ServletContainerInitializer与ServletContextInitializer学习笔记

ServletContainerInitializer加载原理及过程

1. Servlet3.x规范利用spi机制，自动加载实现ServletContainerInitializer接口的类,并调用onStartup方法。
2. SpringServletContainerInitializer根据@HandlesTypes(WebApplicationInitializer.class)进行class筛选。
3. 在onStartup中调用所有WebApplicationInitializer对象的onStart方法。
4. SpringServletContainerInitializer代码实现

   @HandlesTypes(WebApplicationInitializer.class)
   public class SpringServletContainerInitializer implements ServletContainerInitializer {
         
         
      @Override
      public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)throws ServletException {
         
         
      	List<WebApplicationInitializer> initializers = new LinkedList<>();
      	if (webAppInitializerClasses != null) {
         
         
      		for (Class<?> waiClass : webAppInitializerClasses) {
         
         
      			if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
      					WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
         
         
      				try {
         
         
      					initializers.add((WebApplicationInitializer)
      					ReflectionUtils.accessibleConstructor(waiClass).newInstance());
      				}
      				catch (Throwable ex) {
         
         
      					throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
      				}
      			}
      		}
      	}
      	if (initializers.isEmpty()) {
         
         
      		servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
      		return;
      	}
      	servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
      	AnnotationAwareOrderComparator.sort(initializers);
      	for (WebApplicationInitializer initializer : initializers) {
         
         
      		initializer.onStartup(servletContext);
      	}
      }
   
   }
   

ServletContextInitializer加载原理及过程

1. ServletWebServerApplicationContext容器刷新方法onRefresh() 调用 createWebServer() 创建Web服务
2. 创建容器时，通过getSelfInitializer() => getServletContextInitializerBeans() 获取所有的ServletContextInitializer
3. 遍历ServletContextInitializer实例并调用onStartup(servletContext)方法

补充说明

1. 在嵌入式容器中ServletContainerInitializer将不会被自动加载，替代方案可使用ServletContextInitializer解决

   @Bean
   public ServletContextInitializer servletContextInitializer(){
      return servletContext ->{
          CharacterEncodingFilter filter = new CharacterEncodingFilter() ;
          FilterRegistration.Dynamic dynamic = servletContext.addFilter("c-filter", filter);
          dynamic.addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST), false,"/");
      } ;
   }
   

2. 在嵌入式容器中@WebServlet不会被自动扫描，替代方案可使用@ServletRegistrationBean解决

   // ServletRegistrationBean 继承ServletContextInitializer，所以这里与2中效果等同
   @Bean
   public ServletRegistrationBean servletRegistrationBean(){
        ServletRegistrationBean registrationBean =
                new ServletRegistrationBean(new AsyncServlet(), "/async-servlet") ;
        return registrationBean ;
    }
   

3. Springboot应用部署到传统servlet容器也是利用ServletContainerInitializer的SPI实现

   public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
         
         
   	protected Log logger; // Don't initialize early
   	private boolean registerErrorPageFilter = true;
   	protected final void setRegisterErrorPageFilter(boolean registerErrorPageFilter) {
         
         
   		this.registerErrorPageFilter = registerErrorPageFilter;
   	}
   	@Override
   	public void onStartup(ServletContext servletContext) throws ServletException {
         
         
   		this.logger = LogFactory.getLog(getClass());
   		WebApplicationContext rootAppContext = createRootApplicationContext(
   				servletContext);
   		if (rootAppContext != null) {
         
         
   			servletContext.addListener(new ContextLoaderListener(rootAppContext) {
         
         
   				@Override
   				public void contextInitialized(ServletContextEvent event) {
         
         
   					// no-op because the application context is already initialized
   				}
   			});
   		}
   		else {
         
         
   			this.logger.debug("No ContextLoaderListener registered, as "
   					+ "createRootApplicationContext() did not "
   					+ "return an application context");
   		}
   	}
   	protected WebApplicationContext createRootApplicationContext(
   			ServletContext servletContext) {
         
         
   		SpringApplicationBuilder builder = createSpringApplicationBuilder();
   		StandardServletEnvironment environment = new StandardServletEnvironment();
   		environment.initPropertySources(servletContext, null);
   		builder.environment(environment);
   		builder.main(getClass());
   		ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
   		if (parent != null) {
         
         
   			this.logger.info("Root context already created (using as parent).");
   			servletContext.setAttribute(
   					WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
   			builder.initializers(new ParentContextApplicationContextInitializer(parent));
   		}
   		builder.initializers(
   				new ServletContextApplicationContextInitializer(servletContext));
   		builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
   		builder = configure(builder);
   		SpringApplication application = builder.build();
   		if (application.getAllSources().isEmpty() && AnnotationUtils
   				.findAnnotation(getClass(), Configuration.class) != null) {
         
         
   			application.addPrimarySources(Collections.singleton(getClass()));
   		}
   		Assert.state(!application.getAllSources().isEmpty(),
   				"No SpringApplication sources have been defined. Either override the "
   						+ "configure method or add an @Configuration annotation");
   		// Ensure error pages are registered
   		if (this.registerErrorPageFilter) {
         
         
   			application.addPrimarySources(
   					Collections.singleton(ErrorPageFilterConfiguration.class));
   		}
   		return run(application);
   	}
   	protected SpringApplicationBuilder createSpringApplicationBuilder() {
         
         
   		return new SpringApplicationBuilder();
   	}
   	protected WebApplicationContext run(SpringApplication application) {
         
         
   		return (WebApplicationContext) application.run();
   	}
   	private ApplicationContext getExistingRootWebApplicationContext(
   			ServletContext servletContext) {
         
         
   		Object context = servletContext.getAttribute(
   				WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
   		if (context instanceof ApplicationContext) {
         
         
   			return (ApplicationContext) context;
   		}
   		return null;
   	}
   	protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
         
         
   		return builder;
   	}
   }