Filter过滤器

Fileter可以实现：
1）客户端的请求访问servlet之前拦截这些请求，对用户请求进行预处理
2）对HttpServletResponse进行后处理；

注意
多个Filter的执行顺序在web.xml配置文件中的配置顺序一致，一般把feilter配置在servlet之前

代码示例

public class MyFilter implements Filter {

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
	System.out.println("过滤器拦截成功");
		
		request.setCharacterEncoding("UTF-8");
		response.setCharacterEncoding("GBK");
		
		//放行请求
		chain.doFilter(request, response);
		
	}

}

web.xml配置Filter

 <!-- 配置filter过滤器 -->
  <filter>
   <!-- 别名 -->
  	<filter-name>my</filter-name>
  	 <!-- 地址 -->
  	<filter-class>code.filter.MyFilter</filter-class>
  </filter>
   <!-- 配置filter映射关系 -->
  <filter-mapping>
  	<filter-name>my</filter-name>
  	 <!-- 设置拦截路径 ，路径为/* 为拦截所有路径-->
  	<url-pattern>/m</url-pattern>
  </filter-mapping>

执行顺序

在web.xml设置初始化参数

getInitParameter（）方法可以获取在web.xml配置文件中配置的init-param参数

//在MyFilter内的init方法验证
@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		System.out.println("filter-init");
		String site = filterConfig.getInitParameter("Site");
		System.out.println(site);
	}

listener监听器

定义

用于监听web项目的常见对象
HttpServletRequest/HttpSession/ServltContext/HttpServletResponse
作用
1）监听web对象的创建与销毁
2）监听web对象属性的变化
3）监听session绑定javabean的操作（Java类）

代码

package code.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

public class ReqListener implements ServletRequestListener{

	@Override
	public void requestDestroyed(ServletRequestEvent sre) {
		System.out.println("listener-req-destroy");
	}

	@Override
	public void requestInitialized(ServletRequestEvent sre) {
		System.out.println("listener-req-init");

	}
	
}

web.xml配置Listener

运行顺序

filter过滤器和listener监听器的区别

![image]
(https://img2022.cnblogs.com/blog/2636359/202211/2636359-20221116205054468-2011387421.png)

WEB-INF目录

利用反射创建BaseServlet实现调用自定义业务方法

BaseServlet不需要再配置web.xml，因为只需要其提供doGet（）doPost（）方法

public class BaseServlet extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
		//获取请求中的method参数，该参数的值，表示业务方法的名字
		String mName = req.getParameter("method");
		
		//this当前类对象，创建反射的Class类对象
		Class c = this.getClass();
		
		//创建反射method对象
		try {
			Method method = c.getDeclaredMethod(mName, HttpServletRequest.class,HttpServletResponse.class);
			
			//执行方法
			method.invoke(this, req, res);
			
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
		doGet(req, res);
	}
	
	public void test(HttpServletRequest req, HttpServletResponse res) throws IOException{
		PrintWriter out = res.getWriter();
		out.write("baseservlet-function-test()");
		out.flush();
		if(out!=null) out.close();
	}
	public void test2(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
		req.getRequestDispatcher("page/demo.jsp").forward(req, res);
	}
	
}

可以将BaseServlet设置doGet（）方法，然后其他的servlet类都可以继承这个类实现调用自定义业务方法

public class BaseServlet extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
		//获取请求中的method参数，该参数的值，表示业务方法的名字
		String mName = req.getParameter("method");
		
		//this当前类对象，创建反射的Class类对象
		Class c = this.getClass();
		
		//创建反射method对象
		try {
			Method method = c.getDeclaredMethod(mName, HttpServletRequest.class,HttpServletResponse.class);
			
			//执行方法
			method.invoke(this, req, res);
			
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
		doGet(req, res);//如果post方法接收的话，也跳转到doGet方法
	}
}




//

//因为 MyServlet 继承 BaseServlet，所以继承了baseservlet的doget方法，所以在运行时BaseServlet内的this关键字相当于指代MyServlet类对象

public class MyServlet extends BaseServlet{
	public void run(HttpServletRequest req, HttpServletResponse res){
		PrintWriter out = res.getWriter();
		out.write("MyServlet-function test()");
		out.flush();
		if(out != null) out.close();
	}
}

原文地址：http://www.cnblogs.com/xiaoto9426/p/16897568.html