SpringBoot 源码解析（七）----- Spring Boot的…

2019-11-22 09:31:19来源：博客园阅读 ()

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

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

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
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getWriter().write("receive by SimpleServlet");
    }
}

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

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;


    public ServletRegistrationBean(Servlet servlet, String... urlMappings) {
        this(servlet, true, urlMappings);
    }

    public ServletRegistrationBean(Servlet servlet, boolean alwaysMapUrl, String... urlMappings) {
        this.urlMappings = new LinkedHashSet();
        this.alwaysMapUrl = true;
        this.loadOnStartup = -1;
        Assert.notNull(servlet, "Servlet must not be null");
        Assert.notNull(urlMappings, "UrlMappings must not be null");
        this.servlet = servlet;
        this.alwaysMapUrl = alwaysMapUrl;
        this.urlMappings.addAll(Arrays.asList(urlMappings));
    }
    
    public void onStartup(ServletContext servletContext) throws ServletException {
        Assert.notNull(this.servlet, "Servlet must not be null");
        String name = this.getServletName();
        if (!this.isEnabled()) {
            logger.info("Servlet " + name + " was not registered (disabled)");
        } else {
            logger.info("Mapping servlet: '" + name + "' to " + this.urlMappings);
            Dynamic added = servletContext.addServlet(name, this.servlet);
            if (added == null) {
                logger.info("Servlet " + name + " was not registered (possibly already registered?)");
            } else {
                this.configure(added);
            }
        }
    }
    
    //略
}

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;

    public FilterRegistrationBean() {
        super(new ServletRegistrationBean[0]);
    }

    public FilterRegistrationBean(Filter filter, ServletRegistrationBean... servletRegistrationBeans) {
        super(servletRegistrationBeans);
        Assert.notNull(filter, "Filter must not be null");
        this.filter = filter;
    }

    public Filter getFilter() {
        return this.filter;
    }

    public void setFilter(Filter filter) {
        Assert.notNull(filter, "Filter must not be null");
        this.filter = 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

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<B> 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。

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>