Java EE入门教程系列第四章Servlet（四）——过滤器

Myth丶恋晨 2022-04-13 06:40 174阅读 0赞

## 4.4 过滤器 ##

### 4.4.1 过滤器（Filter）简介 ###

Servlet过滤器是服务器与客户端请求和响应的中间层组件。在实际项目开发中，Servlet过滤器主要用于拦截浏览器与服务间的请求与响应，根据过滤器内部的设置，查看、提取或修改交互的数据，之后再转给下一个资源。

主要作用有：

1）对请求的访问进行预处理，如防止乱码、添加必要的安全信息或安全处理等

2）对被过滤资源进行身份验证，实现一定程度上的权限控制

3）对请求进行合理的转发指派，降低服务器负载，提高服务器效率

可以把过滤器看成银行大堂经理，当用户去办理业务时，大堂经理会询问客户需要做什么，根据业务内容进行引导。如果表格填写错误，还会帮忙修正，如果用户去错了银行，也会进行提醒。

从工作原理看，过滤器可以改变一个request或修改一个response。过滤器并不是一个Servlet，它不会生成response，只是在request要离开Servlet时再处理response。以一种Servlet链的方式完成响应。一个过滤器的处理过程包括：

1）在客户端发起对Servlet文件的request时截获请求

2）在Servlet被调用前检查request

3）根据过滤器的设计，修改request头和request数据

4）根据过滤器的设计，修改response头和response数据

5）在Servlet被调用之后截获这个对象

开发者在创建自己的Filter时，必须要实现javax.servlet.Filter这个接口。该接口包含3个抽象方法：init（）、doFilter（）、destroy（）。大多数情况下，Filter的销毁使用Java系统的自动回收机制，如果没有初始化配置的特殊要求，init（）方法也不需要重载。

### 4.4.2 创建过滤器 ###

首先创建被过滤文件First.java

package cy.filter;
    
    import java.io.IOException;
    import java.io.PrintWriter;
    
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    /**
     * Servlet implementation class First
     */
    @WebServlet("/First")
    public class First extends HttpServlet {
    	private static final long serialVersionUID = 1L;
           
        /**
         * @see HttpServlet#HttpServlet()
         */
        public First() {
            super();
            // TODO Auto-generated constructor stub
        }
    
    	/**
    	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		response.setContentType("text/html;charset=UTF-8");
    		request.setCharacterEncoding("UTF-8");
    		PrintWriter out=response.getWriter();
    		try {
    			  String getInfo=(String) getServletContext().getAttribute("Message");
    				out.println("<html");
    				out.println("<head>");
    				out.println("<title></title>");
    				out.println("</head>");
    				out.println("<body>");
    				out.println("lalala");
    				
    				out.println("</body>");
    				out.println("</html");
    			}finally {
    				out.close();
    			}
    		System.out.println("执行当前的Servlet文件");
    	}
    
    	/**
    	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		doGet(request, response);
    	}
    
    }

右键项目名-new-Filter-填写类和包名

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70][]

添加被过滤文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70 1][]

package cy.filter;
    
    import java.io.IOException;
    import javax.servlet.FilterChain;
    import javax.servlet.FilterConfig;
    import javax.servlet.ServletException;
    import javax.servlet.ServletRequest;
    import javax.servlet.ServletResponse;
    import javax.servlet.annotation.WebFilter;
    
    /**
     * Servlet Filter implementation class Filter
     */
    @WebFilter(urlPatterns = { "/First"}, filterName = "Filter", servletNames = { "First" })
    public class Filter implements javax.servlet.Filter {
    
        /**
         * Default constructor. 
         */
        public Filter() {
            // TODO Auto-generated constructor stub
        }
    
    	/**
    	 * @see Filter#destroy()
    	 */
    	public void destroy() {
    		// TODO Auto-generated method stub
    	}
    
    	/**
    	 * @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)
    	 */
    	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    		// TODO Auto-generated method stub
    		// place your code here
    		System.out.println("执行doFilter之前");
    		// pass the request along the filter chain
    		chain.doFilter(request, response);
    		System.out.println("执行doFilter之后");
    	}
    
    	/**
    	 * @see Filter#init(FilterConfig)
    	 */
    	public void init(FilterConfig fConfig) throws ServletException {
    		// TODO Auto-generated method stub
    	}
    
    }

执行First.java：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70 2][]

从执行结果可以好好体会一下过滤过程。

下面举一个相对复杂一些的例子。实际上网过程中经常遇到这样的情况，没有登录浏览贴吧帖子时，看到帖子点击回复按钮时系统会弹出错误提示，提醒用户登录后才可以回复。这个功能可以使用过滤器方式实现。

下面将通过3个文件模拟上述过程：

回帖文件sendInfo.java，需要输入title和id两个参数，然后显示出来

package cy.filter;
    
    import java.io.IOException;
    import java.io.PrintWriter;
    
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    /**
     * Servlet implementation class First
     */
    @WebServlet("/sendInfo")
    public class sendInfo extends HttpServlet {
    	private static final long serialVersionUID = 1L;
           
        /**
         * @see HttpServlet#HttpServlet()
         */
        public sendInfo() {
            super();
            // TODO Auto-generated constructor stub
        }
    
    	/**
    	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		response.setContentType("text/html;charset=UTF-8");
    		PrintWriter out=response.getWriter();
    		String title=request.getParameter("title");
    		String id=request.getParameter("id");
    		try {
    			  
    				out.println("<html");
    				out.println("<head>");
    				out.println("<title></title>");
    				out.println("</head>");
    				out.println("<body>");
    				out.println("一个测试的信息：”"+title+"”来自于"+id);
    				
    				out.println("</body>");
    				out.println("</html");
    			}finally {
    				out.close();
    			}
    		System.out.println("执行当前的Servlet文件");
    	}
    
    	/**
    	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		doGet(request, response);
    	}
    
    }

可以先运行一下这个文件，不过要注意这个文件不是直接运行的，回忆一下前面URL传参的知识，在地址栏给出参数即可得到信息：

![20181128002246136.png][]

过滤器文件MyFilter.java，过滤第一个文件，依据是检查里面的参数id的值是否不为空且值为“Hedy”，如果是则不作任何处理，让

Servlet向浏览器发送内容，如果不是，则拦截请求，直接重定向到错误文件

package cy.filter;
    
    import java.io.IOException;
    import javax.servlet.Filter;
    import javax.servlet.FilterChain;
    import javax.servlet.FilterConfig;
    import javax.servlet.ServletException;
    import javax.servlet.ServletRequest;
    import javax.servlet.ServletResponse;
    import javax.servlet.annotation.WebFilter;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    /**
     * Servlet Filter implementation class MyFilter
     */
    @WebFilter(urlPatterns = { "/sendInfo" },filterName="MyFilter", servletNames = { "sendInfo" })
    public class MyFilter implements Filter {
    
        /**
         * Default constructor. 
         */
        public MyFilter() {
            // TODO Auto-generated constructor stub
        }
    
    	/**
    	 * @see Filter#destroy()
    	 */
    	public void destroy() {
    		// TODO Auto-generated method stub
    	}
    
    	/**
    	 * @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)
    	 */
    	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    		// TODO Auto-generated method stub
    		// place your code here
    		HttpServletRequest req=(HttpServletRequest)request;//对这两个参数进行强制转换的原因是：doFilter()方法本身的请求与响应的参数类型为ServletRequest和ServletResponse，而Servlet请求与响应的参数类型为HttpServletRequest和HttpServletResponse.为了让过滤器拦截请求的同时可以获取客户端传过来的参数id，需要转换这个类型，从而通过getParameter（）方法获取参数值。
    		HttpServletResponse res=(HttpServletResponse)response;
          String isLog=req.getParameter("id");
          System.out.println(isLog);
          if((isLog!=null)&&(isLog.equals("Hedy"))) {
        	// pass the request along the filter chain
      		chain.doFilter(request, response); 
          }
          else {
        	  res.sendRedirect("/JSPLab/error.jsp");
          }
    	}
    
    	/**
    	 * @see Filter#init(FilterConfig)
    	 */
    	public void init(FilterConfig fConfig) throws ServletException {
    		// TODO Auto-generated method stub
    	}
    
    }

错误页面error.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
        pageEncoding="UTF-8"%>
    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
    <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Insert title here</title>
    </head>
    <body>
    <h2>出错啦！请先登录</h2>
    </body>
    </html>

这时再次运行sendInfo.java，修改id参数值不为Hedy:

![20181128004445431.png][]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70]: /images/20220413/4f06f87019e24556b6506d9e1bec55aa.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70 1]: /images/20220413/887dc0008c6745b0bd99591644003163.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwOTk2MDQx_size_16_color_FFFFFF_t_70 2]: /images/20220413/2df3d49a5c79488c83f5301a21e141d3.png
[20181128002246136.png]: /images/20220413/ca5ff70f27ba46d298ce4062c35c70b3.png
[20181128004445431.png]: /images/20220413/a60ffe5e5f9a4c09bf017292b1997caa.png