servlet的动态注册

Myth丶恋晨 2022-12-29 14:00 191阅读 0赞

说实话,对servlet 3动态加载servlet的机制有些失望,本来期望着可以在运行时完成对servlet的注册和销毁,但现状是,**只能在webapp启动在初始化时进行完成注册**,可能是为了安全考虑吧。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，很细心。下面谈谈动态注册Servlet，**但不要希望太高，只能在初始化时进行注册**。在运行时为了安全原因，无法完成注册。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文件中放入实现ServletContainerInitializer接口的初始化器  
先说在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"); 
        }

很简单，难度不大。  
再说说在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！