spring WebApplicationInitializer

    在spring中能夠經過代碼的方式配置原來的xml文件.在用java 代碼配置web.xml時候,spring文檔說配置類要實現WebApplicationInitializer接口.關於爲何要實現org.springframework.web.WebApplicationInitializer接口,記錄下.

實際上是對servlet規範的一種使用.servlet早就支持用代碼實現相關的xml配置.

servlet相關接口:

    ServletContainerInitializer   

    @HandlesTypes:

    解釋:實現了ServletContainerInitializer 接口的類在web程序啓動的時候會被容器掃描加載而且調用其onStartup方法(在方法中註冊servlet或者filters等web.xml中會進行的配置).可是實現此接口的類必須在某個jar包中的META-INF/services路徑下進行聲明(java spi).spring 中的實現類爲org.springframework.web.SpringServletContainerInitializer,該類在springweb jar包中聲明:

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
	@Override
	public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
			throws ServletException {

		List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();

		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) 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);
		}
	}

}

SpringServletContainerInitializer上的註解:@HandlesTypes(WebApplicationInitializer.class)

意思是向該類注入HandlesTypes指定的類或者實現該接口的類.因此咱們通常實現WebApplicationInitializer,經過代碼註冊servlet就能夠使用了.

因此流程是spring 中的SpringServletContainerInitializer onStartup()方法被容器調用,而後onStartup()方法最終調用的是咱們實現類中的方法.

參考servlet3.1規範

      spring-framework-reference

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增長篇文章:http://stefan-isele.logdown.com/posts/201646  (2017年11月10日10:35:44)