Servlet——Servlet过滤器和监听器详解编程语言

过滤器（Filter）是能够为请求和响应的header和内容进行操作的一种特殊的web组件。特殊之处在于其本身并不直接生成web响应，而是拦截过滤web请求和响应。下面直接用代码说明Servlet过滤器

** 

 * 完成请求的拦截，以及资源的统一管理功能 

 * 定义 

 * @author Song X. 

 * @date 2020/02/16 

public class MyFilter1 implements Filter {


 * @param servletResponse 

 * @param filterChain filterChain用于调用过滤器链中的一系列过滤器，这是责任链设计模式的具体体现 

 * @throws IOException 

 * @throws ServletException 

 @Override 

 public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

 //开始执行filter 

 System.out.println("我是filter1逻辑处理"); 

 //为过滤的Servlet设置统一编码格式 

 servletRequest.setCharacterEncoding("utf-8"); 

 servletResponse.setCharacterEncoding("gbk"); 

 //调用下一个过滤器，若没有下一个过滤器则执行对应的Servlet 

 filterChain.doFilter(servletRequest,servletResponse); 

 //Servlet处理完后才会输出这句话 

 System.out.println("filter1处理完成"); 

 /** 

 * 完成Filter的销毁功能，在关闭服务器时执行 

 @Override 

 public void destroy() {

 System.out.println("我是filter1销毁功能"); 

public class MyFilter2 implements Filter {

 @Override 

 public void init(FilterConfig config) throws ServletException {

 System.out.println("我是filter2 init"); 

 @Override 

 public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain) throws ServletException, IOException {

 System.out.println("我是filter2逻辑处理"); 

 filterChain.doFilter(req, resp); 

 System.out.println("filter2处理完成"); 

 @Override 

 public void destroy() {

 System.out.println("我是filter2销毁功能"); 

public class FilterServlet1 extends HttpServlet {


 @Override 

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 this.doGet(request,response); 

 @Override 

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 System.out.println("我是FilterServlet1"); 

 response.getWriter().write("我是FilterServlet1"); 

public class FilterServlet2 extends HttpServlet {


 @Override 

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 this.doGet(request,response); 

 @Override 

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 System.out.println("我是FilterServlet2"); 

 response.getWriter().write("我是FilterServlet2"); 

 !--有多个针对同一Servlet的过滤器时，过滤器链的顺序按照web.xml中配置Filter的顺序 

 这里针对所有Servlet，Filter的过滤顺序是先MyFilter1，后MyFilter2 

 url-pattern表示要过滤的Servlet的url-pattern-- 

 filter 

 filter-name MyFilter1 /filter-name 

 filter-class home.frank.filterProject.MyFilter1 /filter-class 

 /filter 

 filter-mapping 

 filter-name MyFilter1 /filter-name 

 url-pattern /* /url-pattern 

 /filter-mapping 

 filter 

 filter-name MyFilter2 /filter-name 

 filter-class home.frank.filterProject.MyFilter2 /filter-class 

 /filter 

 filter-mapping 

 filter-name MyFilter2 /filter-name 

 url-pattern /* /url-pattern 

 /filter-mapping 

Servlet监听器可以理解为一直观察Servlet的监视器，当Servlet发生一些行为时，监听器作出一些必要的响应。实际上所有监听器都是这样。比如，有监听ServletContext的ServletContext，有监听session的HttpSessionListener，有监听request的ServletRequestListener。

下面以Request为例，代码演示说明各监听器

public class ListenerServlet extends HttpServlet {


 @Override 

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 this.doGet(request,response); 

 @Override 

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

 System.out.println("listenerServlet逻辑处理"); 

 request.setCharacterEncoding("utf-8"); 

 response.setCharacterEncoding("gbk"); 

 request.setAttribute("listen","listen"); 

 request.removeAttribute("listen"); 

 response.getWriter().write("listen study"); 

/** 

 * 监听器：主要用于监听作用域对象的创建，销毁，以及属性设置，可以添加一些公共属性设置，做逻辑判断 

 * 主要监听三种： 

 * Request：主要有两个监听接口: ServletRequestListener, ServletRequestAttributeListener 

 * ServletRequestListener: 监听request对象的创建和销毁 

 * ServletRequestAttributeListener：监听request作用域属性的添加删除和修改 

 * ServletContext：主要有两个监听接口: ServletContextListener, ServletContextAttributeListener 

 * ServletContextListener: 监听ServletContext对象的创建和销毁 

 * ServletContextAttributeListener：监听ServletContext作用域属性的添加删除和修改 

 * Session：主要有四个监听接口: HttpSessionListener, HttpSessionAttributeListener, HttpSessionActivationListener, HttpSessionBindingListener 

 * HttpSessionListener：监听Session对象的创建和销毁 

 * HttpSessionAttributeListener：监听Session对象作用域属性的添加删除和修改 

 * HttpSessionActivationListener：监听Session对象中数据从内存中持久化要硬盘，或从硬盘读入数据 

 * HttpSessionBindingListener：与HttpSessionListener类似，只不过HttpSessionBindingListener的使用需要实例化这么个监听器对象， 

 * 然后以session.setAttribute的方式绑定到一个session上实行监控。也就是说HttpSessionListener是对所有session的监控， 

 * 而HttpSessionBindingListener可以实现一对一的监控 

 * 下面代码以request为例，简单演示监听器的作用 

 * @author Song X. 

 * @date 2020/02/16 

public class MyListen implements ServletRequestListener, ServletRequestAttributeListener {


 @Override 

 public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {

 System.out.println("向request作用域中添加数据"); 

 System.out.println(servletRequestAttributeEvent.getName()); 

 System.out.println(servletRequestAttributeEvent.getValue()); 

 @Override 

 public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {

 System.out.println("向request作用域删除数据"); 

 System.out.println(servletRequestAttributeEvent.getName()); 

 System.out.println(servletRequestAttributeEvent.getValue()); 

 @Override 

 public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {

 System.out.println("向request作用域替换数据"); 

 System.out.println(servletRequestAttributeEvent.getName()); 

 System.out.println(servletRequestAttributeEvent.getValue()); 

 servlet 

 servlet-name ListenerServlet /servlet-name 

 servlet-class home.frank.listenerProject.ListenerServlet /servlet-class 

 /servlet 

 servlet-mapping 

 servlet-name ListenerServlet /servlet-name 

 url-pattern /listen /url-pattern 

 /servlet-mapping 

 listener 

 listener-class home.frank.listenerProject.MyListen /listener-class 

 /listener 

到此为止，Servlet全部回顾完毕。Servlet虽然现在在开发中已经很少使用了，但是了解它的原理，对学习Java EE框架底层源码原理，有很大的帮助。

原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/20598.html