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>
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
Java 面试宝典是大明哥全力打造的 Java 精品面试题，它是一份靠谱、强大、详细、经典的 Java 后端面试宝典。它不仅仅只是一道道面试题，而是一套完整的 Java 知识体系，一套你 Java 知识点的扫盲贴。
它的内容包括：
- 大厂真题：Java 面试宝典里面的题目都是最近几年的高频的大厂面试真题。
- 原创内容：Java 面试宝典内容全部都是大明哥原创，内容全面且通俗易懂，回答部分可以直接作为面试回答内容。
- 持续更新：一次购买，永久有效。大明哥会持续更新 3+ 年，累计更新 1000+，宝典会不断迭代更新，保证最新、最全面。
- 覆盖全面：本宝典累计更新 1000+，从 Java 入门到 Java 架构的高频面试题，实现 360° 全覆盖。
- 不止面试：内容包含面试题解析、内容详解、知识扩展，它不仅仅只是一份面试题，更是一套完整的 Java 知识体系。
- 宝典详情：https://www.yuque.com/chenssy/sike-java/xvlo920axlp7sf4k
- 宝典总览：https://www.yuque.com/chenssy/sike-java/yogsehzntzgp4ly1
- 宝典进展：https://www.yuque.com/chenssy/sike-java/en9ned7loo47z5aw
目前 Java 面试宝典累计更新 400+ 道，总字数 42w+。大明哥还在持续更新中，下图是大明哥在 2024-12 月份的更新情况：
想了解详情的小伙伴，扫描下面二维码加大明哥微信【daming091】咨询
同时，大明哥也整理一套目前市面最常见的热点面试题。微信搜[大明哥聊 Java]或扫描下方二维码关注大明哥的原创公众号[大明哥聊 Java] ，回复【面试题】 即可免费领取。
