spring WebApplicationInitializer

    在spring中能够经过代码的方式配置原来的xml文件.在用java 代码配置web.xml时候,spring文档说配置类要实现WebApplicationInitializer接口.关于为何要实现org.springframework.web.WebApplicationInitializer接口,记录下.

实际上是对servlet规范的一种使用.servlet早就支持用代码实现相关的xml配置.

servlet相关接口:

    ServletContainerInitializer   

    @HandlesTypes:

    解释:实现了ServletContainerInitializer 接口的类在web程序启动的时候会被容器扫描加载而且调用其onStartup方法(在方法中注册servlet或者filters等web.xml中会进行的配置).可是实现此接口的类必须在某个jar包中的META-INF/services路径下进行声明(java spi).spring 中的实现类为org.springframework.web.SpringServletContainerInitializer,该类在springweb jar包中声明:

@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上的注解:@HandlesTypes(WebApplicationInitializer.class)

意思是向该类注入HandlesTypes指定的类或者实现该接口的类.因此咱们通常实现WebApplicationInitializer,经过代码注册servlet就能够使用了.

因此流程是spring 中的SpringServletContainerInitializer onStartup()方法被容器调用,而后onStartup()方法最终调用的是咱们实现类中的方法.

参考servlet3.1规范

      spring-framework-reference

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