servlet的动态注册

只能在webapp启动时进行注册.
在Servlet3.0中可以动态注册Servlet,Filter,Listener，在ServletContext对应注册API为：

  /** 
   * 添加Servlet 
   */ 
  public ServletRegistration.Dynamic addServlet(String servletName, 
      String className); 

  public ServletRegistration.Dynamic addServlet(String servletName, 
      Servlet servlet); 

  public ServletRegistration.Dynamic addServlet(String servletName, 
      Class<? extends Servlet> servletClass); 

  /** 
   * 添加Filter 
   */ 
  public FilterRegistration.Dynamic addFilter(String filterName, 
      String className); 

  public FilterRegistration.Dynamic addFilter(String filterName, Filter filter); 

  public FilterRegistration.Dynamic addFilter(String filterName, 
      Class<? extends Filter> filterClass); 

  /** 
   * 添加Listener 
   */ 
  public void addListener(String className); 

  public <T extends EventListener> void addListener(T t); 

  public void addListener(Class<? extends EventListener> listenerClass);

每个组件注册都提供三个方法。
只能在初始化时进行注册。在初始化情况下的注册Servlet组件有两种方法：
1.实现ServletContextListener接口,在contextInitialized方法中完成注册.
2.在jar文件中放实现ServletContainerInitializer接口的初始化器
在ServletContextListener监听器中完成注册。

    public void contextInitialized(ServletContextEvent sce) { 

        ServletContext sc = sce.getServletContext(); 

        // Register Servlet 
        ServletRegistration sr = sc.addServlet("DynamicServlet", 
            "web.servlet.dynamicregistration_war.TestServlet"); 
        sr.setInitParameter("servletInitName", "servletInitValue"); 
        sr.addMapping("/*"); 

        // Register Filter 
        FilterRegistration fr = sc.addFilter("DynamicFilter", 
            "web.servlet.dynamicregistration_war.TestFilter"); 
        fr.setInitParameter("filterInitName", "filterInitValue"); 
        fr.addMappingForServletNames(EnumSet.of(DispatcherType.REQUEST), 
                                     true, "DynamicServlet"); 

        // Register ServletRequestListener 
        sc.addListener("web.servlet.dynamicregistration_war.TestServletRequestListener"); 
    }

在jar文件中的servlet组件注册，需要在jar包含META-INF/services/javax.servlet.ServletContainerInitializer文件，文件内容为已经实现ServletContainerInitializer接口的类：

com.learn.servlet3.jardync.CustomServletContainerInitializer

该实现部分代码：

@HandlesTypes({ JarWelcomeServlet.class }) 
public class CustomServletContainerInitializer implements 
    ServletContainerInitializer { 
  private static final Log log = LogFactory 
      .getLog(CustomServletContainerInitializer.class); 

  private static final String JAR_HELLO_URL = "/jarhello"; 

  public void onStartup(Set<Class<?>> c, ServletContext servletContext) 
      throws ServletException { 
    log.info("CustomServletContainerInitializer is loaded here..."); 

    log.info("now ready to add servlet : " + JarWelcomeServlet.class.getName()); 

    ServletRegistration.Dynamic servlet = servletContext.addServlet( 
        JarWelcomeServlet.class.getSimpleName(), 
        JarWelcomeServlet.class); 
    servlet.addMapping(JAR_HELLO_URL); 

    log.info("now ready to add filter : " + JarWelcomeFilter.class.getName()); 
    FilterRegistration.Dynamic filter = servletContext.addFilter( 
        JarWelcomeFilter.class.getSimpleName(), JarWelcomeFilter.class); 

    EnumSet<DispatcherType> dispatcherTypes = EnumSet 
        .allOf(DispatcherType.class); 
    dispatcherTypes.add(DispatcherType.REQUEST); 
    dispatcherTypes.add(DispatcherType.FORWARD); 

    filter.addMappingForUrlPatterns(dispatcherTypes, true, JAR_HELLO_URL); 

    log.info("now ready to add listener : " + JarWelcomeListener.class.getName()); 
    servletContext.addListener(JarWelcomeListener.class); 
  } 
}

其中@HandlesTypes注解表示CustomServletContainerInitializer 可以处理的类，在onStartup 方法中，可以通过Set<Class<?>> c 获取得到。
jar文件中不但可以包含需要自定义注册的servlet，也可以包含应用注解的servlet，具体怎么做，视具体环境而定。
把处理某类事物的servlet组件打包成jar文件，有利于部署和传输，功能不要了，直接去除掉jar即可，方便至极！