对于Spring Boot的IOC容器——ServletWebServerApplicationContext，其中的Filter bean，每个Filter bean都会被独立的注册成为Servlet的Filter。大概的注册过程分成2步：
- IOC容器——ServletWebServerApplicationContext将Filter接口的实现类封装成FilterRegistrationBean，放到ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)中
- Spring 容器（ServletWebServerApplicationContext）从ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)，中获取所有的ServletContextInitializer实现类，调用它们的onStartUp函数，FilterRegistrationBean的onStartup函数就是在调用ServletContext的addFilter函数向Servlet添加Filter
1. IOC容器——ServletWebServerApplicationContext将Filter接口的实现类封装成FilterRegistrationBean，放到ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)中
- Spring容器——ServletWebServerApplicationContext的refresh()函数被调用。
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
- ServletWebServerApplicationContext的createWebServer()函数，
- 在函数中会调用getSelfInitializer()函数获取所有的ServletContextInitializer实例，
- 而获取所有的ServletContextInitializer实例的过程中，会查找所有实现了Filter接口的bean，并注册成FilterRegistrationBean（实现了ServletContextInitializer接口，所以也属于ServletContextInitializer实例），
- 然后把FilterRegistrationBean放到initializers变量(LinkedMultiValueMap)中
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
ServletWebServerFactory factory = getWebServerFactory();
createWebServer.tag("factory", factory.getClass().toString());
// getSelfInitializer()函数中获取所有ServletContextInitializer实例，调用onStartup方法完成注册
this.webServer = factory.getWebServer(getSelfInitializer());
......
}
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
return this::selfInitialize;
}
private void selfInitialize(ServletContext servletContext) throws ServletException {
prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
// 获取所有ServletContextInitializer实例，调用onStartup方法
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
beans.onStartup(servletContext);
}
}
// 传入BeanFactory，new 一个ServletContextInitializerBeans对象，这个对象实现了Collection接口，是一个集合，
// 集合内的元素是需要内置在Servlet Context上的ServletContextInitializer bean
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
return new ServletContextInitializerBeans(getBeanFactory());
}
- ServletContextInitializerBeans的构建
public ServletContextInitializerBeans(ListableBeanFactory beanFactory, Class<? extends ServletContextInitializer>... initializerTypes) {
this.initializers = new LinkedMultiValueMap<>();
this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
: Collections.singletonList(ServletContextInitializer.class);
// 找直接定义为ServletContextInitializer的bean（比如FilterRegistrationBean），DelegatingFilterProxyRegistrationBean等，
// 添加到initializers中
addServletContextInitializerBeans(beanFactory);
// 找实现了Filter接口的bean，将他们封装成ServletContextInitializer bean（对于Filter的实现类也就是封装成FilterRegistrationBean），
// 添加到initializers中
addAdaptableBeans(beanFactory);
//排序
List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
.flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
.collect(Collectors.toList());
this.sortedList = Collections.unmodifiableList(sortedInitializers);
logMappings(this.initializers);
}
protected void addAdaptableBeans(ListableBeanFactory beanFactory) {
MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);
// 获取实现了Servlet接口的bean
addAsRegistrationBean(beanFactory, Servlet.class, new ServletRegistrationBeanAdapter(multipartConfig));
// 查找所有实现了Filter接口的bean，并注册成FilterRegistrationBean，
// 然后把FilterRegistrationBean放到initializers变量(LinkedMultiValueMap)中
addAsRegistrationBean(beanFactory, Filter.class, new FilterRegistrationBeanAdapter());
for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
addAsRegistrationBean(beanFactory, EventListener.class, (Class<EventListener>) listenerType,
new ServletListenerRegistrationBeanAdapter());
}
}
所以我们可以看到ServletContextInitializerBeans构造函数是怎么构造一个包含
ServletContextInitializer bean集合的：
- 找直接定义为ServletContextInitializer的bean（比如FilterRegistrationBean），
- DelegatingFilterProxyRegistrationBean等，添加到initializers中
找实现了Filter接口的bean，将他们封装成ServletContextInitializer bean（对于Filter的实现类也就是封装成FilterRegistrationBean），添加到initializers中 - 排序然后一并返回
所以ServletContextInitializerBeans实例表示的是：一个从ListableBeanFactory bean容器中获得的ServletContextInitializer实例的集合。这个集合中的每个元素来自容器中定义的每个如下类型的bean :
- 实现了ServletContextInitializer接口的bean
具体可能以ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean的形式存在。这些 bean 设计的目的是用来注册相应的 Servlet/Filter/EventListener bean 到 ServletContext
- **实现了Servlet/Filter/EventListener接口的 bean**
这些 bean直接以实现裸的Servlet/Filter/EventListener接口的 bean的形式存在，但是Springboot会将它们封装成相应的 RegistrationBean(也是ServletContextInitializer ),然后也注册到ServletContext。
也就是说，Spring帮我们将实现裸的Servlet/Filter/EventListener接口的 bean，封装成ServletRegistrationBean/FilterRegistrationBean/EventListenerRegistrationBean。
2. Spring 容器（ServletWebServerApplicationContext）从ServletContextInitializerBeans实例的成员变量initializers变量(LinkedMultiValueMap)，中获取所有的ServletContextInitializer实现类，调用它们的startUp函数，FilterRegistrationBean的onStartup函数就是在调用ServletContext的addFilter函数向Servlet添加Filter
- 还是要回到上面1.2中的createWebServer()函数中，这次我们不看getSelfInitializer()，而是看factory.getWebServer(...)函数。
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
ServletWebServerFactory factory = getWebServerFactory();
createWebServer.tag("factory", factory.getClass().toString());
// getSelfInitializer()函数中获取所有ServletContextInitializer实例，调用onStartup方法完成注册
this.webServer = factory.getWebServer(getSelfInitializer());
......
}
- TomcatServletWebServerFactory的getWebServer(...)函数
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
......
// 将前面获取到的所有ServletContextInitializer实例作为参数，传下去
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
- TomcatServletWebServerFactory的prepareContext(...)
protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
......
// 合并一下
ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
host.addChild(context);
// 将前面获取到的所有ServletContextInitializer实例作为参数，传下去
configureContext(context, initializersToUse);
postProcessContext(context);}
- tomcatServletWebServerFactory的configureContext（...），创建了TomcatStarter，并将TomcatStarter绑定到Tomcat上，这样Tomcat启动时就会回调TomcatStarter的onStartup函数。
protected void configureContext(Context context, ServletContextInitializer[] initializers) {
// 将前面获取到的所有ServletContextInitializer实例作为参数，传下去
TomcatStarter starter = new TomcatStarter(initializers);
// 下面这一系列操作是将TomcatStarter绑定到Tomcat上，这样TomcatStarter就会在Tomcat启动时被回调
if (context instanceof TomcatEmbeddedContext) {
TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;
embeddedContext.setStarter(starter);
embeddedContext.setFailCtxIfServletStartFails(true);
}
context.addServletContainerInitializer(starter, NO_CLASSES);
......
}
- TomcatStarter的startUp函数被调用的时候，就会变量所有的ServletContextInitializer实例的onStartup函数，
public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
try {
for (ServletContextInitializer initializer : this.initializers) {
initializer.onStartup(servletContext);
}
}
catch (Exception ex) {
this.startUpException = ex;
// Prevent Tomcat from logging and re-throwing when we know we can
// deal with it in the main thread, but log for information here.
if (logger.isErrorEnabled()) {
logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: " + ex.getMessage());
}
}
}
- 回顾第一章：“Filter接口的实现类封装成FilterRegistrationBean”，而FilterRegistrationBean又是ServletContextInitializer接口的实现类，所以上面第5步，也会回调FilterRegistrationBean的onStartup函数，
//FilterRegistrationBean
public final void onStartup(ServletContext servletContext) throws ServletException {
String description = getDescription();
if (!isEnabled()) {
logger.info(StringUtils.capitalize(description) + " was not registered (disabled)"); return;
}
// 这里
register(description, servletContext);
}
- DynamicRegistrationBean.class
protected final void register(String description, ServletContext servletContext) {
//这里
D registration = addRegistration(description, servletContext);
if (registration == null) {
logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");
return;
}
configure(registration);
}
- AbstractFilterRegistrationBean.class， 看到没，最终还是用ServletContext的addFilter函数，向Servlet容器中添加Filter。
protected Dynamic addRegistration(String description, ServletContext servletContext) {
// getFilter()在FilterRegistrationBean中被实现，返回的就是那个Filter接口的实现类。
Filter filter = getFilter();
// 将Filter接口的实现类添加到Servlet容器中
return servletContext.addFilter(getOrDeduceName(filter), filter);
}