)">(servletRegistrationBeans);
    Assert.notNull(filter, </span>"Filter must not be null"<span style="color: rgba(0, 0, 0, 1)">);
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.filter =<span style="color: rgba(0, 0, 0, 1)"> filter;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span><span style="color: rgba(0, 0, 0, 1)"> Filter getFilter() {
    </span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.filter;
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> setFilter(Filter filter) {
    Assert.notNull(filter, </span>"Filter must not be null"<span style="color: rgba(0, 0, 0, 1)">);
    </span><span style="color: rgba(0, 0, 255, 1)">this</span>.filter =<span style="color: rgba(0, 0, 0, 1)"> filter;
}

}
abstract class AbstractFilterRegistrationBean extends RegistrationBean {
private static final EnumSet<DispatcherType> ASYNC_DISPATCHER_TYPES;
private static final EnumSet<DispatcherType> NON_ASYNC_DISPATCHER_TYPES;
private static final String[] DEFAULT_URL_MAPPINGS;
private Set<ServletRegistrationBean> servletRegistrationBeans = new LinkedHashSet();
private Set<String> servletNames = new LinkedHashSet();
private Set<String> urlPatterns = new LinkedHashSet();
//重写 onStartup 方法
public void onStartup(ServletContext servletContext) throws ServletException {
Filter filter = this.getFilter();
Assert.notNull(filter, "Filter must not be null");
String name = this.getOrDeduceName(filter);
if (!this.isEnabled()) {
this.logger.info("Filter" + name + "was not registered (disabled)");
} else {
Dynamic added = servletContext.addFilter(name, filter);
if (added == null) {
this.logger.info("Filter" + name + "was not registered (possibly already registered?)");
} else {
this.configure(added);
}
}
}
//略...
}

我们看到 FilterRegistrationBean 中也保存了目标 Filter 对象，并且继承了RegistrationBean

ServletListenerRegistrationBean

public class ServletListenerRegistrationBean<T extends EventListener> extends RegistrationBean { //存放了目标 listener private T listener;
public ServletListenerRegistrationBean() { } public ServletListenerRegistrationBean(T listener) { Assert.notNull(listener, "Listener must not be null"); Assert.isTrue(isSupportedType(listener), "Listener is not of a supported type"); this.listener = listener; } public void setListener(T listener) { Assert.notNull(listener, "Listener must not be null"); Assert.isTrue(isSupportedType(listener), "Listener is not of a supported type"); this.listener = listener; } public void onStartup(ServletContext servletContext) throws ServletException { if (!this.isEnabled()) { logger.info("Listener " + this.listener + " was not registered (disabled)"); } else { try { servletContext.addListener(this.listener); } catch (RuntimeException var3) { throw new IllegalStateException("Failed to add listener '" + this.listener + "' to servlet context", var3); } } } //略...

}

ServletListenerRegistrationBean 也是一样，那我们来看看 RegistrationBean 这个类

public abstract class RegistrationBean implements ServletContextInitializer, Ordered {...} public interface ServletContextInitializer { void onStartup(ServletContext var1) throws ServletException; }

我们发现RegistrationBean 实现了ServletContextInitializer 这个接口，并且有一个 onStartup 方法，ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean 都实现了onStartup 方法。

ServletContextInitializer是 Servlet 容器初始化的时候，提供的初始化接口。所以，Servlet 容器初始化会获取并触发所有的FilterRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean实例中onStartup 方法

那到底是何时触发这些类的 onStartup 方法呢？

当 Tomcat 容器启动时，会执行callInitializers，然后获取所有的ServletContextInitializer，循环执行onStartup方法触发回调方法。那FilterRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean实例是何时加入到Initializers 集合的呢？这要回顾一下我们上一篇文章 Tomcat 的启动过程

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Servlet 容器的启动

大家可以看看我上一篇文章，我这里简单的复制一下代码

EmbeddedWebApplicationContext

1 @Override 2 protected void onRefresh() { 3 super.onRefresh(); 4 try { 5 //核心方法：会获取嵌入式的 Servlet 容器工厂，并通过工厂来获取 Servlet 容器 6 createEmbeddedServletContainer(); 7 } 8 catch (Throwable ex) { 9 throw new ApplicationContextException("Unable to start embedded container", ex); 10 } 11 } 12 13 private void createEmbeddedServletContainer() { 14 EmbeddedServletContainer localContainer = this.embeddedServletContainer; 15 ServletContext localServletContext = getServletContext(); 16 if (localContainer == null && localServletContext == null) { 17 //先获取嵌入式 Servlet 容器工厂 18 EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory(); 19 //根据容器工厂来获取对应的嵌入式 Servlet 容器 20 this.embeddedServletContainer = containerFactory.getEmbeddedServletContainer(getSelfInitializer()); 21 } 22 else if (localServletContext != null) { 23 try { 24 getSelfInitializer().onStartup(localServletContext); 25 } 26 catch (ServletException ex) { 27 throw new ApplicationContextException("Cannot initialize servlet context",ex); 28 } 29 } 30 initPropertySources(); 31 }

关键代码在第 20 行，先通过 getSelfInitializer()获取到所有的 Initializer，传入 Servlet 容器中，那核心就在 getSelfInitializer() 方法：

1 private ServletContextInitializer getSelfInitializer() { 2 //只是创建了一个 ServletContextInitializer 实例返回 3 //所以 Servlet 容器启动的时候，会调用这个对象的 onStartup 方法 4 return new ServletContextInitializer() { 5 public void onStartup(ServletContext servletContext) throws ServletException { 6 EmbeddedWebApplicationContext.this.selfInitialize(servletContext); 7 } 8 }; 9 }

我们看到只是创建了一个 ServletContextInitializer 实例返回，所以 Servlet 容器启动的时候，会调用这个对象的 onStartup 方法，那我们来分析其 onStartup 中的逻辑，也就是 selfInitialize 方法，并将 Servlet 上下文对象传进去了

selfInitialize

1 private void selfInitialize(ServletContext servletContext) throws ServletException { 2 prepareWebApplicationContext(servletContext); 3 ConfigurableListableBeanFactory beanFactory = getBeanFactory(); 4 ExistingWebApplicationScopes existingScopes = new ExistingWebApplicationScopes(beanFactory); 5 WebApplicationContextUtils.registerWebApplicationScopes(beanFactory,getServletContext()); 6 existingScopes.restore(); 7 WebApplicationContextUtils.registerEnvironmentBeans(beanFactory,getServletContext()); 8 //这里便是获取所有的 ServletContextInitializer 实现类，会获取所有的注册组件 9 for (ServletContextInitializer beans : getServletContextInitializerBeans()) { 10 //执行所有 ServletContextInitializer 的 onStartup 方法 11 beans.onStartup(servletContext); 12 } 13 }

关键代码在第 9 和第 11 行，先获取所有的 ServletContextInitializer 实现类，然后遍历执行所有 ServletContextInitializer 的 onStartup 方法

获取所有的 ServletContextInitializer

我们来看看 getServletContextInitializerBeans 方法

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() { return new ServletContextInitializerBeans(getBeanFactory()); }

ServletContextInitializerBeans 对象是对ServletContextInitializer的一种包装：

1 public class ServletContextInitializerBeans extends AbstractCollection<ServletContextInitializer> { 2 private final MultiValueMap<Class<?>, ServletContextInitializer> initializers = new LinkedMultiValueMap(); 3 //存放所有的 ServletContextInitializer 4 private List<ServletContextInitializer> sortedList; 5 6 public ServletContextInitializerBeans(ListableBeanFactory beanFactory) { 7 //执行 addServletContextInitializerBeans 8 this.addServletContextInitializerBeans(beanFactory); 9 //执行 addAdaptableBeans 10 this.addAdaptableBeans(beanFactory); 11 List<ServletContextInitializer> sortedInitializers = new ArrayList(); 12 Iterator var3 = this.initializers.entrySet().iterator(); 13 14 while(var3.hasNext()) { 15 Entry<?, List<ServletContextInitializer>> entry = (Entry)var3.next(); 16 AnnotationAwareOrderComparator.sort((List)entry.getValue()); 17 sortedInitializers.addAll((Collection)entry.getValue()); 18 } 19 this.sortedList = Collections.unmodifiableList(sortedInitializers); 20 } 21 22 private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) { 23 Iterator var2 = this.getOrderedBeansOfType(beanFactory, ServletContextInitializer.class).iterator(); 24 25 while(var2.hasNext()) { 26 Entry<String, ServletContextInitializer> initializerBean = (Entry)var2.next(); 27 this.addServletContextInitializerBean((String)initializerBean.getKey(), (ServletContextInitializer)initializerBean.getValue(), beanFactory); 28 } 29 30 } 31 32 private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer, ListableBeanFactory beanFactory) { 33 if (initializer instanceof ServletRegistrationBean) { 34 Servlet source = ((ServletRegistrationBean)initializer).getServlet(); 35 this.addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source); 36 } else if (initializer instanceof FilterRegistrationBean) { 37 Filter source = ((FilterRegistrationBean)initializer).getFilter(); 38 this.addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); 39 } else if (initializer instanceof DelegatingFilterProxyRegistrationBean) { 40 String source = ((DelegatingFilterProxyRegistrationBean)initializer).getTargetBeanName(); 41 this.addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); 42 } else if (initializer instanceof ServletListenerRegistrationBean) { 43 EventListener source = ((ServletListenerRegistrationBean)initializer).getListener(); 44 this.addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source); 45 } else { 46 this.addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory, initializer); 47 } 48 49 } 50 51 private void addServletContextInitializerBean(Class<?> type, String beanName, ServletContextInitializer initializer, ListableBeanFactory beanFactory, Object source) { 52 this.initializers.add(type, initializer); 53 if (source != null) { 54 this.seen.add(source); 55 } 56 57 if (logger.isDebugEnabled()) { 58 String resourceDescription = this.getResourceDescription(beanName, beanFactory); 59 int order = this.getOrder(initializer); 60 logger.debug("Added existing" + type.getSimpleName() + "initializer bean'"+ beanName +"'; order=" + order + ", resource=" + resourceDescription); 61 } 62 63 } 64 65 private void addAdaptableBeans(ListableBeanFactory beanFactory) { 66 MultipartConfigElement multipartConfig = this.getMultipartConfig(beanFactory); 67 this.addAsRegistrationBean(beanFactory, Servlet.class, new ServletContextInitializerBeans.ServletRegistrationBeanAdapter(multipartConfig)); 68 this.addAsRegistrationBean(beanFactory, Filter.class, new ServletContextInitializerBeans.FilterRegistrationBeanAdapter(null)); 69 Iterator var3 = ServletListenerRegistrationBean.getSupportedTypes().iterator(); 70 71 while(var3.hasNext()) { 72 Class<?> listenerType = (Class)var3.next(); 73 this.addAsRegistrationBean(beanFactory, EventListener.class, listenerType, new ServletContextInitializerBeans.ServletListenerRegistrationBeanAdapter(null)); 74 } 75 76 } 77 78 public Iterator<ServletContextInitializer> iterator() { 79 //返回所有的 ServletContextInitializer 80 return this.sortedList.iterator(); 81 } 82 83 //略... 84 }

我们看到 ServletContextInitializerBeans 中有一个存放所有 ServletContextInitializer 的集合 sortedList，就是在其构造方法中获取所有的 ServletContextInitializer，并放入 sortedList 集合中，那我们来看看其构造方法的逻辑，看到第 8 行先调用

addServletContextInitializerBeans 方法：　　

1 private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) { 2 //从 Spring 容器中获取所有 ServletContextInitializer.class 类型的 Bean 3 for (Entry<String, ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory, ServletContextInitializer.class)) { 4 //添加到具体的集合中 5 addServletContextInitializerBean(initializerBean.getKey(),initializerBean.getValue(), beanFactory); 6 } 7 }

我们看到先从 Spring 容器中获取所有ServletContextInitializer.class 类型的 Bean，这里我们自定义的 ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean 就被获取到了，然后调用 addServletContextInitializerBean 方法：

1 private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer, ListableBeanFactory beanFactory) { 2 //判断 ServletRegistrationBean 类型 3 if (initializer instanceof ServletRegistrationBean) { 4 Servlet source = ((ServletRegistrationBean)initializer).getServlet(); 5 //将 ServletRegistrationBean 加入到集合中 6 this.addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source); 7 //判断 FilterRegistrationBean 类型 8 } else if (initializer instanceof FilterRegistrationBean) { 9 Filter source = ((FilterRegistrationBean)initializer).getFilter(); 10 //将 ServletRegistrationBean 加入到集合中 11 this.addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); 12 } else if (initializer instanceof DelegatingFilterProxyRegistrationBean) { 13 String source = ((DelegatingFilterProxyRegistrationBean)initializer).getTargetBeanName(); 14 this.addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source); 15 } else if (initializer instanceof ServletListenerRegistrationBean) { 16 EventListener source = ((ServletListenerRegistrationBean)initializer).getListener(); 17 this.addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source); 18 } else { 19 this.addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory, initializer); 20 } 21 22 } 23 24 private void addServletContextInitializerBean(Class<?> type, String beanName, 25 ServletContextInitializer initializer, ListableBeanFactory beanFactory, Object source) { 26 //加入到 initializers 中 27 this.initializers.add(type, initializer); 28 }

很明显，判断从 Spring 容器中获取的 ServletContextInitializer 类型，如 ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean，并加入到 initializers 集合中去，我们再来看构造器中的另外一个方法addAdaptableBeans(beanFactory)：

1 private void addAdaptableBeans(ListableBeanFactory beanFactory) { 2 //从 beanFactory 获取所有 Servlet.class 和 Filter.class 类型的 Bean，并封装成 RegistrationBean 对象，加入到集合中 3 this.addAsRegistrationBean(beanFactory, Servlet.class, new ServletContextInitializerBeans.ServletRegistrationBeanAdapter(multipartConfig)); 4 this.addAsRegistrationBean(beanFactory, Filter.class, new ServletContextInitializerBeans.FilterRegistrationBeanAdapter(null)); 5 } 6 7 private <T, B extends T> void addAsRegistrationBean(ListableBeanFactory beanFactory, Class<T> type, Class beanType, ServletContextInitializerBeans.RegistrationBeanAdapter<T> adapter) { 8 //从 Spring 容器中获取所有的 Servlet.class 和 Filter.class 类型的 Bean 9 List<Entry<String, B>> beans = this.getOrderedBeansOfType(beanFactory, beanType, this.seen); 10 Iterator var6 = beans.iterator(); 11 12 while(var6.hasNext()) { 13 Entry<String, B> bean = (Entry)var6.next(); 14 if (this.seen.add(bean.getValue())) { 15 int order = this.getOrder(bean.getValue()); 16 String beanName = (String)bean.getKey(); 17 //创建 Servlet.class 和 Filter.class 包装成 RegistrationBean 对象 18 RegistrationBean registration = adapter.createRegistrationBean(beanName, bean.getValue(), beans.size()); 19 registration.setName(beanName); 20 registration.setOrder(order); 21 this.initializers.add(type, registration); 22 if (logger.isDebugEnabled()) { 23 logger.debug("Created" + type.getSimpleName() + "initializer for bean'"+ beanName +"'; order=" + order + ", resource=" + this.getResourceDescription(beanName, beanFactory)); 24 } 25 } 26 } 27 28 }

我们看到先从 beanFactory 获取所有 Servlet.class 和 Filter.class 类型的 Bean，然后通过ServletRegistrationBeanAdapter 和FilterRegistrationBeanAdapter 两个适配器将 Servlet.class 和 Filter.class 封装成RegistrationBean

private static class ServletRegistrationBeanAdapter implements ServletContextInitializerBeans.RegistrationBeanAdapter<Servlet> { private final MultipartConfigElement multipartConfig;
ServletRegistrationBeanAdapter(MultipartConfigElement multipartConfig) { this.multipartConfig = multipartConfig; } public RegistrationBean createRegistrationBean(String name, Servlet source, int totalNumberOfSourceBeans) { String url = totalNumberOfSourceBeans == 1 ? "/" : "/" + name + "/"; if (name.equals("dispatcherServlet")) { url = "/"; } //还是将Servlet.class实例封装成ServletRegistrationBean对象 //这和我们自己创建ServletRegistrationBean对象是一模一样的 ServletRegistrationBean bean = new ServletRegistrationBean(source, new String[]{url}); bean.setMultipartConfig(this.multipartConfig); return bean; }

}

private static class FilterRegistrationBeanAdapter implements ServletContextInitializerBeans.RegistrationBeanAdapter<Filter> {
private FilterRegistrationBeanAdapter() {
}

public RegistrationBean createRegistrationBean(String name, Filter source, int totalNumberOfSourceBeans) { //Filter.class实例封装成FilterRegistrationBean对象 return new FilterRegistrationBean(source, new ServletRegistrationBean[0]); }

}

代码中注释很清楚了还是将 Servlet.class 实例封装成 ServletRegistrationBean 对象，将 Filter.class 实例封装成 FilterRegistrationBean 对象，这和我们自己定义 ServletRegistrationBean 对象是一模一样的，现在所有的 ServletRegistrationBean、FilterRegistrationBean

Servlet.class、Filter.class 都添加到 List<ServletContextInitializer> sortedList 这个集合中去了，接着就是遍历这个集合，执行其onStartup方法了

ServletContextInitializer 的onStartup方法

ServletRegistrationBean

public class ServletRegistrationBean extends RegistrationBean { private static final Log logger = LogFactory.getLog(ServletRegistrationBean.class); private static final String[] DEFAULT_MAPPINGS = new String[]{"/*"}; private Servlet servlet;
public void onStartup(ServletContext servletContext) throws ServletException { Assert.notNull(this.servlet, "Servlet must not be null"); String name = this.getServletName(); //调用ServletContext的addServlet Dynamic added = servletContext.addServlet(name, this.servlet); } //略...

}

private javax.servlet.ServletRegistration.Dynamic addServlet(String servletName, String servletClass, Servlet servlet, Map<String, String> initParams) throws IllegalStateException {
if (servletName != null && !servletName.equals("")) {
if (!this.context.getState().equals(LifecycleState.STARTING_PREP)) {
throw new IllegalStateException(sm.getString("applicationContext.addServlet.ise", new Object[]{this.getContextPath()}));
} else {
Wrapper wrapper = (Wrapper)this.context.findChild(servletName);
if (wrapper == null) {
wrapper = this.context.createWrapper();
wrapper.setName(servletName);
this.context.addChild(wrapper);
} else if (wrapper.getName() != null && wrapper.getServletClass() != null) {
if (!wrapper.isOverridable()) {
return null;
}

wrapper.setOverridable(false); } if (servlet == null) { wrapper.setServletClass(servletClass); } else { wrapper.setServletClass(servlet.getClass().getName()); wrapper.setServlet(servlet); } if (initParams != null) { Iterator i$ = initParams.entrySet().iterator(); while(i$.hasNext()) { Entry<String, String> initParam = (Entry)i$.next(); wrapper.addInitParameter((String)initParam.getKey(), (String)initParam.getValue()); } } return this.context.dynamicServletAdded(wrapper); } } else { throw new IllegalArgumentException(sm.getString("applicationContext.invalidServletName", new Object[]{servletName})); }

}

看到没，ServletRegistrationBean 中的 onStartup 先获取 Servlet 的 name，然后调用 ServletContext 的 addServlet 将 Servlet 加入到 Tomcat 中，这样我们就能发请求给这个 Servlet 了。

AbstractFilterRegistrationBean

public void onStartup(ServletContext servletContext) throws ServletException { Filter filter = this.getFilter(); Assert.notNull(filter, "Filter must not be null"); String name = this.getOrDeduceName(filter); //调用 ServletContext 的 addFilter Dynamic added = servletContext.addFilter(name, filter); }

AbstractFilterRegistrationBean 也是同样的原理，先获取目标 Filter，然后调用 ServletContext 的addFilter将 Filter 加入到 Tomcat 中，这样 Filter 就能拦截我们请求了。

回到顶部
DispatcherServletAutoConfiguration

最熟悉的莫过于，在 Spring Boot 在自动配置 SpringMVC 的时候，会自动注册 SpringMVC 前端控制器：DispatcherServlet，该控制器主要在DispatcherServletAutoConfiguration自动配置类中进行注册的。DispatcherServlet 是 SpringMVC 中的核心分发器。DispatcherServletAutoConfiguration 也在 spring.factories 中配置了

DispatcherServletConfiguration

1 @Configuration 2 @ConditionalOnWebApplication 3 // 先看下 ClassPath 下是否有 DispatcherServlet.class 字节码 4 // 我们引入了 spring-boot-starter-web，同时引入了 tomcat 和 SpringMvc, 肯定会存在 DispatcherServlet.class 字节码 5 @ConditionalOnClass({DispatcherServlet.class}) 6 // 这个配置类的执行要在 EmbeddedServletContainerAutoConfiguration 配置类生效之后执行 7 // 毕竟要等 Tomcat 启动后才能往其中注入 DispatcherServlet 8 @AutoConfigureAfter({EmbeddedServletContainerAutoConfiguration.class}) 9 protected static class DispatcherServletConfiguration { 10 public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet"; 11 public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration"; 12 @Autowired 13 private ServerProperties server; 14 15 @Autowired 16 private WebMvcProperties webMvcProperties; 17 18 @Autowired(required = false) 19 private MultipartConfigElement multipartConfig; 20 21 // Spring 容器注册 DispatcherServlet 22 @Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME) 23 public DispatcherServlet dispatcherServlet() { 24 // 直接构造 DispatcherServlet，并设置 WebMvcProperties 中的一些配置 25 DispatcherServlet dispatcherServlet = new DispatcherServlet(); 26 dispatcherServlet.setDispatchOptionsRequest(this.webMvcProperties.isDispatchOptionsRequest()); 27 dispatcherServlet.setDispatchTraceRequest(this.webMvcProperties.isDispatchTraceRequest()); 28 dispatcherServlet.setThrowExceptionIfNoHandlerFound(this.webMvcProperties.isThrowExceptionIfNoHandlerFound()); 29 return dispatcherServlet; 30 } 31 32 @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME) 33 public ServletRegistrationBean dispatcherServletRegistration() { 34 // 直接使用 DispatcherServlet 和 server 配置中的 servletPath 路径构造 ServletRegistrationBean 35 // ServletRegistrationBean 实现了 ServletContextInitializer 接口，在 onStartup 方法中对应的 Servlet 注册到 Servlet 容器中 36 // 所以这里 DispatcherServlet 会被注册到 Servlet 容器中，对应的 urlMapping 为 server.servletPath 配置 37 ServletRegistrationBean registration = new ServletRegistrationBean(dispatcherServlet(), this.server.getServletMapping()); 38 registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME); 39 if (this.multipartConfig != null) { 40 registration.setMultipartConfig(this.multipartConfig); 41 } 42 return registration; 43 } 44 45 @Bean // 构造文件上传相关的 bean 46 @ConditionalOnBean(MultipartResolver.class) 47 @ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME) 48 public MultipartResolver multipartResolver(MultipartResolver resolver) { 49 return resolver; 50 } 51 52 }

先看下 ClassPath 下是否有 DispatcherServlet.class 字节码，我们引入了 spring-boot-starter-web，同时引入了 tomcat 和 SpringMvc, 肯定会存在 DispatcherServlet.class 字节码，如果没有导入 spring-boot-starter-web，则这个配置类将不会生效

然后往 Spring 容器中注册 DispatcherServlet 实例，接着又加入 ServletRegistrationBean 实例，并把 DispatcherServlet 实例作为参数，上面我们已经学过了 ServletRegistrationBean 的逻辑，在 Tomcat 启动的时候，会获取所有的 ServletRegistrationBean，并执行其中的 onstartup 方法，将 DispatcherServlet 注册到 Servlet 容器中，这样就类似原来的 web.xml 中配置的 dispatcherServlet。

<servlet> <servlet-name>dispatcherServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>dispatcherServlet</servlet-name> <url-pattern>/</url-pattern> </servlet-mapping>

所以只要导入了 spring-boot-starter-web 这个 starter，SpringBoot 就有了 Tomcat 容器，并且往 Tomcat 容器中注册了 DispatcherServlet 对象，这样就能接收到我们的请求了

SpringBoot 源码解析（七）----- Spring Boot的核心能力 - SpringBoot如何实现SpringMvc的？

自定义 Servlet、Filter、Listener

Spring 容器中声明 ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean

@ServletComponentScan 注解和 @WebServlet、@WebFilter 以及 @WebListener 注解配合使用

在 Spring 容器中声明 Servlet、Filter 或者 Listener

ServletRegistrationBean、FilterRegistrationBean、ServletListenerRegistrationBean

ServletRegistrationBean

FilterRegistrationBean

ServletListenerRegistrationBean

Servlet 容器的启动

获取所有的 ServletContextInitializer

ServletContextInitializer 的onStartup方法

DispatcherServletAutoConfiguration

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