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 {
    
 
    /** 
     * 完成Filter的初始化功能，服务器开启时就执行 
     * @param filterConfig 
     * @throws ServletException 
     */ 
    @Override 
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println("我是filter1 init"); 
    } 
 
    /** 
     * 完成Filter的功能，每执行Filter对应的Servlet时执行一次 
     * @param servletRequest 
     * @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 requestInitialized(ServletRequestEvent servletRequestEvent) {
 
System.out.println("request对象被创建" +  System.currentTimeMillis()); 
} 
@Override 
public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
 
System.out.println("request对象被销毁" +  System.currentTimeMillis()); 
} 
@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。