16.1、动态注册
为了使动态注册成为可能,ServletContext接口中还添加了以下方法,用来动态地创建Web对象:
<T extends Filter>createFilter(Java.lang.Class<T> clazz)
<T extends java.util.EventListener> createListener(java.lang.Class<T> clazz)
<T extends Servlet> createServlet(java.lang.Class<T> clazz)
例如,假设MyServlet是一个可以直接或间接实现javax.servlet.Servlet的类,那么可以通过调用createServlet方法将MyServlet实例化:
Servlet myServlet = createServlet(MyServlet.class)
创建好一个Web对象之后,可以利用以下任意一个方法将它添加到ServletContext中,这也是Servlet3的一项新特性。
FilterRegistration.Dynamic addFilter(java.lang.String filterName, Filter filter)
<T extends java.util.EventListener> void addListener(T t)
ServletRegistration.Dynamic addServlet(java.lang.String servletName, Servlet servlet)
同样地,也可以通过在ServletContext中调用以下任意一个方法,在创建Web对象的同时,将它添加到ServletContext中。
在创建或添加监听器时,传给第一个addListener覆盖方法的类必须实现以下其中一个或多个接口:
1、ServletContextAttributeListener
2、ServletRequestListener
3、ServletRequestAttributeListener
4、HttpSessionListener
5、HttpSessionAttributeListener
如果将ServletContext传给ServletContextInitializer的onStartup方法,那么监听器类也可以实现ServletContextListener接口。
下面这个例子没有用注解,也没有用部署描述符进行声明。
FirstServlet.java
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- package servlet;
- import java.io.IOException;
- import java.io.PrintWriter;
- import javax.servlet.ServletException;
- import javax.servlet.http.HttpServlet;
- import javax.servlet.http.HttpServletRequest;
- import javax.servlet.http.HttpServletResponse;
- public class FirstServlet extends HttpServlet {
- private static final long serialVersionUID = 1L;
- private String name ;
- @Override
- public void doGet(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- response.setContentType("text/html");
- PrintWriter writer = response.getWriter() ;
- writer.println("<html><head><title>First servlet" +
- "</title></head><body>" + name);
- writer.println("</body></html>") ;
- }
- public void setName(String name) {
- this.name = name ;
- }
- }
DynRegListener.java
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- package listener;
- import javax.servlet.Servlet;
- import javax.servlet.ServletContext;
- import javax.servlet.ServletContextEvent;
- import javax.servlet.ServletContextListener;
- import javax.servlet.ServletException;
- import javax.servlet.ServletRegistration;
- import javax.servlet.annotation.WebListener;
- import servlet.FirstServlet;
- @WebListener
- public class DynRegListener implements ServletContextListener {
- @Override
- public void contextDestroyed(ServletContextEvent arg0) {
- }
- @Override
- public void contextInitialized(ServletContextEvent sce) {
- ServletContext servletContext = sce.getServletContext() ;
- Servlet firstServlet = null ;
- try {
- firstServlet = servletContext.createServlet(FirstServlet.class) ;
- } catch (ServletException e) {
- e.printStackTrace();
- }
- if(firstServlet != null && firstServlet instanceof FirstServlet){
- ((FirstServlet) firstServlet).setName("Dynamically registered servlet");
- }
- ServletRegistration.Dynamic dynamic = servletContext.addServlet("firstServlet", firstServlet) ;
- dynamic.addMapping("/dynamic") ;
- }
- }
当应用程序启动时,容器会调用监听器的contextInitialized方法,结果是创建了一个FirstServlet实例,并注册和映射到/dynamic。可以利用下面这个路径访问FirstServlet
http://localhost:8080/filedowmload/dynamic
16.2、Servlet容器初始化
如果你使用过像Struts或Struts2这类Java Web框架,就应该知道,在使用框架之前必须先配置应用程序。一般来说,需要通过修改部署描述符,告诉Servlet容器你正在使用一个框架。例如,要想在应用程序中使用Struts2,可以将以下标签添加到部署描述符中:
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- <filter>
- <filter-name>struts2</filter-name>
- <filter-class>org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter</filter-class>
- </filter>
- <filter-mapping>
- <filter-name>struts2</filter-name>
- <url-pattern>/*</url-pattern>
- </filter-mapping>
但在Servlet3中,就不再需要这些了。框架可以进行打包,自动完成Web对象的首次注册。
Servlet容器初始化的核心是javax.servlet.ServletContainerInitializer接口。这是一个简单的接口,它只有一个方法:onStartup。在执行任何ServletContext监听器之前,由Servlet容器调用这个方法。
实现ServletContainerInitializer的类必须用@HandleTypes注解进行标注,以便声明初始化程序可以处理这些类型的类。
下面是个例子。要把下面的类和文件打成jar包。
FirstServlet.java
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- package servlet;
- import java.io.IOException;
- import java.io.PrintWriter;
- import javax.servlet.ServletException;
- import javax.servlet.http.HttpServlet;
- import javax.servlet.http.HttpServletRequest;
- import javax.servlet.http.HttpServletResponse;
- public class FirstServlet extends HttpServlet {
- private static final long serialVersionUID = 1L;
- private String name = "wuhaixu" ;
- @Override
- public void doGet(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- response.setContentType("text/html");
- PrintWriter writer = response.getWriter() ;
- writer.println("<html><head><title>First servlet" +
- "</title></head><body>" + name);
- writer.println("</body></html>") ;
- }
- }
javax.servlet.ServletContainerInitializer
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- initializer.MyServletContainerInitializer
MyServletContainerInitializer.java
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- package initializer;
- import java.util.Set;
- import javax.servlet.ServletContainerInitializer;
- import javax.servlet.ServletContext;
- import javax.servlet.ServletException;
- import javax.servlet.ServletRegistration;
- import javax.servlet.annotation.HandlesTypes;
- import servlet.FirstServlet;
- @HandlesTypes({FirstServlet.class})
- public class MyServletContainerInitializer implements ServletContainerInitializer {
- @Override
- public void onStartup(Set<Class<?>> classes, ServletContext servletContext)
- throws ServletException {
- System.out.println("onStartup");
- ServletRegistration registration = servletContext.addServlet("firstServlet", "servlet.FirstServlet");
- registration.addMapping("/f");
- System.out.println("leaving onStartup");
- }
- }
把这些文件和类放在下面的包中,在cmd进行jar包压缩
结果为: