传统的Servlet注册是配置在/WEB-INF/web.xml下,在容器启动的时候注册Servlet。Servlet3规范为我们又提供了一种更加方便的注册方法。容器启动的时候扫描jar包,找到jar包下一个文件路径为下面这个的文件。
META-INF/services/javax.servlet.ServletContainerInitializer
该文件的内容必须是一个实现了javax.servlet.ServletContainerInitializer接口的实现类,该接口下有一个方法会自动被调用。
public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException
其中第一个参数是一个Set<Class>,当该实现类上有注解@HandlesTypes,注解的value属性是一个Class[],这个数组及其子类(或实现类)会被注入到webAppInitializerClasses参数中。
下面我们看看Spring是如何利用这种方式完成DispatcherServlet的注册。
在spring-web模块下,META-INF/services/javax.servlet.ServletContainerInitializer文件中的内容为:
org.springframework.web.SpringServletContainerInitializer
该类实现了ServletContainerInitializer接口,并标有@HandlesTypes注解。
@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) { // Be defensive: Some servlet containers provide us with invalid classes, // no matter what @HandlesTypes says... 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); } } }
onStartup方法会将WebApplicationInitializer的实现类实例放到List中排序,依次调用onStartup()方法。Spring为我们提供了一个WebApplicationInitializer的抽象实现类AbstractAnnotationConfigDispatcherServletInitializer,通常我们只需要继承此类来完成DispatcherServlet与spring context的注册。
AbstractAnnotationConfigDispatcherServletInitializer的父类AbstractDispatcherServletInitializer覆盖了onStartup()方法,
public void onStartup(ServletContext servletContext) throws ServletException { super.onStartup(servletContext); registerDispatcherServlet(servletContext); }
首先调用父类的方法完成root WebApplicationContext的初始化,然后调用registerDispatcherServlet()方法完成DispatcherServlet的注册。
protected void registerDispatcherServlet(ServletContext servletContext) { String servletName = getServletName(); Assert.hasLength(servletName, "getServletName() must not return empty or null"); WebApplicationContext servletAppContext = createServletApplicationContext(); Assert.notNull(servletAppContext, "createServletApplicationContext() did not return an application " + "context for servlet [" + servletName + "]"); FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext); dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers()); ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet); Assert.notNull(registration, "Failed to register servlet with name '" + servletName + "'." + "Check if there is another servlet registered under the same name."); registration.setLoadOnStartup(1); registration.addMapping(getServletMappings()); registration.setAsyncSupported(isAsyncSupported()); Filter[] filters = getServletFilters(); if (!ObjectUtils.isEmpty(filters)) { for (Filter filter : filters) { registerServletFilter(servletContext, filter); } } customizeRegistration(registration); }
暂时只关注Servlet的注册,Spring容器相关的以后介绍,createDispatcherServlet()方法创建了一个DispatcherServlet实例,然后通过servletContext.addServlet()方法完成DispatcherServlet的注册。
protected FrameworkServlet createDispatcherServlet(WebApplicationContext servletAppContext) { return new DispatcherServlet(servletAppContext); }
以上就是DispatcherServlet的注册过程。