springboot 2.1.3.RELEASE添加filter,servlet源码学习

Servlet规范中，通过ServeltContext来注册Filter、Servlet，这里分析Filter，Servlet是相同逻辑

springboot2.0中，我们通过

FilterRegistrationBean将指定得filter来实现ServeltContext注册filter

FilterRegistrationBean的实例化过程public FilterRegistrationBean(T filter, ServletRegistrationBean... servletRegistrationBeans) {
        super(servletRegistrationBeans);
        Assert.notNull(filter, "Filter must not be null");
        this.filter = filter;
}super的实例化
AbstractFilterRegistrationBean(ServletRegistrationBean... servletRegistrationBeans) {
        Assert.notNull(servletRegistrationBeans, "ServletRegistrationBeans must not be null");
        Collections.addAll(this.servletRegistrationBeans, servletRegistrationBeans);
}

可知FilterRegistrationBean得实例化过程就是将Filter保存到servletRegistrationBeans（一个set）中

再分析FilterRegistrationBean类

FilterRegistrationBean的父类是一个ServletContextInitializer，他有一个方法onStartup(ServletContext servletContext)

其结果最终会调用

servletContext.addFilter(this.getOrDeduceName(filter), filter)

现在看看ServletContextInitializer.onStartup的调用地方

springboot启动
new SpringApplication(XXApplication.class).run(XXApplication.class,args)
调用
refreshContext(context); -> refresh(context); -> ((AbstractApplicationContext) applicationContext).refresh();
-> ServletWebServerApplicationContext.onRefresh(); -> createWebServer();WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
}

再分析getSelfInitializer方法

prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(),servletContext);
for (ServletContextInitializer beans :getServletContextInitializerBeans()) {
　　　　　　　　　　　　　　//调用ServletContextInitializer的onStartup方法 

　　　　　　　　　　　　　　beans.onStartup(servletContext); 
} 
-> getServletContextInitializerBeans()

-> new ServletContextInitializerBeans(getBeanFactory())
-> addAdaptableBeans(beanFactory)

addAdaptableBeans(ListableBeanFactory beanFactory) 代码如下：
MultipartConfigElement multipartConfig = getMultipartConfig(beanFactory);
//将servlet类的bean包装为ServletRegistrationBean
addAsRegistrationBean(beanFactory, Servlet.class,new ServletRegistrationBeanAdapter(multipartConfig));
//将Filter的bean包装为FilterRegistrationBean
addAsRegistrationBean(beanFactory, Filter.class,new FilterRegistrationBeanAdapter());
for (Class<?> listenerType : ServletListenerRegistrationBean.getSupportedTypes()) {
　　addAsRegistrationBean(beanFactory, EventListener.class,(Class<EventListener>) listenerType,new ServletListenerRegistrationBeanAdapter());
}

这里提一下，如果Servlet的bean实例名为dispatcherServlet，且该实例在seen集合中（seen集合为自定义配置的实例，通过ServletContextInitializerBeans.addServletContextInitializerBean入口可以查看源码），则设置默认值为  /

String url = (totalNumberOfSourceBeans != 1) ? "/" + name + "/" : "/";
if (name.equals(DISPATCHER_SERVLET_NAME)) {
url = "/"; // always map the main dispatcherServlet to "/"
}
ServletRegistrationBean<Servlet> bean = new ServletRegistrationBean<>(source,url);
bean.setName(name);
bean.setMultipartConfig(this.multipartConfig);
return bean;

总结：springboot启动时，会将所有的 FilterRegistrationBean、ServletRegistrationBean以及被spring管理的Servlet和Filter的实例，通过ServletContext注册Servlet以及Filter

如果不希望filter自动注册，可以通过以下方式：

 @Bean
    public FilterRegistrationBean registration(@Qualifier("loginFilter") LoginFilter loginFilter) {
        FilterRegistrationBean registration = new FilterRegistrationBean(loginFilter);
        registration.setEnabled(false);
        return registration;
    }